Journal of Medical Physics
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   2020| January-March  | Volume 45 | Issue 1  
    Online since March 13, 2020

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Volumetric arc therapy treatment plan dosimetry correction method to account patient weight loss during a course of radiation therapy
Harold DSouza, Henry Weatherburn, Abhishek Dwivedi, Tharmarnadar Ganesh
January-March 2020, 45(1):1-6
Aim: This study aims to validate volumetric arc therapy (VMAT) plan correction method for a patient's lost weight during the course of radiotherapy. Materials and Methods: VMAT plans of prostate and head and neck cancers were considered to evaluate dosimetric effects due to external surface changes caused by patient's weight loss during treatment. Accepted VMAT treatment plan was recalculated on the planning computed tomography (CT) with a newly created external contour from cone-beam CT and was compared with the original plan. Monitor unit (MU) correction was applied based on a simple formalism, and doses were recalculated. Dose statistics were compared with the original plan. Ten patients with significant weight loss were considered to validate proposed MU correction method by comparing the dose statistics before and after MU corrections. Results: We observed 3.7%–5.2% change in the plan maximum dose for one cm change in path length to isocenter with increased planning target volume dose, D95 by 4%. The organs at risk (OAR) doses increased as high as 6.8%. Using MU correction method, target volume and OARs dose changes were reduced to <1% when compared with the original plan. The correction method brought down the maximum plan dose and volume of 95% isodose (V95) cloud below an acceptable range of 1%–2% in 10 patients treatment plans. Conclusion: Image-guided radiation therapy process detects the weight loss, which affects the treatment plan's dose distribution and should be corrected. Applying the correction method described here keeps the patient dosimetry within 1% of the original plan, which is clinically acceptable. The process of plan dosimetry correction to address weight loss can be completed within 30 min without repeating imaging and planning process.
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Development and validation of a matlab software program for decoding the treatment errors in real-time position management Gating-generated breathing trace
Naveen Kumawat, Anil Kumar Shrotriya, Malhotra Singh Heigrujam, Kartikeswar Patro, Satendra Kumar, Anil Kumar Bansal, Ram Kishan Munjal, Anil Kumar Anand
January-March 2020, 45(1):16-23
Introduction: The Real-time Position Management (RPM) is used as a motion management tool to reduce normal tissue complication. However, no commercial software is available to quantify the “beam-on” errors in RPM-generated breathing traces. This study aimed to develop and validate an in-house-coded MATLAB program to quantify the “beam-on” errors in the breathing trace. Materials and Methods: A graphical user interface (GUI) was developed using MATLAB (Matrix Laboratory Ra2016) software. The GUI was validated using two phantoms (Varian-gated phantom and Brainlab ET gating phantom) with three regular motion profiles. Treatment time delay was calculated using regular sinusoidal motion profile. Ten patient's irregular breathing profiles were also analyzed using this GUI. Results: The beam-on comparison between the recorded reference trace and irradiated trace profile was done in two ways: (1) beam-on time error and (2) beam-on displacement error. These errors were ≤1.5% with no statistical difference for phase- and amplitude-based treatments. The predicated amplitude levels of reference phase-based profiles, and the actual amplitude levels of amplitude-based irradiated profiles were almost equal. The average treatment time delay was 47 ± 0.003 ms. The irregular breathing profile analysis showed that the amplitude-based gating treatment was more accurate than phase based. Conclusion: The developed GUI gave the same and acceptable results for all regular profiles. These errors were due to the lag time of the linear accelerator with gating treatment. This program can be used as to quantifying the intrafraction “beam-on” errors in breathing trace with both mode of gating techniques for irregular breathing trace, and in addition, it is capable to convert phase-based gating parameters to amplitude-based gating parameters for treatment.
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Dosimetric Comparison of treatment plans computed with finite size pencil beam and monte carlo algorithms using the incise™ multileaf collimator-equipped cyberknife® system
Kalpani Nisansala Udeni Galpayage Dona, Charles Shang, Theodora Leventouri
January-March 2020, 45(1):7-15
Purpose: InCise™ multileaf collimator (MLC) was introduced for CyberKnife® (CK) Robotic Radiosurgery System (CK-MLC) in 2015, and finite size pencil beam (FSPB) was the only available dose computation algorithm for treatment plans of CK-MLC system. The more advanced Monte Carlo (MC) dose calculation algorithm of lnCise™ was initially released in 2017 for the CK Precision™ treatment planning system (TPS) (v1.1) with new graphic processing unit (GPU) platform. GPU based TPS of the CK offers more accurate, faster treatment planning time and intuitive user interface with smart three-dimensional editing tools and fully automated autosegmentation tools. The MC algorithm used in CK TPS simulates the energy deposited by each individual photon and secondary particles to calculate more accurate dose. In the present study, the dose disparities between MC and FSPB algorithms for selected Stereotactic Ablative Radiation Therapy (SABR) CK-MLC treatment plans are quantified. Materials and Methods: A total of 80 CK-MLC SABR plans computed with FSPB were retrospectively reviewed and compared with MC computed results, including plans for detached lung cancer (or tumors fully surrounded by lung tissues, n = 21), nondetached lung cancer (or tumor touched the chest wall or mediastinum, n = 23), intracranial (n = 21), and pancreas lesions (n = 15). Dosimetric parameters of each planning target volume and major organs at risk (OAR) are compared in terms of normalized percentage deviations (Ndev). Results: This study revealed an average of 24.4% overestimated D95values in plans using FSPB over MC for detached lung (n = 21) and 14.9% for nondetached lung (n = 23) lesions. No significant dose differences are found in intracranial (0.3%, n = 21) and pancreatic (0.9%, n = 15) cases. Furthermore, no significant differences were found in Ndevof OARs. Conclusion: In this study, it was found that FSPB overestimates dose to inhomogeneous treatment sites. This indicates, the employment of MC algorithm in CK-MLC-based lung SABR treatment plans is strongly suggested.
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Investigation of different factors affecting the quality of spect images: A simulation study
Mahsa Noori-Asl
January-March 2020, 45(1):44-51
Background: Monte Carlo (MC) simulation codes are used extensively for modeling the nuclear medicine imaging systems, such as single photon emission computed tomography (SPECT) and positron emission tomography (PET). By using these codes, it is possible to set different imaging parameters and do various studies in the field of nuclear medicine imaging. Aims and Objectives: The aim of this study is to investigate the effective factors in improvement of the SPECT image quality by using MC simulation. Materials and Methods: In this study, we used the SIMIND MC simulation code and Jaszczak phantom containing six spheres with different diameters placed into a water-filled cylindrical phantom for consideration of the effects of different factors on quality of the images obtained from Tc-99m SPECT imaging system. The assessment criteria used to investigate these factors included image contrast, signal-to-noise ratio (SNR) and relative noise of the background (RNB). Results: The results of this study show that the right choice of the arc of rotation, the image matrix size, the number of angular views, type of the collimators, and also filters used in the image reconstruction affect the quality of SPECT images. Also, we show that use of scatter correction methods can improve the image quality. Conclusion: The MC simulation is a suitable tool for investigation of different factors affecting the quality of SPECT images, essentially in the studies based on the energy spectrum, such as the evaluation of the scatter correction methods.
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A method of high-resolution radiotherapy delivery fluences with a pair of fields with orthogonal collimator settings: A study on ten head-and-neck cancer patients
Stipe Galic, Marin Kovacevic, Ivan Lasic, Hrvoje Brkic, Dario Faj
January-March 2020, 45(1):36-43
Context: Introduction of dual-layer multileaf collimator (MLC) radiotherapy linear accelerators into clinical practice is an important development in advanced external beam radiotherapy. A method of delivering comparable high-resolution fluences with a single-layer MLC is presented. Aims: The aims of this study are to present new algorithms and approaches to define high-resolution hypermodulated fluences, obtain orthogonal decomposition of fluences, and deliver them on a linear accelerator with single MLC from two perpendicular collimator settings. Materials and Methods: High-resolution fluences were defined using Monte Carlo (MC) calculation. A novel use of a limited-memory, bounded, Broyden–Fletcher–Goldfarb–Shanno algorithm was used to decompose such fluences to ones deliverable with a pair of fields with mutually orthogonal collimator settings. Such a technique, here named cross motion leaf calculator (XMLC), is compared against single sliding window (SSW) technique typically used in intensity-modulated radiation therapy (IMRT). An electronic portal imaging device (EPID) is used, and the results were compared with gamma analysis. Furthermore, MC was used to determine dose distributions for computed tomography images of ten head-and-neck cancer patients. Results: Gamma analysis (3%, 3 mm) against ideal fluence is considerably more favorable to XMLC (94% ± 4%) versus SSW (76% ± 5%). Furthermore, the dose–volume histogram (DVH) analysis showed that XMLC enables delivery of fluences superior to that of IMRT and these results in clinically relevant enhancements in DVH results. Conclusions: At the time of writing of this study, there were more than 12,000 medical linear accelerators in clinical use, and XMLC can prove itself useful wherever linac is equipped with MLC but cannot delivery latest techniques, such as volumetric modulated arc therapy.
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Validation of PRIMO monte carlo model of clinac®ix 6mv photon beam
B Sarin, B Bindhu, B Saju, Raguram K Nair
January-March 2020, 45(1):24-35
Purpose: This study aims to model 6MV photon of Clinac®iX linear accelerator using PRIMO Monte Carlo (MC) code and to assess PRIMO as an independent MC-based dose verification and quality assurance tool. Materials and Methods: The modeling of Clinac®iX linear accelerator has been carried out by using PRIMO simulation software (Version The simulated beam parameters were compared against the measured beam data of the Clinac®iX machine. The PRIMO simulation model of Clinac®iX was also validated against Eclipse® Acuros XB dose calculations in the case of both homogenous and inhomogeneous mediums. The gamma analysis method with the acceptance criteria of 2%, 2 mm was used for the comparison of dose distributions. Results: Gamma analysis shows a minimum pass percentage of 99% for depth dose curves and 95.4% for beam profiles. The beam quality index and output factors and absolute point dose show good agreement with measurements. The validation of PRIMO dose calculations, in both homogeneous and inhomogeneous medium, against Acuros® XB shows a minimum gamma analysis pass rate of 99%. Conclusions: This study shows that the research software PRIMO can be used as a treatment planning system-independent quality assurance and dose verification tool in daily clinical practice. Further validation will be performed with different energies, complex multileaf collimators fields, and with dynamic treatment fields.
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