Uniform Dose Distribution Research Articles

Comparison of radiotherapy techniques with flattening filter and flattening filter-free in lung radiotherapy according to the treatment volume size

In external radiotherapy (RT), the use of flattening filter-free (FFF) radiation beams obtained by removing the flattening filter (FF) in standard linear accelerators is rapidly increasing, and the benefits of clinical use are the issue of research. Advanced treatment techniques have increased the interest in the operation of linear accelerators in FFF mode. The differences of the beams with non-uniform dose distribution created by removing FF compared to the beams with uniform dose distribution used as a standard were examined. These differences were compared in the treatment plans of lung patients who have different planning target volumes (PTV). Clinac IX linear accelerator units were used. Twenty patients with previously completed treatment were divided into two groups depending on the size of the target volume. All patients underwent two different intensity-modulated RT (IMRT) plans using FF and FFF beams. The Wilcoxon Signed-Rank test was used to compare two different techniques (Significance p < 0.05). There was no statistically significant difference between the two techniques when looking at the D2%(Gy), D98%(Gy), D50%(Gy), homogeneity (HI), and conformity index (CI) data for both groups. When the critical organ doses were evaluated, there was a statistically significant difference only in the V20(%) values of the lungs, but these differences were not very large. Monitor unit (MU) data were found to be lower in FF planning, and treatment time was lower in FFF planning. Except for shorter treatment times, and of the lungs V20(%) value, in standard fractionated RT of lung cancer, there was no significant difference between the use of FFF and FF techniques for large and small target volumes.

Development of the scanning power supply for 300 keV electron accelerator

This paper describes the development and study of the scanning power supply. This locally designed low energy electron accelerator with the former energy of 140 keV will be upgraded to 300 keV. As the setup, scanning power supply is required to excite the scanning coils and deflect the energetic electron beam across a titanium foil window in horizontal and vertical direction respectively. The excitation current of the scanning coils is determined by the energy of the electron beam and the deflected length. Therefore, the scanning power supply must be functioning automatically and continuously to ensure the uniform dose distribution and unavoidable heat issue. As the result, the scanning power supply using the combination of signal generator and power amplifier has been developed and functioning as expected.

Abstract P4-12-10: Clinical implementation of the machine learning-based automated treatment planning tool for whole breast radiotherapy

Abstract Objectives: The machine learning (ML)-based automated treatment planning tool has been developed and evaluated for whole breast radiotherapy. This study implemented the tool in clinic and compared plan quality and planning efficiency with the manual treatment process for whole breast radiotherapy using irregular surface compensator technique. Methods: This study involved two phases. The 1st phase was to evaluate and the 2nd phase was to implement in the clinic. In the 1st phase, thirty whole breast or chest wall cases were planned by using the irregular surface compensator technique with fluence maps manually and iteratively edited to achieve uniform dose distribution to the target by planners. Patients were treated with these manual plans after physician’s approval. The evaluation phase thus compared our in-house ML-based automated treatment planning tool implemented in Eclipse Scripting Application Programming Interface (ESAPI) to these manual plans. The ML-based planning tool generated the fluence maps with the same beam parameters such as beam energy, gantry angle, collimator angle, and aperture shape as manual plans. Breast or chest wall clinical target volume (CTV) coverage based on the percentage CTV volume receiving 95% of the prescribed dose (V95%) and high-dose volume based on V105% were compared to evaluate the plan quality as well as the planning efficiency. Two-tailed Wilcoxon Signed-Rank test was performed to test the null hypothesis that the two planning schemes yield equivalent plan quality. In the 2nd phase, the planners used the automated planning tool for fourteen plans (twelve patients) followed by manual fluence modification as needed. Physicians reviewed and approved the plans, and patients were treated. Results: For the 1st phase, the mean planning time was 110.2 min with standard deviation (SD) of 62.8 min for the manual planning with the range from 25 to 270 min, and 6.4 min with SD of 2.1 min for the automated planning with the range from 4 to 12 min (p&amp;lt;0.01). CTV mean V95% was 96.7% (SD: 5.0%) for the manual planning and 96.7% (SD: 4.8%) for the automated planning (p=0.89). CTV mean V105% was 21.6% (SD: 29.8%) for the manual planning and 20.4% (SD: 30.5%) for the automated planning (p=0.22). Differences in doses to heart and lungs were negligible between the paired plans as the two planning schemes used the same beam parameters. For the 2nd phase, the mean planning time was 16.4 min (SD: 9.1 min) and the mean time for additional manual editing was 12.7 min (SD: 12.5 min). The mean total treatment planning time was 29.1 min (SD: 14.8 min).). CTV mean V95% was 97.2% (SD: 4.2%) and mean V105% was 8.2 % (SD: 3.6%). The manual post modifications were added by the planners with intention to improve the target coverage or to reduce high doses, yet, the difference between the plans without and without the manual post modification was negligible. Conclusion: The ML-based automated treatment planning tool through Varian ESAPI has been successfully implemented for clinical use going through two phases of study. Abiding to the same plan quality as manual process, the automated tool significantly reduced the planning time as the ML-based tool automate the iterative fluence editing process. Citation Format: Sua Yoo, Yang Sheng, Rachel Blitzblau, Suzanne Catalano, Jay Morrison, Leigh O'Neill, Fangfang Yin, Q. Jackie Wu. Clinical implementation of the machine learning-based automated treatment planning tool for whole breast radiotherapy [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P4-12-10.

The perturbation effect of fiducial marker on 3D dose distribution at external surrogate’s radiotherapy

Introduction: Tumor motion is a challenging issue in radiotherapy, which complicates the process of tumor delineation, localization, and dose delivery. External surrogate radiotherapy is one of the available strategies that provides motion dataset for a consistent prediction model to track tumor motion using internal-external markers. Regarding this, the present study was conducted to investigate the effect of implanted fiducial on 3D uniform dose distribution. Material and Methods: For the purpose of the study, a Monte Carlo code was utilized to simulate clip with different dimensions and material structures against four therapeutic beams. Moreover, a combinational clip made of golden and covered by polymethyl methacrylate (PMMA) was proposed to be used with lower dose perturbation. Finally, it was proposed to implant a clip outside tumor volume at specific distances from tumor site to keep dose uniformity on tumor volume. To investigate this issue, the correlation coefficient parameter was calculated as the metric among the motion dataset of tumor and clip. Results: Based on the results, dose perturbation caused by implanted clip was remarkable at hadron therapy depending on its size and material, mainly at the downstream part of the clip. Conclusion: As the findings indicated, the golden marker covered with PMMA could remarkably reduce dose perturbation. The most important concern in this domain is the presence of a possible correlation between tumor motion and motion of the clip implanted outside the tumor volume. The results of the correlation coefficient revealed a close relationship between tumor motion and clip motion.

Influence of alumina feeding dose distribution on superheat based on computational simulation

In Hall-Héroult cell, alumina dissolution has minimum requirement for superheat, however the superheat may not be uniform because the heat dissipation at the end parts is obviously higher than the middle part, which leads to the lower temperature at the end parts. In this study, the influences of alumina feeding dose distribution on the alumina concentration and the bath temperature near the feeder were simulated in computational numerical simulation, and then several cases with different feeding dose distribution were assessed by comparing the superheat per unit feeding dose. Furthermore, a methodology was proposed to optimise the alumina feeding dose distribution. It shows that the cases with lower alumina feeding dose at the end parts have better superheat distribution than the cases with uniform alumina feeding dose distribution. The optimal one is the case with feeding dose distribution (1.6, 1.8, 2.0, 2.0, 1.8, 1.6 kg) of alumina.

Influence of alumina feeding dose distribution on superheat based on computational simulation

In Hall-Heroult cell, alumina dissolution has minimum requirement for superheat, however the superheat may not be uniform because the heat dissipation at the end parts is obviously higher than the middle part, which leads to the lower temperature at the end parts. In this study, the influences of alumina feeding dose distribution on the alumina concentration and the bath temperature near the feeder were simulated in computational numerical simulation, and then several cases with different feeding dose distribution were assessed by comparing the superheat per unit feeding dose. Furthermore, a methodology was proposed to optimise the alumina feeding dose distribution. It shows that the cases with lower alumina feeding dose at the end parts have better superheat distribution than the cases with uniform alumina feeding dose distribution. The optimal one is the case with feeding dose distribution (1.6, 1.8, 2.0, 2.0, 1.8, 1.6 kg) of alumina.

Determining whether multiple needles are necessary in interstitial brachytherapy for thoracic tumors: A dosimetry analysis.

In interstitial brachytherapy, needles must be inserted in a regular, parallel arrangement to ensure a uniform target dose distribution and conformal distribution to the target. It is generally difficult to achieve this in thoracic tumors because of obstruction by the ribs. Furthermore, insertion of multiple needles may cause the patient considerable harm and could expose him/her to additional risks. Thus, we propose the single-dwell-position method, discuss its applicability, and compare it with the actual multiple-needle method using dosimetry. The aim of this study was to evaluate the necessity for multiple needles with irregular alignment in interstitial brachytherapy for thoracic tumors. Twelve patients' interstitial brachytherapy plans were reviewed. The single-dwell-position interstitial brachytherapy plans, wherein one needle was hypothetically inserted, were compared with the actual multiple-needle plans. Dose parameters, including clinical target volume (CTV) and volumes of the lung, spinal cord, heart, and ribs, were compared. We also evaluated the correlation between CTV size and dose difference in the lungs. The nonparametric Wilcoxon test was used. There were no statistically significant differences in the doses achieved with the single-dwell-position plans and actual multiple-needle plans. The correlation between the CTV size and dose difference in the lungs was weak. Irregularly arranged multiple-needle interstitial brachytherapy does not provide superior doses to the lung, heart, spinal cord, or ribs compared with single-dwell-position plans. If regular arrangement of multiple needles is difficult to achieve, the multiple-needle scheme is not the only viable option.

Dynamic MLC quality assurance program using EPID

The introduction of intensity modulated radiotherapy (IMRT) allowed delivering high uniform dose distributions in the target, while sparing the surrounding healthy tissue. This technique is performed thanks to multileaf collimators (MLCs) in continuous motion. Therefore, it is crucial for patient safety to carry out a routinely quality assurance (QA) program of the MLC to ensure that leaf motion is precise, smooth and reproducible on every fraction. In this work a set of tests was gathered in order to quantitatively examine the stability of the electronic portal imaging device (EPID) signal, the accuracy of leaf positioning, the steadiness of leaf velocity and gap sizes and the influence of gravity for different gantry angles. These tests were accomplished using EPID (amorphous silicon aS1000, Varian) calibrated for dosimetric purposes for a 120-leaf Millennium MLC. The detector consists of matrix of 1024 ×784 pixels for 40 ×30 cm2 area at the isocenter, providing a resolution of 0.39 mm/pixel. Hence, the EPID not only presents an adequate resolution to detect potential faults in the MLC performance but it is also agile and accessible. Additionally, it allows exporting .dxf files for each of the tests to be quantitatively assessed with help of a series of routines developed in Matlab. Corroboration of the implemented routines was performed using a detector array. Both methods showed an exceptional agreement. In conclusion, a MLCQA program using the EPID was presented to provide a reduction in time of quantitatively analysis of the tests and an early detection of errors.

Electron beam absorption in 3D-printed polymer samples with different infill densities

In this work, we study the efficiency of electron absorption by the plastic samples produced using 3D printing with different infill densities. We investigate the influence of the print layer orientation relative to the electron beam axis on the radiation dose distribution. It is possible to produce plastic samples with different infill by fused deposition modelling. Ten polymer test samples with the infill density ranging from 10% to 100% are printed and studied experimentally using a 6 MeV electron beam of an MIB-6E betatron. GafChromic EBT3 films are used for the dose measurement. When the infill is above 70%, the difference of dose distribution uniformity cannot be distinguished for the two print layer orientations. Therefore, these samples can be used for electron beam formation.

Analysis of different evaluation indexes for prostate stereotactic body radiation therapy plans: conformity index, homogeneity index and gradient index

AbstractObjectiveThis study analyzed different definitions of the conformity index (CI), homogeneity index (HI), and gradient index (GI) used in evaluation of prostate cancer stereotactic body radiation therapy.MethodsA total of 10 patients with localized prostate cancer staged T1–T2a were selected randomly, from which two stereotactic body radiation therapy plans were designed with CyberKnife and EDGE systems for each patient based on the same images and contours. CI, HI, and GI with different definitions were calculated based on a dose‐volume histogram of treatment plans.ResultsFor four definitions of CI, the results showed that the values calculated by the Radiation Therapy Oncology Group, Lomax and Scheib, and Van't Riet were closer to 1 for EDGE plans, except for that by the Saint‐Anne, Lariboisiere, and Tenon group. Meanwhile, CI values of Van't Riet et al were lower than other definitions, which showed high accuracy to describe the conformity between the target volume and prescribed isodose line. For five definitions of HI, CyberKnife plans showed lower values than EDGE plans. HI calculated based on the D2 (D5) and D98 (D95) were lower than that calculated by the Dmax to Dmin, whereas the standard deviation was higher than others. For the dose GI, the results of effective radius and modified GI showed that EDGE plans had a steeper dose fall‐off, which is inverse to the conventional GI.ConclusionsThese indexes can all be objective tools for the evaluation of plan quality. Our results showed better conformity of dose distribution and dose gradient for EDGE plans, but better uniformity for CyberKnife plans. The conformation number and S‐index were recommended to accurately describe the conformity and uniformity of dose distribution. The ∆Riso and modified GI were also recommended to calculate the dose gradient.

Approaches of Sparing Lung in SBRT for Treating Tumors in Lungs with Volumetric Arc Therapy

Sparing lung in Stereotactic Body Radiotherapy (SBRT) for treating tumors in lungs is critical to reduce the radiation induced toxicities, such as radiation pneumonitis and late fibrosis. A retrospective study was performed with a new combination of approaches to improve lung sparing in SBRT plans with VMAT and Monte-Carlo dose calculation algorithm. Twenty two SBRT lung plans were optimized using traditional LINAC MLC based approaches: an uniform dose distribution in the target with an average P-IDL (prescription isodose line) of (88.8±1.9)% (The error bar of all the data is the 95% confidence interval (CI)) and clinically delivered. The plans were re-optimized using a new combination of approaches with variation of P-IDL and evaluated with: 1. R50%, the ratio of 50% prescription isodose volume to the plan target volume (PTV); 2. V20, the volume of lung within 20Gy; 3. PCI, the Paddick comformity index; 4. D2cm, the maximum dose at 2 cm from PTV in any direction; 5. Dmean, the mean dose in total lung volume. It was found that the optimal P-IDL was in the range of 75%∼80%. With the new optimization strategies and the optimal P-IDL, the average PCI was increased by (10.3±2.1)%; the average R50%, V20, D2cm and Dmean were decreased by (30.2±4.1)%, (27±5.4)%, (13.4±4.3)% and (16.6±2.3)%, respectively; the dose distributions in the targets also were changed from concave to convex. The new set of optimization approaches can significantly improve the lung sparing in SBRT VMAT plans with Monte-Carlo dose calculation algorithm for treating tumors in lungs while increasing the dose to the target. The new plans were more conformal in both high and intermediate dose regions. The optimal P-IDL was found in the range of 75-80%.

Implementation of Machine Learning-Based Treatment Planning Tool for Whole Breast Radiotherapy Using Irregular Surface Compensator Technique

To implement the machine learning (ML)-based treatment planning tool and compare plan quality and planning efficiency with the manual treatment process for whole breast radiotherapy using irregular surface compensator technique. Thirty whole breast or chest wall cases were planned by using the irregular surface compensator technique with fluence maps manually and iteratively edited to achieve uniform dose distribution to the target. Patients were treated with these manual plans after physician’s approval. For each case, an in-house ML-based automated treatment planning tool was implemented in a programming interface and generated the fluence maps with the same beam parameters such as beam energy, gantry angle, collimator angle, and aperture shape as manual plans.An analytical algorithm software with 0.25cm grid size was used in dose calculation for all plans. Breast or chest wall clinical target volume (CTV) coverage based on the percentage CTV volume receiving 95% of the prescribed dose (V95%) and high-dose volume based on V105% were compared to evaluate the plan quality as well as the planning efficiency. Two-tailed Wilcoxon Signed-Rank test was performed to test the null hypothesis that the two planning schemes yield equivalent plan quality. The mean planning time was 110.2 min with standard deviation (SD) of 62.8 min for the manual planning with the range from 25 to 270 min, and 6.4 min with SD of 2.1 min for the ML-based planning with the range from 4 to 12 min (p<0.01). CTV mean V95% was 96.7% (SD: 5.0%) for the manual planning and 96.7% (SD: 4.8%) for the ML-based planning (p=0.89). CTV mean V105% was 21.6% (SD: 29.8%) for the manual planning and 20.4% (SD: 30.5%) for the ML-based planning (p=0.22). Differences in doses to heart and lungs were negligible between the paired plans as the two planning schemes used the same beam parameters. This study successfully implemented the ML-based automated treatment planning tool through scripting software. Abiding to the same plan quality as manual process, the automated tool significantly reduced the planning time as the ML-based tool automate the iterative fluence editing process. This technique demonstrated promising potential to significantly improve efficiency of the treatment planning for whole breast or chest wall radiotherapy.

Sparse proportional re-scanning with hadron beams

Spot Scanning is a well-established technique to deliver the dose with hadron therapy systems. For many years re-scanning (called also re-painting) has been used to achieve uniform dose distribution in particular for moving organs, although it leads to an increase of the treatment time. Reducing this time is a major focus of present research. In this paper, after reviewing the current re-scanning techniques, sparse proportional re-scanning is defined and applied to 29 proton patient cases for a total of 54 fields. In this technique, only the highest weighted spot in the whole target is visited a number of times that is equal to the number N of re-scans. The number of visits of the beam spot to all remaining spots is scaled down proportionally to their weight. Sparse proportional re-scanning is advantageous especially in volumetric re-scanning. In order to quantify the potential advantages of this technique in terms of treatment time, a reduction factor of the number of scanned spots has been introduced, evaluated and analysed for 54 proton fields. The conclusion is that the reduction factor is a function of N (having values equal to 2.8 ± 0.3 and 3.6 ± 0.4 for N = 5 and N = 12 respectively) and does not depend either on the shape and volume of the target or on the distance between the scanned layers and the spot grid. The same values are approximately valid also for carbon ion treatments.

A new fast algorithm to achieve the dose uniformity around high dose rate brachytherapy stepping source using Tikhonov regularization.

The dose optimization algorithm based on anatomical points is developed to produce rapidly uniform doses over target distances generated on the target volume edges in high-dose-rate (HDR) brachytherapy stepping source application for a treatment length of 6cm. Monte Carlo modeling of the 60Co HDR brachytherapy source and the surrounding medium were performed using PHITS code. The source dwell times were optimized using Tikhonov regularization in order to obtain uniform dose distribution at the anatomical points located at predefined target distances. The computed dose rates at distances from 0.25 up to 20cm away from the source were first verified with the literature data sets. Then, the simulation results of the optimization process were compared to the calculations of commercial treatment planning system (TPS) SagiPlan. As a result, the dose uniformity was observed in the isodose curves at the target distances of 10 and 15mm of the treatment length and the prescribed dose achieved the anatomical points uniformly. The algorithm developed in the present study can be applied for achieving the dose uniformity around the brachytherapy stepping source as a quicker tool for different treatment lengths and different target distances while maintaining the high quality of the treatment plans, saving time by avoiding the manual isodose shaping and then better suitable treatment for patients.

The impact of pitch and modulation factor to Tomotherapy treatment plan optimization for motion target

The influence on target motion resulted from the respiratory process, the pitch and modulation factor presented in Tomotherapy. This study was aimed to investigate the effect of motion management using CT-images obtained from the fusion process between the static and dynamic scanning mode on the CIRS Thorax Phantom. The images were created using axial and helical modes which amplitudes were varied from 10 mm, 15 mm, and 20 mm. Then, the organ structures were planned using TomoPlan TPS with the variation of pitch ranging from 0.25 to 0.5, and the modulation factor (MF) in the range 2 to 3. Furthermore, evaluation of radiotherapy planning was performed using dose parameters on target, the organ at risk, homogeneity index (HI), and mean leaf open time (LOT). Target volume for static mode was 3.53 cc, whereas target volume has changed to 10.9 cc, 10.3 cc, 16.1 cc for axial and 6.6 cc, 8.5 cc, 14.9 cc for helical scanning. The HI value obtained was less than 0.07 and lower HI was resulted for a larger MF. It means that the higher modulation factor values will provide a uniform dose distribution to the target and lower dose to the OAR. The greater value of pitch gave the greater mean LOT. The optimum parameters were at the pitch of 0.5 and MF of 3 based on the evaluation of mean LOT and HI. Axial scanning mode provides a larger target volume compared to a helical scanning mode. Therefore, the increment of the target volume in delineating organ should be considered.

Modification of the 4 MeV electron beam from a linear accelerator for irradiation of small superficial skin tumors.

Uniform dose distribution with steep lateral gradient within depth range of 0-0.5 cm is crucial to be able to treat small skin lesions. The standard nominal 4 MeV electron beam from Elekta Versa HD linear accelerator was modified with degrading filter to remove the lateral scatter from treatment head and minimize the penumbra. The energy degrading method was verified based on dosimetric properties and output factors (OFs) with comparison of four types of measurement methods. The properties of degraded 4 MeV electron beam and developed electron applicators seem optimal for treating small targets near the skin surface.

Deep X-ray lithography system with a uniform and high-accuracy fabrication area established in beamline BL11 at NewSUBARU.

A new lithography system to fabricate high-aspect-ratio 3D microstructures was developed at the NewSUBARU synchrotron radiation facility (University of Hyogo, Japan). The X-ray beam generated by this system has high parallelism (horizontal and vertical divergence angles of 278 µrad and 14 µrad, respectively) and high photon flux (31 mW mm-2 at a beam current of 300 mA). The high photon flux and exposure area of the system were validated and a beam-scan method for a large exposure area with a uniform dose distribution has been proposed. In addition, the deep X-ray lithography performance was characterized using a conventional photosensitive material and the synchrotron-radiation-induced direct etching of polytetrafluoroethylene (PTFE) was demonstrated. An enlargement of the microfabrication area up to 100 mm × 100 mm while contemporarily ensuring high uniformity was achieved.

Impact of varying planning parameters on proton pencil beam scanning dose distributions in four commercial treatment planning systems.

In pencil beam scanning proton therapy, target coverage is achieved by scanning the pencil beam laterally in the x- and y-directions and delivering spots of dose to positions at a given radiological depth (layer). Dose is delivered to the spots on different layers by pencil beams of different energy until the entire volume has been irradiated. The aim of this study is to investigate the implementation of proton planning parameters (spot spacing, layer spacing and margins) in four commercial proton treatment planning systems (TPSs): Eclipse, Pinnacle3 , RayStation and XiO. Using identical beam data in each TPS, plans were created on uniform material synthetic phantoms with cubic targets. The following parameters were systematically varied in each TPS to observe their different implementations: spot spacing, layer spacing and margin. Additionally, plans were created in Eclipse to investigate the impact of these parameters on plan delivery and optimal values are suggested. It was found that all systems except Eclipse use a variable layer spacing per beam, based on the Bragg peak width of each energy layer. It is recommended that if this cannot be used, then a constant value of 5mm will ensure good dose homogeneity. Only RayStation varies the spot spacing according to the variable spot size with depth. If a constant spot spacing is to be used, a value of 5mm is recommended as a good compromise between dose homogeneity, plan robustness and planning time. It was found that both Pinnacle3 and RayStation position spots outside of the defined volume (target plus margin). All four systems are capable of delivering uniform dose distributions to simple targets, but their implementation of the various planning parameters is different. In this paper comparisons are made between the four systems and recommendations are made as to the values that will provide the best compromise in dose homogeneity and planning time.

Three-dimensional-printed vaginal applicators for electronic brachytherapy of endometrial cancers.

Vaginal applicators for a novel miniature x-ray tube were developed using three-dimensional (3D) printing to be used in brachytherapy of endometrial cancers. Cylindrical vaginal applicators with various diameters, lengths, and infill percentages (IFPs) were fabricated using a 3D printer. X-ray dose distributions and depth-dose profiles were calculated using a Monte Carlo simulation. The performances of the applicators were evaluated by measuring and analyzing the dosimetric characteristics of x rays generated from the miniature x-ray tube equipped with the applicators. Quite uniform dose distributions around the applicators were achieved by optimizing the dwell positions and the dwell times of the miniature x-ray tube inside the applicators. In addition, identical absolute dose and depth-dose profiles were obtained through the control of the IFP values even though different-sized applicators are used. The presented 3D printing technique provides an efficient approach to provide vaginal applicators with optimal IFPs that allow consistent treatment time for patients of varying vaginal canal size.

Predictive gamma passing rate by dose uncertainty potential accumulation model.

Intensity-modulated radiation therapy (IMRT) utilizes many small fields for producing a uniform dose distribution. Therefore, there are many field junctions in the target region, and resulting dose uncertainties are accumulated. However, such accumulation of the dose uncertainty has not been implemented in the current practice of IMRT dose verification. The purpose of this study is to develop a method to predict the gamma passing rate (GPR) using a dose uncertainty accumulation model. Thirty-three intensity-modulated (IM) beams for head-and-neck cases with step-and-shoot techniques were used in this study. The treatment plan was created using the XiO treatment planning system (TPS). The IM beam was produced by the ONCOR Impression Plus linear accelerator. MapCHECK was used to measure the dose distribution. The distribution of a dose uncertainty potential (DUP) was generated by in-house software that accumulated field shapes weighted by a segmental monitor unit, followed by Gaussian folding. The width of the Gaussian was determined from the width of the lateral penumbra. The dose difference between the calculated and measured doses was compared with the estimated DUP at each point. The GPR of each beam was predicted for 2%/2-mm, 3%/2-mm, and 3%/3-mm tolerances by its own DUP histogram and a GPR-vs-DUP correlation of other beams using the leave-one-out cross-validation method. The predicted GPR was compared with the measured GPR to evaluate the performance of this prediction method. The criteria for the predicted GPR corresponding to a measured GPR≥90% were estimated to examine the feasibility of estimating the measured GPR by this GPR prediction method. The DUP was confirmed to have proportionality to the standard deviation (SD) of the dose difference. The SDs of the difference between the measured and predicted GPRs were 3.1, 1.7, and 1.4% for 2%/2-mm, 3%/2-mm, and 3%/3-mm tolerances, respectively. The criteria of the predicted GPR corresponding to the measured GPR≥90% were 94.1 and 95.0% with confidence levels of 99 and 99.9%, respectively. In this study, we confirmed the good proportionality between the dose difference and the estimated DUP. The results showed a feasibility to predict the dose difference from DUP as estimated by a DUP accumulation model. The predicted GPR developed in this study showed good accuracy for planar dose distributions of head and neck IMRT. The prediction method developed in this study is considered to be feasible as a substitute for the current practice of measurement-based verification of the dose distribution with gamma analysis.