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ABSTRACTS
Year : 2017  |  Volume : 42  |  Issue : 5  |  Page : 46-48
 

Teaching Session



Date of Web Publication24-Oct-2017

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How to cite this article:
. Teaching Session. J Med Phys 2017;42, Suppl S1:46-8

How to cite this URL:
. Teaching Session. J Med Phys [serial online] 2017 [cited 2020 Jun 2];42, Suppl S1:46-8. Available from: http://www.jmp.org.in/text.asp?2017/42/5/46/217114





   TS-2: Technology of Advanced Radiotherapy Equipment Including Ion Beam Therapy Equipment Top


S. D. Sharma

Radiological Physics and Advisory Division, Bhabha Atomic Research Centre, CT and CRS, Mumbai, Maharashtra, India. E-mail: sdsbarc@gmail.com

In recent years, several technological developments have taken place in the design and capabilities of radiotherapy equipment especially beam delivery devices. Radiotherapy beam delivery devices can in general be classified as teleisotope machines, medical electron linear accelerators and ion beam accelerators (proton and heavy ion). Among teleisotope machines, we have conventional telecobalt unit which has recently been equipped with multileaf collimator (MLC), specialised telecobalt machine such as Gamma Knife (various variates) and super specialised machine such as View Ray. The conventional telecobalt machine, which contains a single high activity source, is still the work horse of middle and lower income countries. Gamma knife, which contains multiple cobalt sources (ranging from 30 to 201) and specialised narrow field collimators, is also being used popularly for treatment of mainly intracranial tumours. Many versions of this equipment are available globally. Super specialised teleisotope machine (the View Ray) is the new addition in the armamentarium of external beam therapy. This device contains three source heads at 120 degree apart and diverging multileaf collimators plus an on-board magnetic resonance (MR) imaging device. This super specialised teleisotope machine can be used for treating varieties of cancer cases by applying the techniues of intensity modulated radiotherapy, stereotactic body radiotherapy and on-couch adaptive radiotherapy.

Various versions of technologically advanced medical electron linear accelerators (LINACs) are also available in the radiotherapy departments globally. Medical electron linear accelerators can be classified in two general categories, namely standard LINAC and specialised LINAC. Technological changes have been incorporated in both the categories of the LINACs. For example, advanced medical LINAC with flattening filter free photon beams as well as on-board magnetic resonance imaging system have been introduced recently. The MR-LINAC is the most recent development. The MR-LINAC can be used for treating various cancer cases. It has the capability of simultaneous dose delivery and fast acquisition of diagnostic quality MR images. This helps in tracking the tumour and visualising the anatomy of the patient during treatment. The constant monitoring of the patient during treatment enables the precise targeting of the tumour and help in minimising the dose to normal tissues/organs at risk. A few unitsof MR-LINAC are in use clinically and short term outcome of the treatment is curiously awaited.

Ion beam (mainly proton and carbon) therapy equipment are also in clinical use at a few radiotherapy centres. Proton beam accelerator (energy in the range of 80 to 250 MeV) is in clinical use from last few decades. This is a highly specialised beam delivery device which contains a cyclotron, beam transport system and treatment gantries. Pristine proton beam curve is hardly used rather Spread Out Bragg Peak (SOBP) is created to match the tumour size in the beams eye view and the dose is delivered. The proton beam accelerators are also equipped with precise beam shaping and beam modulating devices plus x-ray imaging system. However, the radiobiological advantage of proton beam is slightly over photon beam and hence heavy ions (e.g. carbon ion) are thought to be a relatively better option as far as the use of high LET radiation is concerned in radiotherapy. Carbon ion accelerators are used at a few centres to generate the clinical data. The technological details of these accelerators will be discussed in detail.


   TS-2: IGRT: Determining Setup Margins and Correction Methods Top


Tharmarnadar Ganesh

Department of Radiation Oncology, Fortis Memorial Research Institute, Gurgaon, Haryana, India.

E-mail: sptgnadar@yahoo.com

The International Commission on Radiation Units and Measurements (ICRU) in its report number 50 defines clinical target volume (CTV) as a tissue volume that contains a GTV and/or subclinical microscopic malignant disease, which has to be eliminated. Going by this definition, CTV is thus an anatomical-clinical concept, that has to be defined before a choice of treatment modality and technique is made. ICRU further adds that margins will have to be added around the CTV to compensate for the effects of organ and patient movements and inaccuracies in beam and patient set up leading to the concept of Planning Target Volume (PTV). The margin is called as CTV-to-PTV margin.

The ICRU Report 62, a supplement to the Report 50, refined the concept of CTV-to-PTV margin. Instead of arriving at PTV from CTV through a single margin that accounts for organ movements and setup inaccuracies, ICRU 62 defines a new process wherein expanding of CTV to PTV is achieved in two steps: first from CTV-to-internal target volume (ITV) through adding of internal margin (IM) and then in the second step from ITV-to-PTV through adding of setup margin (SM). While the IM accounts for the effects of internal organ movements, the setup margin accounts for inaccuracies in patient setup. The need for the splitting of CTV-to-PTV margin into IM and SM arises from the differences between the factors that control these margins. Since the IM is to account for the movements of internal organ, one has practically no, or very little, control over its magnitude. On the other hand the setup margin accounts for uncertainties in setup which are largely controllable and in a carefully designed workflow its magnitude can be significantly reduced. Thus, the two margins have to be viewed from different perspectives if the overall margin is to be kept low.

The setup margin, calculated from van Herk formula, is given as SM = 2.5Σ + 0.7σ, where Σ is the systematic error and σ is the random error. The systematic error is repeated in every treatment fraction and the random error, as the name suggests, randomly affects individual fractions. The errors are computed from the measurements of translational positional shifts in the three directions. They can be computed for a single patient and for a population of patients. While the former helps in alleviating the SM required for an individual patient, the latter helps in deducing a SM required for the concerned treatment site in that clinic that can be incorporated at the time of contouring. The SM is thus a department specific parameter and cannot be and shall not be taken from text books or literature.

After determining the SMs for different sites in a clinic, the next challenge is their meaningful implementation to make the workflow smooth and efficient. This is a crucial step for every department. Each department should carefully design its image guidance protocols for different sites that shall be driven by the SMs determined. The protocols shall clearly define the roles and responsibilities of every team member involved in the process. Action levels, and what actions should be taken if these are exceeded, shall be made known to everyone in the team. Online and offline imaging protocols are the commonly practiced. If these are not practiced correctly, due to failure in understanding them, it can severely hamper the department's workflow efficiency.

The presentation would cover in depth two important practical aspects: (i) determining the SM for a population of patients and (ii) how to clinically implement that SM in a manner improving the efficiency.


   TS-3: Portal Dosimetry Top


Raghavendra Holla

Department of Medical Physics and Radiation Safety, Amrita Institute of Medical Science and Research Center, Kochi, Kerala, India. E-mail: raghavendra.holla@gmail.com

Introduction: Electronic Portal Imaging Device (EPID) is a flat-panel radiation detector, mounted on a linear accelerator (linac). EPID originally designed for geometric verification of patient set-up during treatment, can also be used to obtain dosimetric information about the radiation delivered because of the favourable characteristics such as fast image acquisition, high resolution and digital format. The most common EPID available today is an array of photodiode detectors on an amorphous silicon (a-Si) glass substrate.

EPID dosimetry also know as portal dosimetry is the method of acquiring images and then using these images to determine the dose delivered to a known point (1D), plane (2D) or volume (3D) within a patient. This is achieved via calibration of the EPID using an absorbed dose standard, image processing and knowledge of the patient anatomy.

Application of EPID for dosimetry can be classified according to whether they are performed during treatment time (i.e. with the patient) or outside of treatment time (i.e. without the patient).

EPID can be used for pre-treatment verification, a procedure comparing the whole or part of the intended treatment plan with measurements of corresponding radiation beams delivered by the linear accelerator without patient with open fields or a phantom. This comparison can focus on different aspects of the planned treatment: e.g. predicted and measured leaf positions, dose delivered to the detector or phantom, or incident energy fluence extracted from measurements.

Use of EPID is demonstrated by many researchers for Treatment verification. In this process comparison of all or part of the planned and the delivered dose distribution based on measurements acquired during radiotherapy of the patient is done. These measurements can be used to determine the dose delivered to the detector or patient, or incident energy fluence obtained from measurements.

If the determination of the dose in the detector (EPID), patient or phantom, or determination of the incident energy fluence, based on measurements without an attenuating medium between the source and the detector is performed then it is termed as Non-transmission (or non-transit) dosimetry.

In Transmission (or transit) dosimetry, determination of the dose at the position of the detector (EPID), patient or phantom, or determination of the incident energy fluence, based on radiation transmitted through the patient or phantom is performed. The measurement or determination of the dose inside a phantom is performed with EPID including the dose at points, lines, planes or volumes within the phantom at In-Phantom Dosimetry.

Ex-vivo dosimetry refers to the measurement or determination of the dose inside the patient using EPID transmitted images acquired during the treatment. The acquired images are used either to predict the dose delivered to the patient in 2D or 3D reconstruction of the dose for the entire volume of treatment.

To use of EPID for dose measurements requires calibration and corrections of EPID images to account for the various detector characteristics. Many studies have been performed to study the dosimetric response of EPIDs. For the flat panel detectors, studies have demonstrated a stable dose response that is independent of dose rate and linear with integrated dose. However, the flat panel response is also dependent on the incident photon energy and is affected by radiation scattered in the detector's many layers. The EPID also exhibits ghosting and image lag effect, an exponential memory effect with sequentially acquired frames. A global calibration model for calibrating EPID images to dose includes correction factors for, (1) Absolute dose calibration, (2) Image lag and Ghosting effect, (3) Field size dependence and beam profile Correction, (4) Buildup and energy spectrum correction.

A calibration model developed using these parameters are SSD dependent and hence a correction model has to be developed for a fixed SSD.

Conclusion: Portal dosimetry is evolving now to an integrated part in the total chain of verification procedures that are implemented in a radiotherapy department. It provides a safety net for advanced treatments as well as a full account of the dose delivered to specific volumes, allowing adaptation of the treatment from the original plan if necessary. The combination of an accurate EPID dosimetric calibration and a reliable interpretation of this EPID dose in terms of patient dose would provide a powerful new form of treatment verification for external beam therapies.


   TS-4: Icru Report 89: Prescribing, Recording, and Reporting Brachytherapy for Cancer of the Cervix Top


Jamema Swamidas, Umesh Mahantshetty1

Departments of Medical Physics and 1Radiation Oncology, Tata Memorial Centre, Mumbai, Maharashtra, India. E-mail: svjamema@gmail.com

The objective of this talk is to summarize the recently published ICRU report-89 for Prescribing Recording and Reporting Brachytherapy (BT) for cancer of the cervix. This ICRU report which was published in 2016, deals with the radiation therapy treatment with special emphasis on BT for cervical cancer. It took more than 3 decades to revise the ICRU 38 published in 1985. The report provides definitions, concepts and terms to enable, valid and a reliable exchange of information regarding radiation therapy methods including external radiation and BT. It contains 234 pages, 13 chapters, an appendix of 9 clinical examples, a five page summary and a concise two page summary of the recommendations at the end.

The report starts with a comprehensive information about the epidemiology, incidence, work-up, basic treatment principles and historical outcomes with radio(chemo)therapy. A comprehensive overview of various BT systems including the historical roots linking with the current practices and recent advances in BT. Subsequent chapters include in detail all the processes involved in BT planning including various imaging modalities, modern BT applicators, adaptive target for external and BT, radiobiology considerations, and advanced BT treatment planning process, reporting of dose volume parameters for uniform prescribing, recording and reporting. The highlight of the report is summary and key messages at the end of each chapter which can be implemented in various environments. Tumor can be precisely assessed and delineated in three dimensions taking into account tumor growth pattern at BT and the topography of the adjacent OAR which has been addressed very well in chapter 5. Chapter-6 introduces radiotherapy related morbidity endpoints and sub volumes of OARs, based on the morbidity profiles as known from the clinical experience. For OARs, two reference volumes D2cm3, D0.1cm3, characterizing maximum exposed region in the adjacent organ walls were considered.

Chapters 9-12 are dedicated to treatment planning aspects from physics point of view which include applicator reconstruction, treatment planning and absorbed dose calculation, especially chapter-10 is dedicated to radiographic dose assessment, which emphasises that a common terminology needs to be established, such that progress in 3D BT can influence further developments in institutions with limited resources. Some of the recommended reference points are taken from the previous ICRU report -38, (bladder, rectum, pelvic wall and lymphatic trapezoid) in addition to the vaginal points. More specifically, this report has adopted the definition of point A as a reference point. This geometrical definition is recommended in order to provide a clear distinction with the anatomically defined target dose volume that has been introduced as a new concept. Chapter-11 describes the dose calculation formalism, includes the source strength specification. For absorbed dose calculation, AAPM TG 43 has been recommended. Further, correlation between TRAK and point A dose irradiated volumes is also indicated.

At the end of the report a short summary in tabular form gives a quick and complete overview of all the recommendations. The reporting is structured at different levels, Level 1 describes the minimum requirements, which should be followed in all centers, for all patients, and represents the minimum standard of treatment; level 2 indicates advanced standards that allow a more complete exchange of information between centers; level 3 is related to research and development for which reporting criteria cannot yet be established.

Appendix A consists of nine clinical examples describing in detail the various clinical, imaging, technical, and biological scenarios. The major recommendations as outlined in this report are applied and specified in these examples. On the website http://icru.org/content/reports/spreadsheets for calculating EQD2 doses are provided. This site also contains a printable form for reproducible clinical drawings as used in this report.






 

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