Journal of Medical Physics
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   Table of Contents - Current issue
October-December 2019
Volume 44 | Issue 4
Page Nos. 231-299

Online since Wednesday, December 11, 2019

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Simple electronic portal imager-based pretreatment quality assurance using Acuros XB: A feasibility study Highly accessed article p. 231
Arjunan Manikandan, Sureka Chandra Sekaran, Biplab Sarkar, Sujatha Manikandan
Objective: This study demonstrates a novel electronic portal imaging device (EPID)-based forward dosimetry approach for pretreatment quality assurance aided by a treatment planning system (TPS). Materials and Methods: Dynamic multileaf collimator intensity-modulated radiation therapy (IMRT) plans were delivered in EPID and fluence was captured on a beam-by-beam basis (FEPID). An open field having dimensions equal to those of the largest IMRT field was used in the TPS to obtain the transmitted fluence. This represented the patient-specific heterogeneity correction (Fhet). To obtain the resultant heterogeneity-corrected fluence, EPID measured fluence was corrected for the TPS generated heterogeneity (FResultant= FEPID× Fhet). Next, the calculated fluence per beam basis was collected from TPS (FTPS). Finally, FResultantand FTPSwere compared using a 3% percentage dose difference (% DD)-3 mm distance to agreement [DTA] gamma analysis in an isocentric plane (two-dimensional [2D]) and multiple planes (quasi three-dimensional [3D]) orthogonal to the beam axis. Results: The 2D heterogeneity-corrected dose reconstruction revealed a mean γ passing of the pelvis, thorax, and head-and-neck cases of 96.3% ±2.0%, 96.3% ±1.8%, and 96.1% ±2.2%, respectively. Quasi-3D γ passing for the pelvis, thorax, and head-and-neck cases were 97.5% ±1.4%, 96.3% ±2.4%, and 97.5% ±1.0%, respectively. Conclusion: EPID dosimetry produced an inferior result due to no heterogeneity corrections for sites such as the lung and esophagus. Incorporating TPS-based heterogeneity correction improved the EPID dosimetry result for those sites with large heterogeneity.
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Estimation of backscatter from internal shielding in electron beam radiotherapy using Monte Carlo simulations (EGSnrc) and Gafchromic film measurements Highly accessed article p. 239
Sukhvir Singh, Manoj Kumar Semwal, CP Bhatt
Purpose: The purpose of the study was to estimate the backscatter electron dose in internal shielding during electron beam therapy using Monte Carlo (MC) simulations and Gafchromic film measurements. Materials and Methods: About 6 and 9 MeV electron beams from a Varian 2100C linac were simulated using BEAMnrc MC code. Various clinical situations of internal shielding were simulated by modeling water phantoms with 2 mm lead sheets placed at different depths. Electron backscatter factors (EBF), a ratio of dose at tissue-shielding interface to the dose at the same point without the shielding, were estimated. The role of 2 mm aluminum in reduction of backscatter was investigated. The measurements were also performed using Gafchromic films and results were compared with MC simulations. Results: For particular beam energy, the EBF value initially increased with depth in the buildup region and then decreased rapidly. The highest value of EBF for both the energies is nearly same though at different depths. Decreased EBF was observed for 9 MeV beam in comparison to the 6 MeV beam for the same depth of shielding placement. Two millimeter aluminum reduced the backscatter by nearly 25% at maximum backscatter condition for both the energies, though the effectiveness slightly decreased at higher energy. The range of backscatter electrons was varying from 5 to 12 mm in the upstream direction from the interface. The Gafchromic film-measured EBF and MC-simulated EBF were matching well within the clinically acceptable limits except in close vicinity of tissue-lead interface. Conclusions: This study provides an important clinical data to design internal shielding at the local clinical setup and confirms applicability of MC simulations in backscatter dose calculations at interfaces where physical measurements are difficult to perform.
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Experimental determination of radial dose function and anisotropy function of GammaMed Plus 192Ir high-dose-rate brachytherapy source in a bounded water phantom and its comparison with egs_brachy Monte Carlo simulation p. 246
Rekha Reddy Buchapudi, Ravikumar Manickam, Varatharaj Chandaraj
Objective: The aim of the present study is to experimentally measure the radial dose function g(r) and anisotropy function F(r,θ) of GammaMed Plus 192Ir high-dose-rate source in a bounded water phantom using thermoluminescent dosimeter (TLD) and film dosimetry and compare the obtained results with egs_brachy Monte Carlo (MC)-calculated values for the same geometry. Materials and Methods: The recently developed egs_brachy is a fast Electron Gamma Shower National Research Council of Canada MC application which is intended for brachytherapy applications. The dosimetric dataset recommended by Task Group 43 update (TG43U1) is calculated using egs_brachy for an unbounded phantom. Subsequently, radial dose function g(r) and anisotropy function F(r,θ) are measured experimentally in a bounded water phantom using TLD-100 and Gafchromic EBT2 film. Results: The TG43U1 dosimetric parameters were determined using the egs_brachy MC calculation and compared with published data which are found to be in good agreement within 2%. The experimentally measured g(r) and F(r,θ) and its egs_brachy MC code-calculated values for a bounded phantom geometry are found to be good in agreement within the acceptable experimental uncertainties of 3%. Conclusion: Our experimental phantom size represents the average patient width of 30 cm; hence, results are closer to scattering conditions in clinical situations. The experimentally measured g(r) and F(r,θ) and egs_brachy MC calculations for bounded geometry are well in agreement within experimental uncertainties. Further, the confidence level of our comparative study is enhanced by validating the egs_brachy MC code for the unbounded phantom with respect to consensus data.
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Dose shadowing and prosthesis involvement for megavoltage photon In vivo diode dosimetry p. 254
Nicholas Ade, Dete Van Eeden, F C. P. Du Plessis
Aim: The aim of the study is to investigate the photon beam perturbations induced by an in vivo diode in combination with prosthesis involvement in a human-like phantom. Materials and Methods: Beam perturbations for 6 MV and 10 MV photons caused by an EDP-203G in vivo diode in combination with prosthesis involvement were studied in a unique water-equivalent pelvic phantom, equipped with bony structures and Ti prosthesis using single fields between 2 × 2 and 15 cm × 15 cm as well as 10 MV lateral opposing fields and a four-field plan. Dose distributions were measured with Gafchromic EBT3 films with and without the diode included in the beams on the prosthesis (prosthetic fields) and non-prosthesis (non-prosthetic fields) sides of the phantom. Differences between prosthetic and non-prosthetic field dose data were determined to assess the effect of the prosthesis on the diode-induced beam perturbations inside the phantom. Results: Photon beam dose perturbations ranged from 2% to 7% and from 5% to 12% for prosthetic and non-prosthetic fields, respectively, with relative differences between 2% and 4%. In addition, d50depths ranging from 8.7 to 11.5 cm and from 11.5 to 15 cm were acquired in the phantom for prosthetic and non-prosthetic fields, respectively, with relative differences between 2% and 5%. Conclusion: On the basis of accuracy requirements in radiotherapy noting that a small underdose to tumors could yield a decrease in the probability of tumor control, the diode-induced beam perturbations in combination with prosthesis involvement in the photon fields may affect treatment outcome, as there would be a reduction in the prescribed target dose during treatment delivery.
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Characterization of the mixed radiation field produced by carbon and oxygen ion beams of therapeutic energy: A Monte Carlo simulation study p. 263
CK Ying, David Bolst, Anatoly Rosenfeld, Susanna Guatelli
Purpose: The main advantages of charged particle radiotherapy compared to conventional X-ray external beam radiotherapy are a better tumor conformality coupled with the capability of treating deep-seated radio-resistant tumors. This work investigates the possibility to use oxygen beams for hadron therapy, as an alternative to carbon ions. Materials and Methods: Oxygen ions have the advantage of a higher relative biological effectiveness (RBE) and better conformality to the tumor target. This work describes the mixed radiation field produced by an oxygen beam in water and compares it to the one produced by a therapeutic carbon ion beam. The study has been performed using Geant4 simulations. The dose is calculated for incident carbon ions with energies of 162 MeV/u and 290 MeV/u, and oxygen ions with energies of 192 MeV/u and 245 MeV/u, and hence that the range of the primary oxygen ions projectiles in water was located at the same depth as the carbon ions. Results: The results show that the benefits of oxygen ions are more pronounced when using lower energies because of a slightly higher peak-to-entrance ratio, which allows either providing higher dose in tumor target or reducing it in the surrounding healthy tissues. It is observed that, per incident particle, oxygen ions deliver higher doses than carbon ions.Conclusions: This result coupled with the higher RBE shows that it may be possible to use a lower fluence of oxygen ions to achieve the same therapeutic dose in the patient as that obtained with carbon ion therapy.
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Establishment of a new three-dimensional dose evaluation method considering variable relative biological effectiveness and dose fractionation in proton therapy combined with high-dose-rate brachytherapy p. 270
Daisuke Kobayashi, Tomonori Isobe, Kenta Takada, Yutaro Mori, Hideyuki Takei, Hiroaki Kumada, Satoshi Kamizawa, Tetsuya Tomita, Eisuke Sato, Hiroshi Yokota, Takeji Sakae
Purpose: The purpose of this study is to evaluate the influence of variable relative biological effectiveness (RBE) of proton beam and dose fractionation has on dose distribution and to establish a new three-dimensional dose evaluation method for proton therapy combined with high-dose-rate (HDR) brachytherapy. Materials and Methods: To evaluate the influence of variable RBE and dose fractionation on dose distribution in proton beam therapy, the depth-dose distribution of proton therapy was compared with clinical dose, RBE-weighted dose, and equivalent dose in 2 Gy fractions using a linear-quadratic-linear model (EQD2LQL). The clinical dose was calculated by multiplying the physical dose by RBE of 1.1. The RBE-weighted dose is a biological dose that takes into account RBE variation calculated by microdosimetric kinetic model implemented in Monte Carlo code. The EQD2LQLis a biological dose that makes the RBE-weighted dose equivalent to 2 Gy using a linear-quadratic-linear (LQL) model. Finally, we evaluated the three-dimensional dose by taking into account RBE variation and LQL model for proton therapy combined with HDR brachytherapy. Results: The RBE-weighted dose increased at the distal of the spread-out Bragg peak (SOBP). With the difference in the dose fractionation taken into account, the EQD2LQLat the distal of the SOBP increased more than the RBE-weighted dose. In proton therapy combined with HDR brachytherapy, a divergence of 103% or more was observed between the conventional dose estimation method and the dose estimation method we propose. Conclusions: Our dose evaluation method can evaluate the EQD2LQLconsidering RBE changes in the dose distribution.
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The impact of positron emission tomography/computed tomography addition to contrast-enhanced computed tomography findings during radiation treatment planning of locally advanced carcinoma esophagus p. 276
Sharad Bhatnagar, Shweta Sharma, Manoj Semwal, Sankalp Singh
Introduction: Does metabolic imaging help in better definition of target during radiation treatment planning by bringing about changes in dimensions of the primary tumor in terms of diameter, length, and picking up new skip lesions or nodal stations which in turn prevents geographic misses by including more 18F-fluorodeoxyglucose avid regions not visible on conventional imaging? Materials and Methods: We compared the length and radial dimensions of the primary tumor as well as changes brought about due to addition of new nodal stations, involved structures, and skip lesions in 50 patients of carcinoma esophagus treated between 2011 and 2013, as seen on contrast-enhanced computed tomography (CT) thorax and positron emission tomography (PET)/CT and drew conclusions regarding the technical changes brought about in treatment planning by the additional input of PET/CT. Results and Conclusions: PET-CT tremendously changes treatment plans by expanding the gross tumor volume and including regions which might otherwise have been missed on purely CT-based plans. Of the 50 patients, it changed the contouring and treatment planning of 35 patients and did not impact the remaining 15. Whether this translates into better long-term controls requires further validation by randomized controlled trials, which was not our present objective.
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4π radiotherapy using a linear accelerator: A misnomer in violation of the solid geometric boundary conditions in three-dimensional Euclidean space p. 283
Biplab Sarkar, Tharmarnadar Ganesh, Anusheel Munshi, Arjunan Manikandan, Bidhu Kalyan Mohanti
Purpose: The concept of 4πc radiotherapy is a radiotherapy planning technique receiving much attention in recent times. The aim of this article is to disprove the feasibility of the 4π radiotherapy using a cantilever-type linear accelerator or any other external-beam delivery machines. Materials and Methods: A surface integral-based mathematical derivation for the maximum achievable solid angle for a linear accelerator was carried out respecting the rotational boundary conditions for gantry and couch in three-dimensional Euclidean space. The allowed movements include a gantry rotation of 0–2πc and a table rotation of πc/2 - 0 - 3πc/2) . Results: Total achievable solid angle by cantilever-type linear accelerator (or any teletherapy machine employing a cantilever design) is 1/r2 π ∫ - θ=0 π ∫ φ=0 (rSinθdφ). (rdθ) = 2πc, which is applicable only for the foot and brain radiotherapy where the allowed table rotation is 90°–0°–270°. For other sites such as pelvis, thorax, or abdomen, achievable solid angle as the couch rotation comes down significantly. Practically, only suitable couch angle is 0° by avoiding gantry–couch–patient collision. Conclusions: Present cantilever design of linear accelerator prevents achieving a 4π radian solid angle at any point in the patient. Even the most modern therapy machines like CyberKnife which has a robotic arm also cannot achieve 4π geometry. Maximum achievable solid angle under the highest allowable boundary condition(s) cannot exceed 2πc, which is restricted for only extremities such as foot and brain radiotherapy. For other parts of the body such as pelvis, thorax, and abdomen, the solid angle is reduced to 1/5th (maximum value) of the 4πc. To obtain a 4πc solid angle in a three-dimensional Euclidean space, the patient has to be a zero-dimensional point and X-ray head of the linear accelerator has a freedom to rotate in every point of a hypothetical sphere of radius 1 m. This article establishes geometrically why it is not possible to achieve a 4πc solid angle.
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A dosimetric analysis of the rectal doses in intracavitary brachytherapy of carcinoma cervix: A prospective study from a single institute p. 287
Aparna Suryadevara, Anil Kumar Talluri, Krishnam Raju Alluri, E Vasundhara, Sudhakar Kumar, N V.N Madhusudhana Sresty
Introduction: Carcinoma cervix is a common gynecologic malignancy in India and is treated with radical chemoradiation where intracavitary brachytherapy (ICR) is an integral part. In ICR of cervix, the two-dimensional (2D) point-based dosimetry cervix is the most common method used in high-volume centers with rectal dose calculation at modified ICRU rectal point with rectal wire placement. The rectal dose measurement using this method underestimates the dose to the rectum, and rectal dose also varies with the type of applicator used. The aim of our study is to compare the rectal dose calculated by ICRU 38 method versus rectal dose calculated by the rectal wire method using Henschke applicator. Materials and Methods: This is a single-institute, dosimetric comparison study done prospectively. Fifty patients were planned for ICR after 2D orthogonal radiograph-based, computer planning by iridium 192 high-dose rate remote afterloading technique after placing the appropriate Henschke applicator. The vaginal packing was done using sterile gauze with contrast material for defining the ICRU 38 rectal point, and a rectal wire was placed for the modified ICRU rectal point. Rectal doses were calculated by both the methods and compared. Results: The modified ICRU rectal point recorded a lower rectal dose (mean of 25%) compared to ICRU 38 rectal point in the study patients. There were ten patients (20%) with either too much or too little contrast material which made the visualization of the rectal point and radiation planning difficult. P value by paired t-test method was 0.0001, which was statistically significant. Conclusion: The modified ICRU rectal point is easier to visualize than ICRU 38 method (100% vs. 80%) for dosimetry, but it underestimates the rectal doses when compared to ICRU 38 rectal point. There needs to be a correction factor applied (25% in our study for Henschke applicator) when evaluating the rectal doses calculated by rectal wire method, to reduce the rectal toxicity.
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Assessment of adult diagnostic reference levels for panoramic radiography in Tamil Nadu region p. 292
Amal Jose, A Saravana Kumar, KN Govindarajan, B Devanand, N Elango
Aim: The aim of this study was to calculate dose area product (DAP) and to determine diagnostic reference level (DRL) for adult panoramic procedures in Tamil Nadu. Materials and Methods: In this study, air kerma on the front side of the secondary collimator was measured with a Black Piranha, RTI Electronics, Sweden and multiplied with the corresponding exposed area to calculate DAP. The obtained DAP values were further analyzed, and DRL was calculated using the Microsoft Excel software. The study was carried out with regular adult exposure parameters. Results: The mean, range, and 3rd quartile values for 67 panoramic scanners in Tamil Nadu, India, were calculated as 94 mGycm2, 41 mGycm2–165 mGycm2, and 114.3 mGycm2, respectively. The results are comparable with other international studies. Conclusion: The present study suggests that further optimization can be achieved in many centers by the recruitment of professionally qualified radiographers and conducting periodic training programs on the optimization of exposure parameters. Considering this as the first study for the dental DRL assessment, further studies are suggested to establish national dental DRL in India.
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A primer on theory and operation of linear accelerators in radiation therapy p. 298
Challapalli Srinivas
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