|Year : 2018 | Volume
| Issue : 5 | Page : 1-13
|Date of Web Publication||26-Oct-2018|
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
. Invited Talks. J Med Phys 2018;43, Suppl S1:1-13
| I-1: Dosimetry of Small Static Fields Used in External Beam Radiotherapy: A Review of Iaea TRS 483|| |
Saskatchewan Cancer Agency, University of Saskatchewan, Saskatoon, Saskatchewan, Canada. E-mail: gavin.[email protected]
In 2008, Alfonso et al. (Med. Phys. 35, 5179-86) published a stage setting paper outlining a well thought out dosimetry formalism for reporting corrected relative output factors for small and non-standard fields. However, many questions remained. Both experimental and Monte Carlo implementations of the proposed formalism helped guide the community in developing a code of practice (COP). The IAEA has now published a COP for small field dosimetry. Report 483 “Dosimetry of small static fields used in external beam radiotherapy – An International code of practice for reference and relative dose determination” follows a productive 10 years of research that addressed the many questions associated with the challenges related to small field dosimetry. A review of small field dosimetry physics, a detailed discussion of the formalism and practical aspects of clinical implementation will be covered.
| I-2: Three Dimensional Dosimetry by Polymer Gel and Solid Plastic Dosimeters|| |
University of Minnesota, Minneapolis, Minnesota, USA
Accurate dose measurement tools are needed to evaluate the radiation dose delivered to patients by using modern and sophisticated radiation therapy techniques. However, the adequate tools which enable us to directly measure the dose distributions in three dimensional (3D) space are not commonly available. One such 3D dose measurement device is the polymer-based dosimeter, which changes the material property in response to radiation. These are available in the gel form such as polymer gel dosimeter (PGD) and ferrous gel dosimeter (FGD) and in the solid form as solid plastic dosimeter (SPD). Those are made of a continuous uniform medium which polymerizes upon irradiation. Hence, the intrinsic spatial resolution of those dosimeters is very high, and it is only limited by the method by which one converts the dose information recorded by the medium to the absorbed dose. The current standard methods of the dose quantification are magnetic resonance imaging, optical computed tomography, and X-ray computed tomography. In particular, magnetic resonance imaging is well established as a method for obtaining clinically relevant dosimetric data by PGD and FGD. Despite the likely possibility of doing 3D dosimetry by PGD, FGD or SPD, the tools are still lacking more extensive clinical applications. In this presentation, I summarize the current status of PGD, FGD, and SPD and discuss the issue faced by these for broader acceptance in radiation oncology clinic and propose some directions for future development.
| I-3: Patient Dosimetry in Diagnostic Radiology|| |
Kalpana M. Kanal
Department of Radiology, University of Washington Medical Center, Seattle, WA 98195-7115, USA. E-mail: [email protected]
Estimating patient dose is an important skill for a Medical Physicist to have since medical physicists are called upon to estimate patient dose in several modalities. These requests for calculating patient dose may originate from patients, providers, or Institutional Review Board within an organization which reviews doses for research studies. Dose estimates may be used for quality assurance and improvement processes, protocol optimization, benchmarking, or patient risk estimates.
In this session, CT, Fluoroscopy and fetal dosimetry as well discuss recently published diagnostic reference levels (DRLs) for adult CT exams in the USA will be discussed.
- CT - Dose metrics displayed on the CT scanner such as CTDIVOL and DLP can be useful tools however each has limitations that must be understood in order to provide an appropriate dose estimate for the circumstance at hand. Evaluation of individual patient risk estimates often requires a more rigorous evaluation.
- Fluoroscopy and Interventional Radiology – Review dose metrics and radiobiology relevant to the modality and work through adult and pediatric dose calculations using a case-based approach
- Fetal dosimetry - The goal here is to provide a review of scientific, regulatory, and educational material on the topic of fetal exposure (and risk calculation).
- Discuss recently published DRLs for the ten most common adult CT exams in the USA.
- Describe the limitations of displayed dose metrics for estimating patient dose
- Review dose metrics and radiobiology relevant to the modality
- Provide a review of fetal dose risk from ionizing radiation
- Provide a review of dose calculation techniques
- Discuss the recently published DRL for adult CT exams in the USA.
| I-4: Modern Technology: Clinician's Feedback to the Medical Physicist|| |
D. N. Sharma
Department of Radiation Oncology, All India Institute of Medical Sciences, New Delhi, India
Radiation oncology acts as a center stage in the treatment of cancer. About 70-80% of cancer patients require radiotherapy, either for cure or palliation, at some point of time during their illness. There has been lot of advances in radiation oncology in the recent years. The major highlights of the last two decades have been the advent of state of the art technologies like IMRT, IGRT, SBRT, proton therapy and image guided brachytherapy etc.
Technological advancement is the key factor behind the advances in radiation oncology. Computerization has played a pivotal role and the advances in imaging technology have helped radiation oncology a lot. Computed tomography is used routinely for radiotherapy planning. The radiation oncology has evolved from two dimensional to three dimensional planning. The auto contouring and auto segmentation tools in the new planning systems are highly useful. With current technology, we can precisely conform high doses to tumor target at the same time protecting the critical structures. This helps in dose escalation without increasing radiation related morbidity thus improving the therapeutic ratio.
However, the modern technology should be assessed whether it has made a significant clinical impact. The radiation oncologists must convey the therapeutic results achieved by the technological and refined dose delivery advances to all the personnel involved in radiation treatment (medical physicists, dosimetrists, radiation therapy technologists etc.). This will help in adopting, rejecting and amending a particular radiation technology. Any given new technological advancement may be measured with at least three yardsticks namely, local control, survival and quality of life.
There have been definite improvements in toxicity outcome with modern precision RT techniques. For head and neck cancer patients, xerostomia rates have significantly come down. The GI toxicity with pelvic IMRT has also been reported to be significantly lower. SBRT has improved the local control rates in early lung tumors and posing a challenge to the traditional treatment like surgery. The survival improvement data across all tumor sites is yet to come with the use of modern technology as pr the literature so far. The risk of secondary cancers induced by dose spillages to OAR in IMRT continue to be an issue although no such data exists as of now. The waiting times have definitely becoming longer due to extra time consumed by the sophisticated RT techniques. This could jeopardize the overall cure rate in a specified population. This aspect may be more relevant in the Indian context where majority of patients still come in advanced stages needing palliative RT. It is to be seen as to what extent the IMRT/IGRT will benefit the palliative group of patients.
These developments also mandate a fresh look at our physician training programs. Appropriate training in image acquisition and interpretation would be highly useful in this scenario to prevent systematic errors in treatment planning. Incorporation of a good quality assurance programme to monitor treatment delivery and execution is another challenge to be met head on. However prudent clinical judgment must be used in applying these new tools as indiscriminate and over enthusiastic usage may not auger too well in this era of evidence-based medicine.
| I-5: Quality Assurance of Specialized Radiotherapy Equipments: Preliminary Report of Atomic Energy Regulatory Board Task Group|| |
S. D. Sharma1,2, G. Sahani3, Smriti Sharma3, S. Jamema4, R. Upreti4, S. Deshpande5, P. K. Dixit3, R. L. Sha3
1Radiological Physics and Advisory Division, Bhabha Atomic Research Centre, 2Homi Bhabha National Institute, Anushaktinagar, 3Radiological Safety Division, Atomic Energy Regulatory Board, 4Department of Medical Physics, Tata Memorial Hospital, 5Department of Radiation Oncology, P. D. Hinduja National Hospital, Mumbai, Maharashtra, India. E-mail: [email protected]
Atomic Energy Regulatory Board (AERB) constituted a Task Group (TG) to develop quality assurance (QA)/Acceptance criteria for specialised radiotherapy equipments (TGQA-SRTE) such as Robotic Radiosurgery Device (tradename-Cyberknife), Helical Beam Delivery device (tradename – Tomotherapy), Gamma Radiosurgery Device (tradename – Gamma knife) and Intra Operative Radiation Therapy units. To accomplish this task, the TG thoroughly reviewed the relevant reports published by international agencies [International Atomic Energy Agency (IAEA), Vienna, Austria; International Electrotechnical Commission (IEC), Geneva, Switzerland], professional societies (e.g. American Association of Physicists in Medicine, USA), and scientific papers published by individual researchers. The technical brochure and acceptance test proforma provided by the manufacturer of these equipments were thoroughly studied and salient features were noted. Specific inputs of manufacturers/suppliers of the equipment were also obtained. It was decided by the committee that general layout followed in Acceptance test proforma developed for standard radiotherapy equipment will be followed and test parameters and their tolerance values were clubbed in the four-general category namely, electrical, mechanical, dosimetry and radiation safety.
The QA proforma of Cyber knife machine includes general technical information about the equipment and the treatment and monitoring accessories; details about the safety systems and interlocks; mechanical performance of couch, collimator and robotic manipulator; dosimetry tests related to beam profile, depth dose, penumbra, relative surface dose, beam quality and performance of the beam quantity monitoring system; radiation safety compliance of the accelerator and facility; and performance of imaging systems. Tolerances of some of the test parameters were also assigned by the TG.
The layout of the QA proforma of Helical beam delivery device (Tomotherapy unit) is also the same and it includes tests on both beam delivery as well as imaging systems. The dosimetry tests include the evaluations of both transverse and longitudinal beam profiles using the criteria for flattening filter free beam (i.e. unflat beam). Radiation safety tests includes the test of the accelerator and the collimator system. The test parameters are selected in such a way that it should be applicable to all the models of this treatment delivery device.
The QA proforma of Gamma Knife unit is somewhat simpler than the above mentioned two specialised radiotherapy equipment. The important mechanical test is the verification of unit centre point radiometrically. Test on collision interlock and safety tools to avoid collision of patient with unit during treatment have also been included in the acceptance test proforma. For output measurement, the methodologies outlined in recently published IAEA technical report series 483 (TRS 483) has been adopted.
In case of intraoperative radiotherapy systems, the QA proforma is being modulated to include test parameters related to all the devices, both based on beam therapy and brachytherapy concepts, and is yet to be finalized by the task group.
In summary, the AERB task group has developed QA proforma for all the specialized radiotherapy equipments for which systematic regulatory protocols were not available. It is expected that the newly developed QA proforma by the AERB TG will be helpful for the user in conducting the performance test of these devices in a systematic manner.
| I-7: Radiation Dosimetry and Clinical Out Come|| |
VS Hospital, Chennai, Tamil Nadu, India
Dosimetry plays a vital role in the field of Radiation Oncology. Any radiation event is a probabilistic function. Hence an error can be expected and accepted. Any error in the dose delivery, dose prescription affects the clinical outcome for the patient. Dose delivery depends on the dose measurement and QA Program of the department. Based on the steep dose response curves for normal tissues and tumour, a less than 5% uncertainty in dose delivered to the patient is required.
In the Absolute dosimetry of the beam, as per IAEA, uncertainty involved in the determination of the different factors, is 0.9%. That's one of the reasons for accepting +/-2% error in absolute dosimetry. This means total of 4% in the total dose can be expected. In the era of IMRT and VMAT, Leakage measurement through the jaws and MLC leaves, becomes important. Choice of dosimeter plays a vital role in the error. Still the studies are going in the field of small field dosimetry and choice of dosimeters, Dose verification systems need to be evaluated carefully based on the type of treatments carried out.
The errors could be random error or systematic error. Random errors cannot be avoided, but has to be accounted. Systematic errors may be evaluated and may be corrected in the future. The systematic error depends on the individual institutional setup, depends on various factors, including the setup of patient.
Dose prescription and reporting mechanism, as per ICRU reports have to be followed for evaluating the clinical outcome any protocol based patients. Even though 4% or 5% looks smaller, when it comes to the deterministic effects of the vital structure, it is critical. If the error is in the positive side, then the damage may be produced. The same error in the dose delivery to the tumour is critical, if it is in the negative side. Based on criticality of the tumour and the critical organ, a balance has to be arrived.
| I-8: Dosimetry of Magnetic Resonance Imaging-Linear Accelerator|| |
Department of Radiation Oncology, AIIMS, New Delhi, India. E-mail: [email protected]
Need for Magnetic Resonance Imaging-Linear Accelerator: The current image-guided radiotherapy systems are sub-optimal in the esophagus, pancreas, rectum, lymph node, kidney, etc. These locations in the body are not easily accessible for fiducials and cannot be visualized sufficiently on cone-beam computed topographies, making daily patient set-up prone to geometrical uncertainties. Additionally, interfraction and intrafraction uncertainties for those locations arise from motion with breathing and organ filling. To allow real-time imaging of all patient tumor locations at the actual treatment position, a fully integrated system combining of on-board magnetic resonance imaging with a linear accelerator (MRI-Linear Accelerator or MR-Linac) is needed.
Magnetic Resonance Imaging-Linear Accelerator Systems: The MRI-Linear accelerator is a hybrid ring-gantry technology combining both MR imaging system and linear accelerator in a single unit which is used for MRI-guided radiotherapy (MRIgRT). The MRI-guided radiotherapy provides real-time images of a patient during treatment and offers more detailed and higher contrast images for the identification of tumors and soft tissues than conventional IGRT techniques. This boosts tumour targeting accuracy, reducing side-effects and increasing survival rates. Hence, accurate radiation dose is crucial to the safe and effective treatment of patients but radiation transport is strongly affected by the magnetic fields used in MRI-Linac.
The superconducting magnets with range from 0.35T to 1.5T magnetic field strength MRI scanners with 45-50 cm FOV are used and share their common isocenter with linear accelerator. The gradient coils, body coils and surface coils are used during imaging with different MRI pulse sequence techniques. The MRI scanner is either of parallel or perpendicular configurations.
The 6MV or 7MV low energy linear accelerators with flattening filter free beam having dose rate of 500-600 MU/min at 90 cm SAD is used. The multileaf collimator (MLC) having field sizes of 27 × 24 cm2 or 57 × 22 cm2 is used for step and shoot IMRT. Treatment planning system with Monte Carlo based algorithms for the dose calculation becomes mandatory in the presence of magnetic fields. The MRIgRT systems such as ViewRay MRIdian Linac and Elekta Unity are commercially available, besides there are two prototypes research system's work is underway.
Radiation Dosimetry in the Presence of Magnetic Fields: Because the isocenter of the linac coincides with that of the MR scanner, dose will be deposited in a magnetic field environment. Secondary electrons released by the photon beam will therefore be deflected by the Lorentz force which causes the particle to curve in a direction perpendicular to the magnetic field. The particle's radius of curvature in a magnetic field is dependent on its kinetic energy and the strength of the magnetic field. This is known as electron return effect (ERE). For a homogeneous phantom, in regions where electron equilibrium is reached, the impact of this effect is negligible. However, for interfaces with a large difference in density, like in an air-filled ionization chamber, this effect cannot be ignored. However, MRI lacks the electron density information and may suffer from geometric distortions, and therefore is not directly suited for dose calculation. Therefore The MRI-linac pose technical challenges such as (i) Radiofrequency (RF) interference between linac and MR and hence MR needs to be isolated which collects weak signal from patient as linac is a significant source of RF. (ii) magnetic field mutual interaction: MR magnet and linac because the electron trajectories perturbed by the presence of B0 Lorentz force (iii) Dose deposition effects in MR's magnetic field (iv) Skin dose effects in MR's magnetic field. The current commercial systems are having limitations such as only static IMRT delivery, co-planar treatment, carbon-fiber couch can't be used as they conduct electric current and plastic couch is used and its attenuations are accounted for in the treatment planning systems.
Clinical Commissiong of Magnetic Resonance Imaging-Linear Accelerator: The radiation dosimetry can be classified as (i) reference or absolute dosimetry (ii) relative dosimetry. The dosimetric and other ancillary equipments used in MRI-linac should be of non-ferromagnetic and MR-safe. The MRI compatible different detectors and phantoms need special considerations while performing dosimetry in the magnetic fields. The ionization chamber, multi-axis ionization chambers, IC-Profiler film, OSLD and plastics phantoms are used in the commissioning and routine quality assurance process. The dosimetry and the dose distributions are highly influenced by the magnetic field of the MR (+/-10%). Therefore the magnetic field correction factors and electrometer correction factor for change in meter reading due to magnetic field while absolute dose determination. For the absolute calibration, the real water as 1D phantom and solid water phantom is not suitable for MRI-linac.
The MRI-compatible 3D water scanning phantom is used for relative dosimetry. In the standard 3D scanning water phantom, any ferromagnetic components have to be replaced with non-ferromagnetic components (e.g. electric stepper motor need to be replaced with ultrasonic motor/transducers etc). The special MRI-compatible daily QA phantom, 4D dynamic motion phantom and additional phantoms filled with water or water gel are used for commissioning dosimetry, patient-specific treatment QA and routine machine QA dosimetry for both MRI system as well as linac system used in MRI-linac.
Conclusion: The MRI-Linac is advancements in IGRT system for MR guided Radiotherapy (MRgRT) which allows the simultaneous use of beams of ionizing radiation and magnetic resonance imaging (MRI) to image a patient during treatment. The MRI-Linac is the only system allows for 2D, 3D, 4D real-time image guided on-line adaptive and stereotactic body radiotherapy basis on the soft tissue and organ at risk with beam gating and motion tracking capabilities. Apart from providing better soft tissue visualization of anatomy, it also provides several functional imaging modalities for measuring biological function and physiology and also used in treatment response assessment as imaging biomarker towards personalized therapy.
| I-9: Radiobiology of Lung Stereotactic Body Radiation Therapy Treatments|| |
T. S. Kehwar
Mercy Fitzgerald and Mercy Philadelphia Hospital, Philadelphia, PA 19143, USA
Purpose: This study reviews radiobiological phenomenon response of lung stereotactic body radiation therapy (SBRT) treatment.
Introduction: Cancer is the second most common cause of death in the USA, exceeded only by heart disease, and also a leading cause of death allover the World, among other cancers, which typically present in locally advanced or metastatic stage. Traditionally, conventional fractionated radiation therapy was the preferred choice of treatment for the patients who were unfit for surgery.
Recently, the SBRT treatment has increasingly been used for surgically inoperable stage I lung cancers. Local control rates for SBRT treatment dramatically improved compared to the conventional fractionation treatment.
Radiobiology of Conventional Radiation Therapy: The response of a tumor to the conventional fractionation schemes have been well explained by 5 R's of radiobiology. First 4 R's were initially described by Withers, and subsequently 5th R was introduced by Steel et al. based on the data that the responsiveness of the tumors to radiotherapy treatment correlated with intrinsic radio-sensitivity of the cells in vitro.
The linear – quadratic (LQ) model, with appropriate modifications, is able to explain above said 5 R's of radiobiology and used as most popular method of fitting experimental results derived from in vitro and in vivo radiation survival experiments and to explain the responses of different dose fractionation schemes.
The LQ model is a low dose approximation and at lower doses the effects of 5 R's of are exploited in the design of various fractionated treatment schedules. The biologically effective dose (BED) for such fractionation schemes may be given by
Where n is the number of fractions, d dose per fraction, and a/b is the tissue specific parameter, that implies the dose at which the component of lethal damage is equal to that of sublethal damage.
Radiobiology of Stereotactic Body Radiation Therapy: The LQ model has been used to interpret treatment outcome within its validity range up to 6 Gy per fraction. The dose – response curves of the LQ model keep on bending beyond its validity range and become inconsistent with in vitro survival curves, which are straight on semilogarithmic plot at high doses used in SRS and SBRT treatments. In the high dose range, this might be due to overloading of the repair enzymes.
To explain radiobiological phenomenon in the high dose region the LQ – L and the USC models were developed by Carlone et al. and Park et al., respectively.
The linear – quadratic – linear model: The LQ model fits appropriately in the conventional fractionation range and smoothly transition to the linearity at a transition dose, Dt, which explained by the LQ – L model. The survival fraction (S) can be written in bipartite form after single dose of radiation as
where a and a/b are the LQ parameters, as explained in previous section, and g is the coefficient of the damage in the final linear portion of the survival curve at high doses, which is the loge cell kill per Gy dose in the linear portion of the survival curve at high doses, and at Dt and can be given by
The universal – survival – curve model: The USC model is a hybrid model derived by combining the LQ mode and the MT model, where the LQ model smoothly transition to an asymptotic linear portion of the MT model at Dt (transition dose). The S of USC model for single dose fraction is given by
Unified LQ-L Model: Unified LQ-L model  uses transition dose Dt derived by equating the dose responses of the LQ and the MT models at transition dose, Dt, which can be given by
Results and Discussion: Sheu et al. reported that the LQ model overestimates the magnitude of the cell kill at larger doses compared to conventional doses, which could affect the accuracy the dose-response results at larger doses. Guerrero and Li  modified the LQ model to address the issue of high dose per fraction regimens and to get best fit at large doses, which exhibit linear-quadratic-linear (LQ-L) behavior. Wang et al.presented a generalized LQ model, which also shows a linear-quadratic-linear behavior. Park et al. proposed a bipartite universal survival curve (USC) model, which is hybrid of the LQ and the historic multi-target (MT).
Astrahan  reported that LQ-L model predicts decreasing fractionation sensitivity for higher doses with increasing dose per fraction. Wennberg & Lax  have shows that at doses of about 15 Gy or higher, the USC model predicted much lower fractionation sensitivity, compared to the LQ model, for both tumor and normal tissues.
Kehwar et al. had studied the best fit survival curves for non-small cell and small cell lung cancer cell lines fitted to the LQ, LQ-L and USC models, and found that the LQ-L model with Dt derived using the MT model provide appropriate fit and is the best to use to predict SRS and SBRT treatments. These radiation cell survival data were used to determine the values of a and b, using the best-fit regression method and an interactive inspection and chi–square best fit to the initial curvature points with R2 ≥ 0.97, respectively, for low dose survival data, and D0 and by the best-fit regression method to the final slope survival data, using MT model. [Table 1] enlists representative LQ and radiation cell survival parameters for SC and NSC lines.
|Table 1: Representative values of the parameters that can be used to predict lung stereotactic body radiation therapy treatments|
Click here to view
Conclusions: The unified LQ-L model provides best fit to the radiation cell survival data with smooth and gradual transition of the LQ model to linear portion of the survival curve at transition dose Dt. The Fitting of the experimental dose response data in the range of high doses, used in SRS and SBRT, to the LQ, LQ-L, and USC models illustrates that the unified LQ-L(Dt) model provides the best explanation of the problem. On the other hand, the LQ model overestimates the severity of response at high doses due to continues to bending of the curve, other models do not transition smoothly to the linear portion of the curve. The transition dose Dt and final slope g, the loge cell kill per unit dose in the final linear portion of the survival curve, can be calculated using D0 and obtained by the best fit exponential regression of experimental or multi-fraction dose response data. Plots of this study show that the unified LQ-L(Dt) model offers a best description of the cell survival data for SC and NSC cell lines in the high dose region well beyond the shoulder, and is a best suitable model for clinical use.
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- Carlone M, Wilkins D, Raaphorst P. The modified linear-quadratic model of Guerrero and Li can be derived from a mechanistic basis and exhibits linear-quadratic-linear behaviour. Phys Med Biol 2005;50:L9-13.
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| I-10: Pet Computed Tomography in Radiotherapy Planning|| |
Apollo Specialty Hospital, Chennai, Tamil Nadu, India
Radiotherapy is one of the most effective modalities for treating malignancies especially the tumors of the head and neck apart from surgery. This is due to the advent and integration of most advanced techniques like intensity modulated radiotherapy (IMRT). These high precision techniques need accurate tumor location and its extent and further the volume of tumor to be irradiated. Radiotherapy planning during the last few years has been solely based on anatomical tumor volumes. This may sometimes under or over estimate the tumor as well as missing of occult primaries with metastatic cervical nodes. Molecular imaging with FDG could help overcoming some of these as well as can be complementary to the conventional imaging.
An essential step in RT planning is to define the tumor location and extent, further to define volume to be irradiated. Commonly used was anatomical tumor target volume with Computed Tomography (CT) which includes Gross tumor volume –GTV, clinical tumor volume –CTV and planning tumor volume –PTV. Incorporating PET with anatomical tumor volume gives metabolic tumor volume MTV. This is described as the volume of tumor tissue with FDG. This volume corresponds to GTV of CT. Studies had shown that this volume was found closest to the pathologic GTV from specimens after surgery. Information obtained from novel tracers like those of hypoxia, proliferation, apoptosis, and receptor expression can be integrated with that of FDG PET imaging, which provides greater insight into the biologic pathways involved in radiation responses. Few tracers of molecular imaging with 18F MISO and 18F FAZA could identify patients who could fall into the radio resistant subgroups. This would help to identify hypoxic cells and to augment higher doses, which are resistant to radiotherapy. This complex cross-sectional dose distributions and the delivery of differentiated dose a within the target, a technique that has been referred to as “dose painting” is possible with IMRT. Contouring the outline of the tumor or metastatic lymph nodes applying PET/CT, the so-called “dose painting”, is still one of the most challenging and controversial issues in radiation therapy planning.
PETCT in RT planning helps in the management of patients when the location of tumor is in the vicinity of complex anatomy and critical organs. MTV guided volume delineation determines the metabolically active component which is generally smaller than that of the morphological appearance there by reducing the gross tumor volume, an unique advantage in sparing the surrounding organs at risk. This would help in those organs with dose constraints. Non enlarged nodes harboring micro metastases which are metabolically active in PETCT are included in the treatment which often not included in anatomical tumor delineation by CT alone. PET/CT has also been found helpful in the management of occult primary head and neck tumors by determining a site of origin of the primary tumor in 60%. This translates into reduced dose distribution to uninvolved mucosal sites compared with the results of CT scan only–based plans. PETCT would also be very useful in differentiating tumor recurrence from postoperative and post RT changes. In addition Whole body PETCT adds complementary information which could modify TNM staging resulting in shift in treatment perspective from curative to palliative because of identification of distant metastases. Several studies have documented that the improved staging with FDG PET can be used to improve patient management and significantly impact RT planning and therefore improve outcome and minimize toxicity.
PETCT can also used in the interim period of an early RT. This would help to find out the response to the treatment. Based on the response a revision of the treatment plan can be done either by reduction or augmentation of the mean dose delivery. This might help in reduction of radiation to non target tissue.
However PET also bears some disadvantages like limited spatial resolution which is overcome by a large extent by the fusion of metabolic images with CT images (Hybrid PETCT) which is the present day equipment and false positive findings because of inflammation and physiological distribution of tracers. Smearing of targets might happen due to motion artifacts which can be eliminated considerably using respiratory gating and 4D PETCT imaging in the place of conventional PETCT images.
To conclude the data from functional imaging greatly improves RTP by enhancing the mean radiation dose to the target and minimizing unnecessary irradiation of normal tissues. The potential of PET to quantify metabolism and identify new imaging targets within tumor tissue such as cellular proliferation, hypoxia, tumor receptors, and gene expression, thereby helping in the biological optimization of dose delivery.
| I-12: Comprehensive Quality Assurance Program for Volumetric Arc Therapy Volumetric Arc Therapy or Rapidarc™ Treatments|| |
P. Atwal, B. Vangenderen, L. Mathew, D. Morton, D. Visagie, M. Sonier, J. A. Pratt, M. Wu, C. Shaw, R. Ramaseshan
Department of Medical Physics, BC Cancer, Abbotsford, British Columbia, Canada
Volumetric arc therapy (VMAT) or Rapidarc™ is an advanced form of the intensity modulated radiation therapy (IMRT) technique in which radiation dose is delivered in an arc of ≤360-degrees while simultaneously modulating the multileaf collimator (MLC) position, dose rate, and gantry speed. The delivered dose is precisely sculpted to provide sharp dose falloff beyond the target, thereby providing dose sparing of normal tissues and critical organs while maintaining high target dose. The use of VMAT in radiotherapy is constantly expanding. In our clinic, the majority of treatments are carried out using the VMAT technique, ranging from very small to large convoluted targets. Rotational beam delivery with simultaneous modulation of MLCs, dose rate, and gantry speed, creates a highly patient-specific plan and poses a challenge for accurately verifying the dose the patient receives during treatment. 3D in vivo dosimetry would be ideal to determine patient dose but is not practical. In order to take full advantage of VMAT treatments a comprehensive quality assurance (QA) program from the treatment plan to the dose delivery at the Linac, along with patient setup verification, is required. We present a comprehensive QA program for VMAT delivery comprised of multiple elements: 1. Treatment planning and plan quality assurance, 2. Treatment delivery specific to patients, 3. Cone-beam computed tomography (CBCT) accuracy verification, and 4. Routine machine QA tailored to VMAT/IMRT.
- Treatment planning and QA of treatment plans: Initial imaging, arc start and stop angles, collimator angle, optimal constraints, treatment and critical structure volumes and their overlaps and field sizes all play an important role in developing optimal plans. We describe the methodology and appropriate treatment plan QA performed to achieve this
- Patient specific treatment delivery and its effects are always challenging. Ideally, BANG gel tied to patient anatomy could provide such verification, but is not practical. Currently there are a number of devices available in the market to measure patient specific QA however none of them are ideal to provide the actual dose delivered to the target and critical structure. We developed a unique method to utilize dynalog files (log files created after each delivery providing leaf, gantry, dose information) to recreate the dose distribution both in Eclipse and Monte Carlo. We also utilized the isocenter wobble from gantry, couch, and collimator motion along with daily variation in the output and fraction to fraction MLC variations in our reconstruction. The dose can be calculated in CBCT used for patient setup. Inclusion of all these factors provide us with an accurate estimate of the dose delivered to the patient and can be compared to the planned dose
- We typically use CBCT 3D-3D matching to setup the patient. We have determined the accuracy of 3D-3D matching and have incorporated it in our planning target volume (PTV), specifically for small targets
- We perform extensive MLC QA regularly to verify the performance of MLC. Some of these tests include MLC backlash test, picket fence test, complex MLC movements, MLC and gantry statistics, and so on
We have also established some tests to verify the accuracy of Dynalog log file recordings. More work is underway to routinely introduce this in the clinic.
| I-14: Quality of Quality Assurance Program in Radiotherapy Practice|| |
Department of Medical Physics, BC Cancer, Abbotsford, British Columbia, Canada
Typically dose delivery to the patient is aimed to be within ±5%. ICRU report 24 considered ± 5% accuracy was required in the delivery of absorbed dose to the target volume, but in critical situations ±2% may be required. Dutrix reported that a difference of 7% dose delivery effected tumor regression and normal tissue reactions. The term accuracy in radiotherapy is often used loosely. Overall uncertainty is combination of type “A” uncertainties assessed by statistical means (Random errors) and type “B” assessed by other means (systematic). The objective of any QA program is to minimize the random error and eliminate systematic error as much as possible. The objective of the talk is to show with some examples as to how to achieve this.
Some of the examples discussed are 1: Absolute dose measurements where the depth of ionization chamber needs to be measured accurately. We have noticed a discrepancy between mechanical position and actual depth and also manual measurement of the depth. 2. Not levelling the water tank affects the absolute dose measurements specifically low energy electrons. 3. Solid water cavity temperature variation can add to the uncertainty in the measurements. 4. MLC backlash, improper MLC QA will affect precision treatments like IMRT and Vmat. 5. Beam modelling in treatment planning system results in systematic error 6. Isocentre wobble can affect functioning and small target treatments 7. Annual QA parameters comparison could create unsuspected errors. The errors associated with these measurements and methods to minimize them are discussed.
| I-16: Gafchromic EBT3 Films Dosimetry for Clinical Beams Output Audits|| |
Wameid Abdel Rahman
King Fahad Medical City Hospital, Riyadh, Saudi Arabia
Introduction: The Radiation Oncology Physics subcommittee of the Saudi Medical Physics Society is interested in launching a service to audit outputs of clinical photon beam with a postal dosimetry system. This service will be available for 14 radiotherapy clinics distributed across the Kingdom of Saudi Arabia. The goal of our work was to study the feasibility of using EBT3 GafChromic films for carrying out this task.
Methods: EBT3 file calibration - Two EBT3 film batches were calibrated using 5 megavoltage x-ray beams (two 6 MV, two 15 MV, and an 18 MV). For each film batch, 2 × 2.5 cm2 film pieces were cut and irradiated to doses in the range between 0 to 350 cGy. The film pieces were scanned prior to and after irradiation using EPSON 10000 XL desktop flat-bed document scanner (Seiko Epson Corporation, Suwa, Nagano, Japan). All scans were carried out using transmission scanning, spatial resolution of 300 dots per inch (dpi) and 48-bit RGB (Red, Green, and Blue) mode. Irradiation of the films was carried out at depth of maximum dose zmax, 10 × 10 cm2 field size, and 100 cm source-surface distance SSD. Dosimetric characterization of the films was based on the red channel colour value of the scanned images. To describe the calibration curve of the EBT3 film batch the following relationship was used:
where D is the dose; b, c, n and are parameters determined from the fit; and OD is the net optical density of the film given by:
where Iunexp and Iexp are the red channel colour values of the unexposed and expose film scans, respectively.
[Figure 1]a shows a typical calibration curve for our GafChromic® EBT3 film obtained in a 6 MV x-ray beam and the parameters b, c, n and for the two film batches for the five calibration x-ray beam are shown in [Figure 1]b, [Figure 1]c [Figure 1]d, respectively.
|Figure 1: (a) Calibration curve for a GafChromic® EBT3 film in 6 MV beam (Red channel); the parameters, and of Eq. (1) as well as their statistical average (lines) for two film batches for five x-ray beams are shown in (b-d), respectively|
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Postal phantom: The proposed postal phantom is a 4 × 4 × 20 cm3 lucite phantom as shown in [Figure 2]. The phantom consists of two parts to allow placement of a 2 × 2.5 cm2 GafChromic® EBT3 film at 8.9 cm depth. Conversion factors - listed in [Table 1] - that relate the dose at 8.9 cm depth in the lucite postal phantom to the dose at 10 cm depth in a large water phantom for 10 × 10 cm2 field size and 100 cm source-surface distance were calculated using DOSXYZnrc/EGSnrcmp Monte Carlo code for a number of clinical x-ray beams.
|Figure 2: The postal phantom is a 4 × 4 × 20 cm3 lucite phantom consisting of two pieces to allow placement of our GafChromic® EBT3 film at 8.9 cm depth|
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|Table 1: Conversion factors that relate the dose at 8.9 cm depth in the luctie postal phantom to the dose at 10 cm depth in large water phantom for 10×10 cm2 field size and 100 cm source-surface|
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Thus, the schematics of the relationship between the dose measured in the lucite phantom to the dose at in water is shown in [Figure 3].
|Figure 3: Schematic representation of parameters relating the dose at zmax in water to dose measured at 8.9 cm depth in the lucite postal phantom|
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Results: The performance of our proposed lucite postal phantom was tested in 15 clinical megavoltage x-ray beams across 4 radiotherapy centres in Saudi Arabia who were participating in postal services provided by either the International Atomic Energy Agency or the Radiological Physics Centre. The measured to stated doses of the 15 clinical beams, listed in [Table 2], are within ± 5%.
|Table 2: Measured output with our lucite phantom to stated output for 15 clinical x-ray beams|
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Conclusion: In this work we have designed and tested a postal dosimetry system based on GafChromic® EBT3 films embedded in a custom made lucite irradiation phantom. Our results shows that outputs of clinical x-ray beams can be monitored with our system. Because of their energy independence at high energies, GafChromic® EBT3 film is a practical dosimeter that may be used for auditing clinical beam outputs.
| I-18: Methodology of Dose Reduction in Interventional Radiology|| |
Kalpana M. Kanal
Department of Radiology, University of Washington Medical Center, Seattle, WA 98195-7115, USA. E-mail: [email protected]
All fluoroscopy equipment marketed in the United States must meet radiation control design specifications as mandated by the FDA. However, no regulation on design can guarantee safe use. Almost all fluoroscopic machines can expose patients to unacceptable and dangerous levels of radiation. In 1994, the FDA issued a public health advisory regarding “Avoidance of Serious X-ray Induced Skin Injuries to Patients During Fluoroscopically-Guided Procedures.” Several key points in this communication are: (1) that all operators of a fluoroscopic system must be trained and understand system operation, including the implications for radiation exposure from each mode of operation, (2) facilities should ensure that physicians performing fluoroscopic procedures have education, and (3) assure appropriate credentials and training for physicians performing fluoroscopy.
As the operator of the equipment, you must know:
- How to properly operate the x-ray machine and how to properly use the features specific to that unit
- How to properly position the patient and the x-ray system for the procedure,
- How to control image quality (by properly selecting image quality and special dose rate controls, magnification, geometry, collimation, etc.),
- How to minimize radiation levels (by employing the same features as in the previous item),
- How personnel should be positioned for minimum radiation exposure, and
- How to properly utilize shielding devices and personnel monitoring devices.
Regardless of who controls the machine, the physician remains responsible for assuring that x-rays are properly/safely applied and appropriate radiation protection measures are followed.
In the talk on fluoroscopy dose reduction, we will review the dose reduction options mentioned above.
| I-19: Addressing Public Concerns About Their Exposure to Low Doses of Anthropogenic Radiation|| |
G. Cranmer-Sargison1, N. D. Priest2
1Saskatchewan Cancer Agency, University of Saskatchewan, Saskatoon, Saskatchewan, 2CANDU Owners Group Inc., Toronto, Ontario, Canada. E-mail: [email protected]
The COG Strategic Research and Development (SRD) Low Dose Radiation (LDR) research program has now embarked on an independent and evidence-based research initiative in response to concerns and worries about the effects of exposure to anthropogenic radiation at low dose levels. A Canadian research team have taken up the challenge and will look to answer the following - What are the public concerns regarding exposures to low dose anthropogenic radiation; To what extent are the public concerns justified by evidence of adverse effects; Why are effects seen or not seen following LDR; How best are the results of studies communicated to the public; How effective have the communications been in reducing concerns? The LDR program is forecast to sustain at $1M per year over several years. A program overview will be presented with a detailed description of the individual research projects provided.
| I-20: Different Magnetic Resonance Image Characteristics for Using Magnetic Resonance Imaging as a Read-Out Technique for 3D Gel Dosimetry|| |
Department of Radiology and Imaging Sciences, RMMCH, Annamalai University, Chidambaram, Tamil Nadu, India
With rapid advances in radiotherapy treatment, 3D dose measurement techniques of great precision are required more than ever before. The fundamental chemical and physical phenomena that occur in 3D gel dosimeters are used for detection and verification of dose distributions. Gel dosimeters are prepared with the help of radiation sensitive chemicals that, upon irradiation with ionising radiation, undergo a fundamental change in their properties as a function of the absorbed radiation dose. Fricke gel dosimeter and polymer gel dosimeters are the two emerging types of gel dosimeters. In order to evaluate the exposed gels different 'read-out' imaging modalities like magnetic resonance imaging (MRI), optical computer tomography, x-ray CT or ultrasound are used. The use of magnetic resonance imaging as a non-destructive measurement of a dosimeter gel was first proposed in 1984 by Gore et al. Different MR characteristics should be investigated before using MRI as a read-out technique for 3D gel dosimetry to avoid dose errors. Hence, in this talk, various MR image characteristics such as T1 and T2 contrasts, gradient imaging coils, spatial encoding methods i.e., slice thickness, frequency and phase encoding, properties of the K-space are discussed. Moreover, Spin Echo family sequences with their important parameters like TR, TE, Turbo factor, Inversion time, NEX, SNR, and Bandwidth are analyzed.
| I-21: Dosimetry of Nuclear Medical Imaging: Current Practices and New Modalities|| |
Department of Physics, University of Saskatchewan, Saskatoon, SK S7N 5E2, Canada. E-mail: [email protected]
Radiation dosimetry is a multi-disciplinary activity of theory, experiment and numerical evaluations, which involves physics, biology, statistics, instrumentation and computer modeling. Of primary interest are biological effects either for the harm they cause to living organisms or otherwise. As biological effects are consequences of chemical transformations, which in turn are due to ionization phenomena, the dosimetry relies on the number of ions created. In the context of medical imaging, the interest is mainly on photon interactions with matter. For practical assessment of dose deposits, great strides have been made in terms of the transducer development, speeds of data processing thanks to technological innovations of the latter half of the 20th century and they continue to evolve. The ever increasing computational power and the activities of high energy physics communities also contributed vastly to the dosimetric evaluations. In this regard, the Monte Carlo simulation codes based on the electron -gamma-showers are worth noting. In the last few decades, MCNP, EGS and the GEANT based GATE are now widely used with precisions and details unimaginable a few decades ago. Despite all these developments, the dosimetry is still missing an important ingredient. The dosimetry's ultimate aim is to assess the biological risk to the patient. However, physical instruments cannot access it.
My talk will highlight these developments and suggest a possible path to achieve accurate estimates of physico chemical phenomena for dose due to various species of radiations of use in medical imaging.
| I-22: Electronic Brachytherapy|| |
Peter MacCallum Cancer Centre, Victoria, Australia
Electronic brachytherapy is an advanced radiation treatment technique specifically designed to deliver high doses of radiation inside or very close to the tumor-bearing tissues. Unlike the traditional radionuclide-based brachytherapy, electronic brachytherapy uses a miniaturised x-ray tube that can produce radiation when energised. Most of the electronic brachytherapy systems operate at 50 kVp, therefore posing less radiation exposure to both patients and staff when compared to the standard Cobalt-60 or Iridium-192 based brachytherapy systems. Electronic brachytherapy systems have broader applications that include the treatment of skin, brain, breast, spinal metastasis, endometrium, and cervix. Most of the electronic brachytherapy systems require less shielding and can be operated in remote centres with minimal radiation shielding facilities. The dose fall-off characteristics of the Xoft Axxent electronic brachytherapy system mimic those of the low energy isotopes, yet the unit still maintains the high dose rate property of an Ir-192 source. Electronic brachytherapy systems have the potential to replace conventional radionuclide-based brachytherapy sources and systems.
| I-23: A Place of Cobalt-60 Units in Reducing the Global Disparity of Access to Quality Radiation Therapy|| |
Chandra P. Joshi1,2, L. John Schreiner1,2
1Department of Oncology and Physics, Queen's University, 2Department of Medical Physics, Cancer Centre of Southeastern Ontario, Kingston Health Sciences Centre, Kingston, Ontario K7L 2V7, Canada
Cobalt-60 (Co-60) units are increasingly being replaced by linear accelerators (linacs) as the radiation therapy (RT) treatment unit of choice. The main reasons driving this change are the perceived disadvantages associated with Co-60 radiation such as lower photon energy, larger penumbra, lower radiation output, periodic source replacements, technological limitations and security issues. In contrast, linacs are considered to offer superior and state-of-the-art solutions including multiple photon and electron beams, intensity modulated RT (IMRT), image guided RT (IGRT) and motion management. Paradoxically, Co-60 technology has been responsible for many major RT milestones ahead of linacs, including, the first treatment of a patient with megavoltage RT (1951), the first stereotactic radiosurgery (Gammaknife, 1967), and the first magnetic resonance guided IGRT (Renaissance™, ViewRay 2014);, in part due to the uncomplicated aspects of Co-60 technology. More recently, specialized Co-60 units such as GammaPod for breast stereotactic body RT (SBRT) (Xcision Medical, USA) and the GB500 total body irradiation unit (Best Theratronics, Canada) have also become available.
Major Co-60 unit manufacturers now provide options for IMRT, IGRT and DICOM connectivity with their conventional units. For example, the Equinox unit (Best Theratronics, Canada) can be equipped with motorized wedge and multi-leaf collimator capable of three-dimensional conformal RT (3DCRT) and IMRT deliveries; the Bhabhatron 3i system (Panacea Medical Technologies, India) which allows for patient positioning with a hexapod couch with six degrees of freedom and low dose kilovoltage 'cone-beam' IGRT. Treatment planning studies have shown that Co-60 units can produce tomotherapy and IMRT treatment plans with quality similar to linacs.,,,, These studies show that Co-60 radiation can provide clinically competitive plans compared to linacs, albeit with relatively less steep dose gradients outside the planning target volumes (PTV). Interestingly, a recent study involving lung patients treated with SBRT suggests that patients receiving higher doses outside the planning target volume (PTV) had less risk of developing distant metastases which highlights the potential pitfalls of treatment plans with steep dose gradients, a disadvantage usually linked with Co-60 radiation.
The global burden of cancer is continuously rising, more than half of cancer cases worldwide occurring in low and middle-income countries (LMICs) (GBD 2016); and >90% patients in low income countries (LICs) and >50% patients in LMICs lacking access to RT., In high income/high resource settings, tremendous efforts are spent on fine-tuning steepness of the shoulder of the dose volume histogram (DVH) for the PTV, whereas the same DVH (PTV) for many patients in LICs/LMICs are either similar to DVHs for spared organs at risk, or don't exist at all (due to no access to RT). These issues have motivated many Co-60 and linac manufacturers to create clinically effective and economically viable solutions to perform quality RT in low resource and high volume settings. This has resulted in availability of “no-frills” Co-60 units including GammaBeam, (Best Theratronics); Bhabhatron II (Panacea Medical), and Halcyon™ linac system (Varian).
Linacs and Co-60 units offer different strengths in terms of infrastructure, maintenance, shielding requirements, staffing, costs, staff training, patient throughput, planning/dosimetry, and ease of clinical use. Co-60 units and linacs thus offer complementary rather than competing technologies. Considering clinical needs, demography, geography, human and economic environments, Co-60 units and economic linacs can be strategically deployed to reduce the global disparity in access to quality RT.
- Johns HE, Bates LM, Epp ER, Cormack DV, Fedorux SO, Morrison A, et al. 1,000-curie cobalt 60 units for radiation therapy. Nature 1951;168:1035-6.
- Mutic S, Dempsey JF. The viewRay system: Magnetic resonance-guided and controlled radiotherapy. Semin Radiat Oncol 2014;24:196-9.
- Van Schelt J, Smith DL, Fong N, Toomeh D, Sponseller PA, Brown DW, et al. A ring-based compensator IMRT system optimized for low- and middle-income countries: Design and treatment planning study. Med Phys 2018;45:3275-86.
- Merna C, Rwigema JC, Cao M, Wang PC, Kishan AU, Michailian A, et al. A treatment planning comparison between modulated tri-cobalt-60 teletherapy and linear accelerator-based stereotactic body radiotherapy for central early-stage non-small cell lung cancer. Med Dosim 2016;41:87-91.
- Dhanesar S, Darko J, Joshi CP, Kerr A, Schreiner LJ. Cobalt-60 tomotherapy: Clinical treatment planning and phantom dose delivery studies. Med Phys 2013;40:081710.
- Joshi CP, Dhanesar S, Darko J, Kerr A, Vidyasagar PB, Schreiner LJ, et al. Practical and clinical considerations in cobalt-60 tomotherapy. J Med Phys 2009;34:137-40.
- Adams EJ, Warrington AP. A comparison between cobalt and linear accelerator-based treatment plans for conformal and intensity-modulated radiotherapy. Br J Radiol 2008;81:304-10.
- Zubizarreta EH, Fidarova E, Healy B, Rosenblatt E. Need for radiotherapy in low and middle income countries – The silent crisis continues. Clin Oncol (R Coll Radiol) 2015;27:107-14.
- GBD 2016 Causes of Death Collaborators. Global, regional, and national age-sex specific mortality for 264 causes of death, 1980-2016: A systematic analysis for the Global Burden of Disease Study 2016. Lancet 2017;390:1151-210.
- Healy BJ, van der Merwe D, Christaki KE, Meghzifene A. Cobalt-60 machines and medical linear accelerators: Competing technologies for external beam radiotherapy. Clin Oncol (R Coll Radiol) 2017;29:110-5.
| I-29: Status of the Radiological and Radionuclide Standards in India|| |
M. S. Kulkarni
Radiation Safety Systems Division, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India. E-mail: [email protected]
Radiation Standards Section, Radiation Safety Systems Division, BARC is the national custodian of radiation standards and also recognized as the Designated Institute (DI) for ionizing radiation metrology in India. It maintains a number of national standards for ionizing radiation and continuously updates them to achieve better accuracy. These standards include primary standards, secondary standards and working standards for radiological quantities, radioactivity, neutron, and chemical dosimetry. BARC has also been recognized as a Secondary Standard Dosimetry Laboratory (SSDL-BARC) by IAEA/WHO. Under this aegis, quality audits are being conducted since 1976 for assessing the dosimetric status of radiotherapy centres, nuclear medicine centres and radiation processing facilities in India.
Radiological standards for X-ray, gamma, neutrons and beta particles are maintained in BARC and their equivalence is established with international standards by participation in the international intercomparison programs organized by BIPM and APMP. Free Air Ionization Chamber is maintained as the primary standard for low and medium energy X-ray beam qualities (upto 300 kV). A graphite calorimeter is being developed as a primary standard for the measurement of absorbed dose to water for therapy level dosimetry. Diagnostic beam qualities have been established for the calibration of instruments used in diagnostic radiology. Reference standard (cylindrical ionization chamber) calibrated at BIPM for absorbed dose to water and air kerma at60 Co energy is used for the calibration of dosimeters of all the Radiotherapy Centres in India. A large volume cylindrical graphite ionization chamber is maintained as the reference standard for brachytherapy reference airkerma measurements. Traceable calibration to more than 300 brachytherapy facilities in the country is provided using this standard.
The SSDL-BARC activities also cover TLD postal dose quality audits for all the radiotherapy centres in the country under reference conditions. End-to-end IMRT Dosimetry audits are being conducted using film and TLDs inserted in a specially designed phantom. Beta dosimetry is carried out using an Extrapolation Chamber. A calibration facility has been developed at BARC for generating Series 1 and Series 2 reference fields as per international standards to calibrate beta measuring instruments.
Radioactivity standards have been established since 1970s in the laboratory. Over the years the 4πβ-γ coincidence method has evolved as the most powerful and widely used primary standard for radioactivity measurements. At present three 4πβ-γ coincidence primary standards with different detection mediums are operational in the laboratory. In addition, well type high pressure ionisation chamber and HPGe detector systems are maintained as secondary standard for radioactivity measurements. A gas flow multi-wire large area proportional counting system has also been developed and established for measurements of large area sources (10 x 15 cm2). Traceability of radioactivity measurements at all the Nuclear Medicine Centres (~250) in the country is established by calibration of dose calibrators at BARC and also by conducting national audit of I-131 activity measurements. The talk will cover the present and future status of the radiological and radioactivity standards in the country.
| I-30: Impact of Measurement Uncertainty in Medical Physics|| |
Radiological Physics & Advisory Division, Homi Bhabha National Institute, Bhabha Atomic Research Centre, Mumbai – 400085, India. E-mail: [email protected]
Introduction: The measurements made within the Medical Physics discipline directly impact on the dose of ionising radiation that patients generally receive. Measurement uncertainty analysis is the process of defining the uncertainty of a measurement value. All measurements have an inherent uncertainty (the true value is never known). There are usually a range of factors that contribute to the uncertainty in the measured value. Each contribution may be large or small, and may contribute to the overall measurement uncertainty in different ways. Different measurements of the same quantity may yield different results, and it is impossible to tell which measurement is best without further information about the measurement. Measurement uncertainty analysis provides this additional information, allowing an uncertainty value to be assigned to each measurement. Measurement uncertainty was well entrenched internationally during mid 1900's and an accepted part of metrology. Historically many different measurement uncertainty techniques have been used to describe measurement uncertainty in various scientific disciplines including Medical Physics. This has led to problems whereby two scientific organizations wishing to compare measurement results have had difficulty because measurement uncertainties have not been calculated and expressed in the same manner. The development of innovative uses of measurement uncertainty analysis in the field of Medical Physics is the key focus of this work. The concept of measurement uncertainty is applied to generate innovative perspectives on issues that are important in the discipline of Medical Physics and the application of medical physics in a clinical setting. Specifically, the work addresses measurement uncertainty analysis to provide improved understandings of quality assurance test methods, effective dose calculation methods, and dosimetry indicators.
Purpose: The Guide to the Expression of Uncertainty in Measurement (GUM) is recognized by many peak international scientific organizations. However, there are shortcomings of GUM; for example, one of the biggest criticisms of the GUM is that it is a complex document to deal with, because it is underpinned with an extensive mathematical basis so that it has the capacity to deal with almost all measurement uncertainty problems. It has been identified that in certain circumstances, the assumption in the GUM that the probability density function of the measurand is a t-distribution (in accordance with the Central Limit Theorem) is not always the case. There has also been criticism that the evaluation of type A uncertainties should be conducted using Bayesian methods in certain circumstances The objective of this work is to employ measurement uncertainty analysis to differentiate effective dose calculation methods in a scientifically rigorous manner. Nevertheless, eight of the world's peak measurement bodies are joint publishers of the GUM and so it is presumed that the guiding principles of the GUM represent the best methodology for calculating and expressing measurement uncertainty. For this reason, the methods of the GUM were chosen to calculate and express measurement uncertainty values for Medical Physics problems. Before proceed further, it is useful to define measurement uncertainty.
Material and Methods:
Definition of Measurement Uncertainty
Measurement uncertainty is the specification of a range within which the 'true' (unknown) value is believed to lie with a specified level of confidence.
Link of measurement uncertainty to medical physics
Medical Physics is the application of physics to medicine. Medical Physicists are involved in radiation therapy, nuclear medicine, medical imaging and radiation protection. A large portion of medical physics measurements involve ionising radiation that is used for imaging and for therapy. It is therefore important that any radiation doses received by patients for imaging purposes are minimized and in the case of therapeutic uses, only the specified amount of radiation is delivered to a specified location within the body. Medical Physicists are therefore intimately involved in making important measurements that determine the health outcomes of patients and so the correct calculation and reporting of measurement uncertainty is paramount.
Results and Discussion: A case study of measurement uncertainty analysis of adult CT head scan dose estimates has been explored in this work. The effective dose (E) in mSv is calculated as product of the weighted CT dose index (CTDIw), measured in phantom (mGy), scans length (L) in cm, and normalized effective dose (Ew, DLP) in mSv/mGy/cm. Dose length product (DLPw) is further calculated by multiplying CTDIw and L. The uncertainty of effective dose estimates calculated using the DLP method is dependent on two factors: DLPw and Ew, DLP. The value DLPw displayed by a particular CT scanner may be either a manufacturer's nominal value, or a calibrated value based on measurements performed on that CT scanner. The value Ew, DLP is an average of the sensitivity of the volume of tissue typically irradiated during a head CT scan. The analysis of the uncertainties was assessed in accordance with the GUM. The uncertainty components were considered as uncorrelated, so the combined standard uncertainty, Ucomb, and effective degrees of freedom, where the uncertainty parameters are standard uncertainty, u, degrees of freedom, μ and sensitivity coefficient, c. DLP values are based on CTDIw values, which in turn are based on two dose measurements within a 160 mm PMMA phantom. The combined uncertainty of two independent dose measurements, each with a standard uncertainty of 2.5% is 3.54%. The standard uncertainty due to using a single Ew, DLP value for all CT scanners is equal to the percent standard deviation of the effective dose to CTDIw ratios for a range of scanners and was found to be 7.3%.
Conclusion: Analysis of measurement uncertainty in medical physics has a greater impact to deliver an accurate dose to patients during radiotherapuatic treatment.
[Figure 1], [Figure 2], [Figure 3]
[Table 1], [Table 2], [Table 3]