Journal of Medical Physics
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   Table of Contents - Current issue
July-September 2019
Volume 44 | Issue 3
Page Nos. 145-229

Online since Friday, September 13, 2019

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An experimental slope method for a more accurate measurement of relative radiation doses using radiographic and radiochromic films and its application to megavoltage small-field dosimetry Highly accessed article p. 145
Raghavendra Holla, D Khanna, Bhaskaran K Pillai, KV Jafar Ali, PS Renil Mon, CO Clinto, Tharmarnadar Ganesh
Purpose: An experimental method using the linear portion of the relative film dose–response curve for radiographic and radiochromic films is presented, which can be used to determine the relative depth doses in a variety of very small, medium, and large radiation fields and relative output factors (ROFs) for small fields. Materials and Methods: The film slope (FS) method was successfully applied to obtain the percentage depth doses (PDDs) for external beams of photon and electrons from a Synergy linear accelerator (Elekta AB, Stockholm, Sweden) under reference conditions of 10 cm × 10 cm for photon beam and nominal 10 cm × 10 cm size applicator for electron beam. For small-field dosimetry, the FS method was applied to EDR2 films (Carestream Health, Rochester, NY) for 6 MV photon beam from a linac (Elekta AB, Stockholm, Sweden) and small, circular radiosurgery cones (Elekta AB, Stockholm, Sweden) with diameters of 5, 7.5, 10, 12.5, and 15 mm. The ROFs for all these cones and central axis PDDs for 5, 10, and 15 mm diameter cones were determined at source-to-surface distance of 100 cm. The ROFs for small fields of CyberKnife system were determined using this technique with Gafchromic EBT3 film (Ashland, NJ, USA). The PDDs and ROFs were compared with ion chamber (IC) and Monte Carlo (MC) simulated values. Results: The maximum percentage deviation of PDDFS with PDDIC for 4, 6, and 15 MV photon beams was within 1.9%, 2.5%, and 1.4%, respectively, up to 20-cm depth. The maximum percentage deviation of PDDFS with PDDIC for electron beams was within 3% for energy range studied of 8–15 MeV. The gamma passing rates of PDDFS with PDDIC were above 96.5% with maximum gamma value of >2, occurring at the zero depths for 4, 6, and 15 MV photons. For electron beams, the gamma passing rates between PDDFS with PDDIC were above 97.7% with a maximum gamma value of 0.9, 1.3, and 0.7 occurring at the zero depth for 8, 12, and 15 MeV. For small field of 5-mm cone, the ROFFS was 0.665 ± 0.021 as compared to 0.674 by MC method. The maximum percentage deviation between PDDFS and PDDMC was 3% for 5 mm and 10 mm and 2% for 15 mm cones with 1D gamma passing rates, respectively, of 95.5%, 96%, and 98%. For CyberKnife system, the ROFFS using EBT3 film and MC published values agrees within 0.2% for for 5 mm cone. Conclusions: The authors have developed a novel and more accurate method for the relative dosimetry of photon and electron beams. This offers a unique method to determine PDD and ROF with a high spatial resolution in fields of steep dose gradient, especially in small fields.
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Out-of-field dose calculation by a commercial treatment planning system and comparison by monte carlo simulation for varian TrueBeam® p. 156
NS Shine, Raghukumar Paramu, M Gopinath, RC Jaon Bos, PM Jayadevan
Purpose: The calculation accuracy of treatment planning systems (TPSs) drops drastically when the points outside the field edges are considered. The real accuracy of a TPS and linear accelerator (linac) combination for regions outside the field edge is a subject which demands more study. In this study, the accuracy of out-of-field dose calculated by a TPS, used with a TrueBeam® (TB) linac, is quantified. Materials and Methods: For dose calculation, Eclipse™ version 13.7 commissioned for TB machine was used. For comparison, Monte Carlo (MC) methods, as well as the measurements, were used. The VirtuaLinac, a Geant 4-based MC program which is offered as a cloud solution, is used for the generation of input phase-space (PS) files. This PS file was imported into PRIMO (PENELOPE based MC program) for the simulation of out-of-field dose. Results: In this study, the accuracy of the out-of-field dose calculated by a TPS for a TB linac was estimated. As per the results in comparison with MC simulations, the TPS underestimated the dose by around 45% on an average for the off-axis-distance range considered in this study. As the off-axis distance increased, the underestimation of the dose also increased. Conclusion: In this work, it was observed that the TPS underestimates doses beyond the edges of treatment fields for a clinical treatment executed on a TB machine. This indicates that the out-of-field dose from TPSs should only be used with a clear understanding of the inaccuracy of dose calculations beyond the edge of the field.
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Electronic portal imaging device-based three-dimensional volumetric dosimetry for intensity-modulated radiotherapy pretreatment quality assurance p. 176
Manikandan Arjunan, Sureka Chandra Sekaran, Biplab Sarkar, Saran Kumar Manavalan
Aim: This study aimed at evaluating the efficacy of treatment planning system (TPS)-based heterogeneity correction for two- and three-dimensional (2D and 3D) electronic portal imaging device (EPID)-based pretreatment dose verification. An experiment was conducted on the EPID back-projection technique and intensity-modulated radiotherapy (IMRT). Materials and Methods: Treatment plans were delivered in EPID without a patient to obtain the fluence pattern (FEPID). A heterogeneity correction plane (Fhet) for an open beam of 30 cm × 30 cm was extracted from the TPS. The heterogeneity-corrected measured fluence is developed by matrix element multiplication (FResultant= FEPID× Fhet). Further planes were summed to develop a 3D dose distribution and exported to the TPS. Dose verifications for 2D and 3D were carried out with the corresponding TPS values using 2D gamma analysis (ɣ) and dose volume histogram (DVH) comparison, respectively. Totally, 33 patients (17 head–neck and 16 thorax cases) were evaluated in this study. Results: The head–neck and thorax plans show a 3-mm-distance to agreement (DTA) 3% DD gamma passing of 96.3% ± 2.0% and 95.4% ± 1.8% points, respectively, between FTPS and FResultant. The comparison of the uncorrected measured fluence (FEPID) with FTPS reveals a gamma passing of 82.2% ± 7.3% and 80.4% ± 8.6% for head–neck and thorax cases, respectively. A total of 87 out of the 102 head–neck and thorax beams exhibit a planner gamma passing of 97.6% ± 2.1%. In the 3D-DVH comparison of thorax and head–neck cases, D5% for planning target volume were −0.5% ± 2.2% and −2.1% ± 3.5%, respectively; D95% varies as 1.0% ± 2.7% and 1.4% ± 1.1% between TPS calculated and heterogeneity-corrected-EPID-based dose reconstruction. Conclusion: The novel TPS-based heterogeneity correction can improve the 2D and 3D EPID-based back projection technique. Structures with large heterogeneities can also be handled using the proposed technique.
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Correlation between biological effective dose and radiation-induced liver disease from hypofractionated radiotherapy p. 185
Angelo M Bergamo, Kevin Kauweloa, Gregory Gan, Zheng Shi, Janeen Daniels, Richard Crownover, Ganesh Narayanasamy, Sotirios Stathakis, Panayiotis Mavroidis, Niko Papanikolaou, Alonso Gutierrez
Background: The prevention of radiation-induced liver disease (RILD) is very significant in ensuring a safe radiation treatment and high quality of life. Aims and Objectives: The purpose of this study is to investigate the correlation of physical and biological effective dose (BED) metrics with liver toxicity from hypo-fractionated liver radiotherapy. Materials and Methods: 41 hypo-fractionated patients in 2 groups were evaluated for classic radiation-induced liver disease (RILD) and chronic RILD, respectively. Patients were graded for effective toxicity (post-treatment minus pre-treatment) using the Common Terminology Criteria for Adverse Events (CTCAE) v4.0. Physical dose (PD) distributions were converted to BED. The V10Gy, V15Gy, V20Gy, V25Gy and V30Gy physical dose-volume metrics were used in the analysis together with their respective BED-converted metrics of V16.7Gy3, V30Gy3, V46.7Gy3, V66.7Gy3 and V90Gy3. All levels were normalized to their respective patient normal liver volumes (NLV) and evaluated for correlation to RILD. Results were measured quantitatively using R2 regression analysis. Results: The classic RILD group had median follow-up time of 1.9 months and the average PD-NLV normalized V10Gy, V15Gy, V20Gy, V25Gy and V30Gy metrics per grade were plotted against RILD yielding R2 correlations of 0.84, 0.72, 0.73, 0.65 and 0.70, respectively while the BED-volume metrics of V16.7Gy3, V30Gy3, V46.7Gy3, V66.7Gy3 and V90Gy3 resulted in correlation values of 0.84, 0.74, 0.66, 0.78 and 0.74, respectively. BED compared to PD showed a statistically significant (p=.03) increase in R2 for the classic RILD group. Chronic RILD group had median follow-up time of 12.3 months and the average PD-NLV normalized V10Gy, V15Gy, V20Gy, V25Gy and V30Gy metrics per grade were plotted against RILD grade yielding R2 correlations of 0.48, 0.92, 0.88, 0.90 and 0.99 while the BED-volume metrics of V16.7Gy3, V30Gy3, V46.7Gy3, V66.7Gy3 and V90Gy3 resulted in correlation values of 0.43, 0.94, 0.99, 0.21 and 0.00, respectively. Conclusion: The strong correlations of the V10Gy and V15Gy PD-volume metrics as well as the V16.7Gy3(BED of V10Gy) to both classic and chronic RILD imply the appropriateness of the current 15Gy evaluation level for liver toxicity with hypo-fractionated treatments.
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A novel design of N-fiducial phantom for automatic ultrasound calibration p. 191
Maria Chatrasingh, Jackrit Suthakorn
Background: Freehand ultrasound (US) is a technique used to acquire three-dimensional (3D) US images using a tracked 2D US probe. Calibrating the probe with a proper calibration phantom improves the precision of the technique and allows several applications in computer-assisted surgery. N-fiducial phantom is widely used due to the robustness of precise fabrication and convenience of use. In principle, the design supports single-frame calibration by providing at least three noncollinear points in 3D space at once. Due to this requirement, most designs contain multiple N-fiducials in unpatterned and noncollinear arrangements. The unpatterned multiple N-fiducials appearing as scattered dots in the US image are difficult to extract, and the extracted data are usually contaminated with noise. In practice, the extraction mostly relied on manual interventions, and calibration with N-fiducial phantom has not yet achieved high accuracy with single or few frame calibrations due to noise contamination. Aims: In this article, we propose a novel design of the N-fiducial US calibration phantom to enable automatic feature extraction with comparable accuracy to multiple frame calibration. Materials and Methods: Along with the design, the Random Sample Consensus (RANSAC) algorithm was used for feature extraction with both 2D and 3D models estimation. The RANSAC feature extraction algorithm was equipped with a closed-form calibration method to achieve automatic calibration. Results: The accuracy, precision, and shape reconstruction errors of the calibration acquired from the experiment were significantly matched with the previous literature reports. Conclusions: The results showed that our proposed method has a high efficiency to perform automatic feature extraction compared to conventional extraction performed by humans.
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A Model-based method for assessment of salivary gland and planning target volume dosimetry in volumetric-modulated arc therapy planning on head-and-neck cancer p. 201
Honglai Zhang, Yijian Cao, Jeffrey Antone, Adam C Riegel, Maged Ghaly, Louis Potters, Abolghassem Jamshidi
This study examined the relationship of achievable mean dose and percent volumetric overlap of salivary gland with the planning target volume (PTV) in volumetric-modulated arc therapy (VMAT) plan in radiotherapy for a patient with head-and-neck cancer. The aim was to develop a model to predict the viability of planning objectives for both PTV coverage and organs-at-risk (OAR) sparing based on overlap volumes between PTVs and OARs, before the planning process. Forty patients with head-and-neck cancer were selected for this retrospective plan analysis. The patients were treated using 6 MV photons with 2-arc VMAT plan in prescriptions with simultaneous integrated boost in dose of 70 Gy, 63 Gy, and 58.1 Gy to primary tumor sites, high-risk nodal regions, and low-risk nodal regions, respectively, over 35 fractions. A VMAT plan was generated using Varian Eclipse (V13.6), in optimization with biological-based generalized equivalent uniform dose (gEUD) objective for OARs and targets. Target dose coverage (D95, Dmax, conformity index) and salivary gland dose (Dmean and Dmax) were evaluated in those plans. With a range of volume overlaps between salivary glands and PTVs and dose constraints applied, results showed that dose D95 for each PTV was adequate to satisfy D95>95% of the prescription. Mean dose to parotid <26 Gy could be achieved with <20% volumetric overlap with PTV58(parotid-PTV58). On an average, the Dmean was seen at 15.6 Gy, 21.1 Gy, and 24.2 Gy for the parotid-PTV58 volume at <5%, <10%, and <20%, respectively. For submandibular glands (SMGs), an average Dmean of 27.6 Gy was achieved in patients having <10% overlap with PTV58, and 36.1 Gy when <20% overlap. Mean doses on parotid and SMG were linearly correlated with overlap volume (regression R2 = 0.95 and 0.98, respectively), which were statistically significant (P < 0.0001). This linear relationship suggests that the assessment of the structural overlap might provide prospective for achievable planning objectives in the head-and-neck plan.
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Calibration of gafchromic EBT film using the Microtek ScanMaker 9800XL plus flatbed scanner with a modified one red-channel after three-channel method p. 207
Liyun Chang, Sheng-Yow Ho, Hueisch-Jy Ding, Shyh-An Yeh, Pang-Yu Chen
Purpose: Using the Microtek ScanMaker 9800XL Plus (9800XL+) flatbed scanner, a method is presented to accurately calibrate EBT film, which cannot be calibrated simply using a general three-channel method because of the nonhomogeneous scanning. Materials and Methods: Through the percentage-depth-dose method, 6-MV photon beams with two different monitor units were delivered to eight EBT2 films, each of which was tightly sandwiched in a 30-cm cubic polystyrene phantom and positioned parallel to the central axis of the beam. Before and after irradiation, all films were scanned using the Microtek 9800XL+ scanner and the pixel values (PVs) were measured along the central axis of the beam on the film and fitted to the corresponding depth doses. Before calibration, the irradiated film image was first modified using a template matrix, which was generated using the prescanned background images. Then, a modified one red-channel after three-channel method was used to calibrate the film. Results: Without a template matrix, the three-channel method cannot be used because the PVs do not correspond to a rational fitting form. Using the proposed method, the difference between the fitted dose and the delivered dose is <2%. The green channel, and not the red, is found to have the largest dynamic range. Conclusion: The proposed technique allows the use of the three-channel method to calibrate film using a Microtek 9800XL+ scanner.
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Evaluation of healthy tissue dose at different regions between volumetric-modulated arc therapy and intensity-modulated radiation therapy plans in the treatment of various cancers p. 213
Jayapalan Krishnan, Suresh Rao, Sanath Hegde, Jayarama Shetty
Background: Radiotherapy plays an important role in the management of cancer. Although the improved technologies increase therapeutic index, different delivery techniques deliver different dose pattern to the healthy tissue within and outside treatment volume. Objective: The objective of this study was to evaluate the low, intermediate, and high dose to healthy tissue within and outside the treatment volume and to find the relation between tumor volume and various doses received healthy tissue volume. Materials and Methods: A total of 150 patients were included. For all patients, planning computed tomography images were acquired. Tumors, critical structures, and healthy tissue volumes at different regions were delineated. Two sets of plans, one with volumetric-modulated arc therapy and another with intensity-modulated radiation therapy (IMRT) were created, optimized for 6 MV photons and dose was calculated. Dosimetry results for tumor, organs at risks (OARs), and healthy tissue from both the techniques were evaluated and compared. Results: Tumor coverage and dose to OARs was significantly better with volumetric-modulated arc therapy (VMAT). Volume of healthy tissue received high-dose within the treatment volume as well as volume of healthy tissue received low and intermediate-dose out of treatment volume were significantly (P < 0.002) lesser with VMAT. Besides, the results showed that as the tumor volume increased, the various dose received healthy tissue volume also increased. Conclusions: VMAT plan can reduce the risk of secondary malignancy while treating different sites of cancer. VMAT is the most appropriate technique than IMRT, especially in the treatment of large tumor volume. Special attention has to be given, especially while treating women and children.
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Evaluation and validation of IBA I'MatriXX array for patient-specific quality assurance of tomotherapy® p. 222
N V. N Madhusudhana Sresty, A Krishnam Raju, B Nagarjuna Reddy, VC Sahithya, Yakub Mohmd, G Deleep Kumar, T Anil Kumar, Tasneem Rushdi, S Aparna, G Durga Prasad, Harjot Kaur Bajwa
TomoTherapy® is a modern radiation treatment technique in which intensity-modulated radiation therapy (IMRT) is delivered in helical fashion. A two-dimensional (2D) array which has been existing for IMRT patient-specific quality assurance (PSQA) verifications for many years is I'MatriXX. Our objectives were to validate this I'MatriXX and to evaluate it for different patient sites and fractionation schedules of TomoTherapy treatment. Twenty-five plans were created with virtual target for different possible pitch values and field widths for validation. Gamma index criteria of 3%/2% dose differences and 3/2 mm distance to agreement were used. QA plans of 26 different treatment sites and different fractionation schedules were used. Results indicated that the matrix response is independent of field width, pitch, and modulation factor of TomoTherapy with 3%, 3 mm criteria. High passing rate ranging from 99.7% to 90.7% was observed for selected patient plans. We found that I'MatriXX 2D array can be utilized for easy and quick TomoTherapy PSQA.
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The physics of CT dosimetry: CTDI and beyond p. 228
Gurpreet Singh
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