Journal of Medical Physics
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   2011| October-December  | Volume 36 | Issue 4  
    Online since November 18, 2011

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Evaluation of pencil beam convolution and anisotropic analytical algorithms in stereotactic lung irradiation
Tania De La Fuente Herman, Kerry Hibbitts, Terence Herman, Salahuddin Ahmad
October-December 2011, 36(4):234-238
DOI:10.4103/0971-6203.89974  PMID:22228933
The aim of this study was to evaluate differences in dose distributions in stereotactic body radiation therapy treatment plans for lung tumors calculated with pencil beam convolution (PBC) algorithm with modified Batho power law (MBPL) versus heterogeneity corrected anisotropic analytical algorithm (AAA) of the Varian Eclipse treatment planning system. The four-dimensional computed tomography images from 20 patients with lung cancer were used to create treatment plans. Plans used five to seven nonopposing coplanar 6 MV beams. Plans generated with the PBC algorithm and MBPL for tissue heterogeneity corrections were optimized to deliver 60 Gy in three fractions to at least 95% of the planned target volume, and the normal tissue doses for spinal cord, esophagus, heart, and ipsilateral bronchus were restricted to less than 18, 27, 30, and 30 Gy, respectively. Plans were recalculated with AAA, retaining identical beam arrangements, photon beam fluences, and monitor units. The pencil beam plans, designed to deliver 60 Gy, delivered on average 51.6 Gy when re-calculated with the AAA, suggesting a reduction of at least 10% to prescription dose is appropriate when calculating with the AAA.
  4 3,513 93
The effects of cutouts on output, mean energy and percentage depth dose of 12 and 14 MeV electrons
Navid Khaledy, Azim Arbabi, Dariush Sardari
October-December 2011, 36(4):213-219
DOI:10.4103/0971-6203.89970  PMID:22228930
Electron field-shaping cerrobend cutouts on the linear accelerator applicator have some effects on the output and percentage depth dose. These effects which arise from the lateral scatter nonequilibrium are particularly evident in higher energies and in cutouts with smaller radius. Dose measurements for circular, square, and triangular cutouts as well as open field was performed in a 10 × 10 cm applicator, using plane parallel type ion chamber with a 100 cm source surface distance. The Percentage Depth Doses curves were drawn and the outputs were measured for each of these cutouts. The output factors, normalized to open 10 × 10 cm field, varied between 0.891 and 0.996 depending on the energy, cutout shape, and cavity area. With the use of cutouts, R 100 shifted toward the surface. The shifts ranged from 9 to 0 mm and from 13 to 0 mm for 12 and 14 MeV, respectively, depending on the shape and cavity area. For R 90 , R 80 , and R 50 the ranges for observed shifts narrowed down and practically no shifts were observed for R 20 . We present these changes in the form of predictive formulas, which would be useful in clinical applications.
  2 3,731 150
Dose volume histogram analysis and comparison of different radiobiological models using in-house developed software
Arun S Oinam, Lakhwant Singh, Arvind Shukla, Sushmita Ghoshal, Rakesh Kapoor, Suresh C Sharma
October-December 2011, 36(4):220-229
DOI:10.4103/0971-6203.89971  PMID:22228931
The purpose of this study is to compare Lyman-Kutcher-Burman (LKB) model versus Niemierko model for normal tissue complication probability (NTCP) calculation and Niemierko model versus Poisson-based model for tumor control probability (TCP) calculation in the ranking of different treatment plans for a patient undergoing radiotherapy. The standard normal tissue tolerance data were used to test the NTCP models. LKB model can reproduce the same complication probability data of normal tissue response on radiation, whereas Niemierko model cannot reproduce the same complication probability. Both Poisson-based and Niemierko models equally reproduce the same standard TCP data in testing of TCP. In case of clinical data generated from treatment planning system, NTCP calculated using LKB model was found to be different from that calculated using Niemierko model. When the fractionation effect was considered in LKB model, the calculated values of NTCPs were different but comparable with those of Niemierko model. In case of TCP calculation using these models, Poisson-based model calculated marginally higher control probability as compared to Niemierko model.
  2 7,669 228
Megavoltage cone beam computed tomography: Commissioning and evaluation of patient dose
Hassan S Abou-elenein, Ehab M Attalla, H Ammar, Ismail Eldesoky, Mohamed Farouk, Mohamed S Zaghloul
October-December 2011, 36(4):205-212
DOI:10.4103/0971-6203.89969  PMID:22228929
The improvement in conformal radiotherapy techniques enables us to achieve steep dose gradients around the target which allows the delivery of higher doses to a tumor volume while maintaining the sparing of surrounding normal tissue. One of the reasons for this improvement was the implementation of intensity-modulated radio therapy (IMRT) by using linear accelerators fitted with multi-leaf collimator (MLC), Tomo therapy and Rapid arc. In this situation, verification of patient set-up and evaluation of internal organ motion just prior to radiation delivery become important. To this end, several volumetric image-guided techniques have been developed for patient localization, such as Siemens OPTIVUE/MVCB and MVision megavoltage cone beam CT (MV-CBCT) system. Quality assurance for MV-CBCT is important to insure that the performance of the Electronic portal image device (EPID) and MV-CBCT is suitable for the required treatment accuracy. In this work, the commissioning and clinical implementation of the OPTIVUE/MVCB system was presented. The geometry and gain calibration procedures for the system were described. The image quality characteristics of the OPTIVUE/MVCB system were measured and assessed qualitatively and quantitatively, including the image noise and uniformity, low-contrast resolution, and spatial resolution. The image reconstruction and registration software were evaluated. Dose at isocenter from CBCT and the EPID were evaluated using ionization chamber and thermo-luminescent dosimeters; then compared with that calculated by the treatment planning system (TPS- XiO 4.4). The results showed that there are no offsets greater than 1 mm in the flat panel alignment in the lateral and longitudinal direction over 18 months of the study. The image quality tests showed that the image noise and uniformity were within the acceptable range, and that a 2 cm large object with 1% electron density contrast can be detected with the OPTIVUE/MVCB system with 5 monitor units (MU) protocol. The registration software was accurate within 2 mm in the anterior-posterior, left-right, and superior-inferior directions. The additional dose to the patient from MV-CBCT study set with 5 MU at the isocenter of the treatment plan was 5 cGy. For Electronic portal image device (EPID) verification using two orthogonal images with 2 MU per image the additional dose to the patient was 3.8 cGy. These measured dose values were matched with that calculated by the TPS-XiO, where the calculated doses were 5.2 cGy and 3.9 cGy for MVCT and EPID respectively.
  1 3,565 149
On the quality assurance and verification of modern radiation therapy treatment
L John Schreiner
October-December 2011, 36(4):189-191
DOI:10.4103/0971-6203.89963  PMID:22228926
  - 2,491 556
On "ruby" in myocardial perfusion imaging
Dilip Gude
October-December 2011, 36(4):239-240
DOI:10.4103/0971-6203.89975  PMID:22228934
  - 1,599 60
A simple plan evaluation index based on the dose to critical structures in radiotherapy
Ramachandran Prabhakar, Goura K Rath
October-December 2011, 36(4):192-197
DOI:10.4103/0971-6203.89965  PMID:22228927
The dose to critical structures plays a very important role in treatment plan evaluation and forms a major challenging parameter in radiotherapy treatment planning. In this study, a simple index, Plan Normal tissue complication Index (PNI) has been proposed for treatment plan evaluation based on the dose to surrounding critical structures. To demonstrate the proposed index, four different critical treatment sites that include the prostate, upper abdominal cancer, lung, and head and neck were selected for this study. A software progam (PNIcalc) has been developed to compute the PNI from the exported dose-volume histogram data and from the tissue tolerance data published by Emami et al. and Kehwar et al. The software also shows the parameters that exceed the threshold limits of dose-volume parameters presented in the QUANTEC recommendations (2010). In all the studied cases, PNI gave an overall picture of the dose received by the critical structures and also indicate the fractional volume exceeding the tolerance limit. The proposed index, PNI gives a quick comparison and selection of treatment plans that result in reduced dose to the critical structures. It can be used as an additional tool for routine treatment plan evaluation in external beam radiotherapy.
  - 8,854 525
Effect of low-density heterogeneities in telecobalt therapy and validation of dose calculation algorithm of a treatment planning system
Anuj Kumar, Sunil Dutt Sharma, AK Arya, Surabhi Gupta, Deepak Shrotriya
October-December 2011, 36(4):198-204
DOI:10.4103/0971-6203.89967  PMID:22228928
Telecobalt machines are still prominently used for the treatment of a variety of cancer cases in developing countries. The human body is a heterogeneous composition of variety of tissues and cavities which vary widely in their physical and radiological properties. The presence of heterogeneities in the path of telecobalt beam presents an altered dose distribution in the region of clinical interests. A computerized treatment planning system (TPS) is generally used for calculating the dose distribution in the patient. Experimental measurements were carried out in a telecobalt beam with the objectives to study the effects of low-density heterogeneities and to verify the ability of the ASHA radiotherapy TPS in predicting the altered dose distribution along the central axis and off-axis of the beam. Locally available kailwood was tested for its lung equivalence and measurements were carried out in a polymethyl methacrylate phantom by introducing lung equivalent and air gap heterogeneities. A comparison of experimentally measured and TPS calculated dose values indicates that the TPS overestimates the dose by 11.6% in lung equivalent (kailwood) heterogeneity along the central axis. Similarly, it was found that the TPS overestimates the dose by 3.9% and 5.9%, respectively, with air heterogeneity of 1.0 and 2.0 cm. While testing the adequacy of TPS in off-axis region, it was found that the TPS calculation does not indicate the widening of the beam profile in the low-density heterogeneity region. This study suggests that the effective path length based algorithm of the ASHA radiotherapy TPS is unable to achieve the recommended 3% accuracy of clinical dose calculation in heterogeneous media.
  - 3,164 203
Effect of surface waves on radiotherapy dosimetric measurements in water tanks
Mohammad Bakhtiari
October-December 2011, 36(4):230-233
DOI:10.4103/0971-6203.89972  PMID:22228932
The effect of surface waves, generated by moving the scanning arms in water phantoms, on radiation dosimetry is studied. It is shown that in large water tanks, high arm speeds can result in dosimetric errors of up to 5%. The measurements that are started after damping the water waves can result in about a 50% improvement in accuracy of measurements. It is shown that the water surfaces at the start of the measurements have high fluctuations that transform to a steady phase by elapsing time.
  - 1,891 124
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