Dose rate correction for the novel 2D diode array MapCHECK 3.

PURPOSE: To investigate the dose rate dependence of MapCHECK3 and its influence on measurement accuracy, as well as the effect of dose rate correction. MATERIALS AND METHODS: The average and instantaneous dose rate dependence of MapCHECK2 and MapCHECK3 were studied. The accuracy of measurements was investigated where the dose rate differed significantly between dose calibration of the MapCHECK and the measurement. Measurements investigated include: the central axis dose for different fields at different depths, off-axis doses outside the field, and off-axis doses along the wedge direction. Measurements using an ion chamber were taken as the reference. Exponential functions were fit to account for average and instantaneous dose rate dependence for MapCHECK3 and used for dose rate correction. The effect of the dose rate correction was studied by comparing the differences between the measurements for MapCHECK (with and without the correction) and the reference. RESULTS: The maximum dose rate dependence of MapCHECK3 is greater than 2.5%. If the dose calibration factor derived from a 10 × 10 cm2 open field at 10 cm depth was used for measurements, the average differences in central diode dose were 0.8% ± 1.0% and 1.0% ± 0.8% for the studied field sizes and measurement depths, respectively. The introduction of wedge would not only induce -1.8% ± 1.3% difference in central diode dose, but also overestimate the effective wedge angle. After the instantaneous dose rate correction, above differences can be changed to 1.9% ± 8.1%, 0.2% ± 0.1%, and 0.0% ± 0.9%. The pass rate can be improved from 98.4% to 98.8%, 98.3%-100.0%, and 96.3%-100.0%, respectively. CONCLUSION: Compared with MapCHECK2 (SunPoint1 diodes), the more pronounced dose rate dependence of MapCHECK3 (SunPoint2 diodes) should be carefully considered. To ensure highly accurate measurement, it is suggested to perform the dose calibration at the same condition where measurement will be performed. Otherwise, the dose rate correction should be applied.

Dosimetric evaluation in Helical TomoTherapy for lung SBRT using Monte Carlo-based independent dose verification software.

PURPOSE: To elucidate the dosimetric errors caused by a model-based algorithm in lung stereotactic body radiation therapy (SBRT) with Helical TomoTherapy (HT) using Monte Carlo (MC)-based dose verification software. METHODS: For 38 plans of lung SBRT, the dose calculation accuracy of a treatment planning system (TPS) of HT was compared with the results of DoseCHECK, the commercial MC-based independent verification software. The following indices were extracted to evaluate the correlation of dosimetric errors: (1) target volume, (2) average computed tomography (CT) value of the planning target volume (PTV) margin, and (3) average CT value of surrounding 2-mm area of the PTV (PTV ring). Receiver operating characteristic (ROC) analyses determined the threshold for 5% of differences in PTV D95%. Then, the 38 plans were classified into two groups using the cutoff values of ROC analysis for these three indices. Dosimetric differences between groups were statistically compared using the Mann-Whitney U test. RESULTS: TPS of HT overestimated by more than 5% in the PTV D95% in 16 of 38 plans. The PTV ring showed the strongest correlation with dosimetric differences. The cutoff value for the target volume, the PTV margin, and the PTV ring was 14.7 cc, -754 HU, and -708 HU, respectively. The area under the curve (AUC) for the target volume, the PTV margin, and the PTV ring were 0.835, 0.878, and 0.932, respectively. Dosimetric errors more than 5% were observed when the PTV volume was less than 15 cc or when the CT value around the target was less than -700 HU. CONCLUSION: The TPS of HT might overestimate the PTV dose by more than 5% if any the three indices in this study were below threshold. Therefore, independent verification with an MC-based algorithm should be strongly recommended for lung SBRT in HT.

[Research on Position Verification of Multi-Leaf Collimator (MLC) and Dose Verification Based on Electronic Portal Imaging Device].

Objective: A quality control (QC) system based on the electronic portal imaging device (EPID) system was used to realize the Multi-Leaf Collimator (MLC) position verification and dose verification functions on Primus and VenusX accelerators. Methods: The MLC positions were calculated by the maximum gradient method of gray values to evaluate the deviation. The dose of images acquired by EPID were reconstructed using the algorithm combining dose calibration and dose calculation. The dose data obtained by EPID and two-dimensional matrix (MapCheck/PTW) were compared with the dose calculated by Pinnacle/TiGRT TPS for γ passing rate analysis. Results: The position error of VenusX MLC was less than 1 mm. The position error of Primus MLC was significantly reduced after being recalibrated under the instructions of EPID. For the dose reconstructed by EPID, the average γ passing rates of Primus were 98.86% and 91.39% under the criteria of 3%/3 mm, 10% threshold and 2%/2 mm, 10% threshold, respectively. The average γ passing rates of VenusX were 98.49% and 91.11%, respectively. Conclusion: The EPID-based accelerator quality control system can improve the efficiency of accelerator quality control and reduce the workload of physicists.

Fan beam CT-guided online adaptive external radiotherapy of uterine cervical cancer: a dosimetric evaluation.

PURPOSE: To discuss the dosimetric advantages and reliability of the accurate delivery of online adaptive radiotherapy(online ART) for uterine cervical cancer(UCC). METHODS AND MATERIALS: Six UCC patients were enrolled in this study. 95% of the planning target volume (PTV) reached 100% of the prescription dose (50.4 Gy/28fractions/6weeks) was required. The patients were scanned with uRT-Linac 506c KV-FBCT then the target volume (TV) and organs at risk (OARs) were delineated by doctors. The dosimeters designed and obtained a routine plan (Plan0). KV-FBCT was used for image guidance before subsequent fractional treatment. The online ART was processed after registration, which acquired a virtual nonadaptive radiotherapy plan (VPlan) and an adaptive plan (APlan). VPlan was the direct calculation of Plan0 on the fractional image, while APlan required adaptive optimization and calculation. In vivo dose monitoring and three-dimensional dose reconstruction were required during the implementation of APlan. RESULTS: The inter-fractional volumes of the bladder and rectum changed greatly among the treatments. These changes influenced the primary gross tumor volume (GTVp) and the position deviation of GTVp and PTV and positively affected the prescription dose coverage of TV. GTVp decreased gradually along with dose accumulation. The Dmax, D98, D95, D50, and D2 of APlan were superior to those of VPlan in target dose distribution. APlan had good conformal index, homogeneity index and target coverage. The rectum V40 and Dmax, bladder V40, the small bowel V40 and Dmax of APlan were better than that of VPlan. The APlan's fractional mean γ passing rate was significantly higher than the international standard and the mean γ passing rate of all cases after the three-dimensional reconstruction was higher than 97.0%. CONCLUSION: Online ART in external radiotherapy of UCC significantly improved the dose distribution and can become an ideal technology to achieve individualized precise radiotherapy.

Quantitative study on dose distribution of Freiburg flap for keloid high-dose-rate brachytherapy based on MatriXX.

PURPOSE: To quantify the dose distribution effect of insufficient scattering conditions in keloid HDR brachytherapy with Freiburg fFlap (FF) applicator. MATERIALS AND METHODS: A phantom composed of FF applicator, MatriXX and solid water slices was designed and scanned for treatment planning. Bolus with different thicknesses were covered to offer different scatter conditions. Planar dose distributions were measured by MatriXX. The maximum value (Max), average value (Avg) and γ passing rate (3 mm/3%) were evaluated by the software MyQA Platform. RESULTS: The maximum and average doses measured by MatriXX were lower than the calculated values. The difference increased as field size decreased. The Max value, found at 0.86 cm level in the two tube case, was -20.0%, and the avg value was -11.9%. All the γ values were less than 95%. This difference gradually decreased with increasing bolus thickness and the γ values were significantly improved. CONCLUSION: MatriXX could be used for dose verification of HDR brachytherapy with an FF applicator. When the FF applicator was applied for keloid, insufficient scattering conditions would cause an insufficient target dose. This difference could be reduced by covering the bolus with different thicknesses on the applicator. The smaller the field, the thicker the bolus required.

Preliminary study on dosimetry characteristics of a novel cylindrical dose verification system.

OBJECTIVE: To develop a novel ionization chamber array dosimetry system, study its dosimetry characteristics, and perform preliminary tests for plan dose verification. METHODS: The dosimetry characteristics of this new array were tested, including short-term and long-term reproducibility, dose linearity, dose rate dependence, field size dependence, and angular dependence. The open field and MLC field plans were designed for dose testing. Randomly select 30 patient treatment plans (10 intensity-modulated radiation therapy [IMRT] plans and 20 volumetric modulated arc therapy [VMAT] plans) that have undergone dose verification using Portal Dosimetry to perform verification measurement and evaluate dose verification test results. RESULTS: The dosimetry characteristics of the arrays all performed well. The gamma passing rates (3%/2 mm) were more than 96% for the combined open field and MLC field plans. The average gamma pass rates were (99.54 ± 0.58)% and (96.70 ± 3.41)% for the 10 IMRT plans and (99.32 ± 0.89)% and (94.91 ± 6.01)% for the 20 VMAT plans at the 3%/2 mm and 2%/2 mm criteria, respectively, which is similar to the Portal Dosimetry's measurement results. CONCLUSIONS: This novel ionization chamber array demonstrates good dosimetry characteristics and is suitable for clinical IMRT and VMAT plan verifications.

Evaluation and improvement of angular response for a commercial 2D detector array for patient-specific QA.

PURPOSE: MatriXX ionization chamber array has been widely used for the composite dose verification of IMRT/VMAT plans. However, in addition to its dose response dependence on gantry angle, there seems to be an offset between the beam axis and measured dose profile by MatriXX for oblique beam incidence at various gantry angles, leading to unnecessary quality assurance (QA) fails. In this study, we investigated the offset at various setup conditions and how to eliminate or decrease it to improve the accuracy of MatriXX for IMRT/VMAT plan verification with original gantry angles. METHODS: We measured profiles for a narrow beam with MatriXX located at various depths in increments of 0.5 mm from the top to bottom of the sensitive volume of the array detectors and gantry angles from 0° to 360°. The optimal depth for QA measurement was determined at the depth where the measured profile had minimum offset. RESULTS: The measured beam profile offset varies with incident gantry angle, increasing from vertical direction to lateral direction, and could be over 3 cm at vendor-recommended depth for near lateral direction beams. The offset also varies with depth, and the minimum offset (almost 0 for most oblique beams) was found to be at a depth of ∼2.5 mm below the vendor suggested depth, which was chosen as the optimal depth for all QA measurements. Using the optimal depth we determined, QA results (3%/2 mm Gamma analysis) were largely improved with an average of 99.4% gamma passing rate (no fails for 95% criteria) for 10 IMRT and VMAT plans with original gantry angles compared to 94.1% using the vendor recommended depth. CONCLUSIONS: The improved accuracy and passing rate for QA measurement performed at the optimal depth with original gantry angles would lead to reduction in unnecessary repeated QA or plan changes due to QA system errors.

A hybrid method to improve efficiency of patient specific SRS and SBRT QA using 3D secondary dose verification.

PURPOSE: Patient Specific QA (PSQA) by direct phantom measurement for all intensity modulated radiation therapy (IMRT) cases is labor intensive and an inefficient use of the Medical Physicist's time. The purpose of this work was to develop a hybrid quality assurance (QA) technique utilizing 3D dose verification as a screening tool to determine if a measurement is necessary. METHODS: This study utilized Sun Nuclear DoseCHECK (DC), a 3D secondary verification software, and Fraction 0, a trajectory log IMRT QA software. Twenty-two Lung stereotactic body radiation therapy (SBRT) and thirty single isocentre multi-lesion SRS (MLSRS) plans were retrospectively analysed in DC. Agreement of DC and the TPS dose for selected dosimetric criteria was recorded. Calculated 95% confidence limits (CL) were used to establish action limits. All cases were delivered and measured using the Sun Nuclear stereotactic radiosurgery (SRS) MapCheck. Trajectory logs of the delivery were used to calculate Fraction 0 results for the same criteria calculated by DC. Correlation of DC and Fraction 0 results were calculated. Phantom measured QA was compared to Fraction 0 QA results for the cases which had DC criteria action limits exceeded. RESULTS: Correlation of DC and Fraction 0 results were excellent, demonstrating the same action limits could be used for both and DC can predict Fraction 0 results. Based on the calculated action limits, zero lung SBRT cases and six MLSRS cases were identified as requiring a measurement. All plans that passed the DC screening had a passing measurement based PSQA and agreed with Fraction 0 results. CONCLUSION: Using 95% CL action limits of dosimetric criteria, a 3D secondary dose verification can be used to determine if a measurement is required for PSQA. This method is efficient for it is part of the normal clinical workflow when verifying any clinical treatment. In addition, it can drastically reduce the number of measurements needed for PSQA.

[Progress in Development of Dose Verification System Software KylinRay-Dose4D].

Advanced radiotherapy technology enables the dose to more accurately conform to the tumor target area of the patient, providing accurate treatment for the patient, but the gradient of the patient's radiation dose at the tumor edge is getting larger, which putting forward higher requirements for radiotherapy dose verification. The dose verification system software KylinRay-Dose4D can verify the patient's pre-treatment plan and the in vivo/on-line dose during the patient's treatment, providing important reference for the physicist to modify the radiotherapy plan and ensuring that the patient receives accurate treatment. This study introduces the overall design and key technologies of KylinRay-Dose4D, and tests the pre-treatment plan dose checking calculation and 2D/3D dose verification through clinical cases. The test results showed that the 2D/3D gamma pass rate (3 mm/3%) of KylinRay-Dose4D reconstructed dose compared with TPS plan dose and measured dose is larger than 95%, which indicating that the reconstructed dose of KylinRay-Dose4D meets the requirement of clinical application.

Statistical process control to monitor use of a web-based autoplanning tool.

PURPOSE: To investigate the use of statistical process control (SPC) for quality assurance of an integrated web-based autoplanning tool, Radiation Planning Assistant (RPA). METHODS: Automatically generated plans were downloaded and imported into two treatment planning systems (TPSs), RayStation and Eclipse, in which they were recalculated using fixed monitor units. The recalculated plans were then uploaded back to the RPA, and the mean dose differences for each contour between the original RPA and the TPSs plans were calculated. SPC was used to characterize the RPA plans in terms of two comparisons: RayStation TPS versus RPA and Eclipse TPS versus RPA for three anatomical sites, and variations in the machine parameters dosimetric leaf gap (DLG) and multileaf collimator transmission factor (MLC-TF) for two algorithms (Analytical Anisotropic Algorithm [AAA]) and Acuros in the Eclipse TPS. Overall, SPC was used to monitor the process of the RPA, while clinics would still perform their routine patient-specific QA. RESULTS: For RayStation, the average mean percent dose differences across all contours were 0.65% ± 1.05%, -2.09% ± 0.56%, and 0.28% ± 0.98% and average control limit ranges were 1.89% ± 1.32%, 2.16% ± 1.31%, and 2.65% ± 1.89% for the head and neck, cervix, and chest wall, respectively. In contrast, Eclipse's average mean percent dose differences across all contours were -0.62% ± 0.34%, 0.32% ± 0.23%, and -0.91% ± 0.98%, while average control limit ranges were 1.09% ± 0.77%, 3.69% ± 2.67%, 2.73% ± 1.86%, respectively. Averaging all contours and removing outliers, a 0% dose difference corresponded with a DLG value of 0.202 ± 0.019 cm and MLC-TF value of 0.020 ± 0.001 for Acuros and a DLG value of 0.135 ± 0.031 cm and MLC-TF value of 0.015 ± 0.001 for AAA. CONCLUSIONS: Differences in mean dose and control limits between RPA and two separately commissioned TPSs were determined. With varying control limits and means, SPC provides a flexible and useful process quality assurance tool for monitoring a complex automated system such as the RPA.

Predicting the complexity of head-and-neck volumetric-modulated arc therapy planning using a radiation therapy planning quality assurance software.

Background/Aim: The more complex the treatment plan, the higher the possibility of errors in dose verification. Recently, a treatment planning quality assurance (QA) software (PlanIQ) with a function to objectively evaluate the quality of volumetric-modulated arc therapy (VMAT) treatment plans by scoring and calculating the ideal dose-volume histogram has been marketed. This study aimed to assess the association between the scores of ideal treatment plans identified using PlanIQ and the results of dose verification and to investigate whether the results of dose verification can be predicted based on the complexity of treatment plans. Materials and methods: Dose verification was performed using an ionization chamber dosimeter, a radiochromic film, and a three-dimensional dose verification system, Delta4 PT. Correlations between the ideal treatment plan scores obtained by PlanIQ and the results of the absolute dose verification and dose distribution verification were obtained, and it was examined whether dose verifications could be predicted from the complexity of the treatment plans. Results: Even when the score from the ideal treatment plan was high, the results of absolute dose verification and dose distribution verification were sometimes poor. However, even when the score from the ideal treatment plan was low, the absolute volume verification and dose distribution verification sometimes yielded good results. Conclusions: Treatment plan complexity can be determined in advance from the ideal treatment plan score calculated by PlanIQ. However, it is difficult to predict the results of dose verification using an ideal treatment plan.

A novel angular dependency model for MatriXX response and its application to true composite dose verification for IMRT plans.

PURPOSE: This paper proposes a model for the angular dependency of MatriXX response and investigates whether MatriXX, with the angular-model-based approach can be applied to true composite dose verification for IMRT plans. METHOD: This model attributes the angular dependence of MatriXX response to dynamical translation of its effective measurement plane (EMP) due to the change of beam angle. Considering this mechanism, true composite dose verifications for IMRT plans specified in AAPM TG 119 report using both MatriXX and Gafchromic EBT3 films were undertook and compared to validate the applicability of MatriXX for patient specific QA of composite beam IMRT plans. Dose verifications using MatriXX with and without angular-model-based approach were performed. RESULTS: MatriXX with angular-model-based approach achieved gamma passing rates with 3%/3 mm and 3%/2 mm criteria better than 98.3% and 98.1% respectively for true composite dose verification of plans in AAPM TG 119 report. The 3%/3 mm and 3%/2 mm gamma passing rates using MatriXX without angular-model-based approach ranged from 85.8% to 98.2% and from 81.3% to 96.5%, respectively. The p-values from the single sided paired t-test indicated no statistical difference between the passing rates from MatriXX with angular-model-based approach and from films, and significant difference between the passing rates from uncorrected MatriXX and from films. CONCLUSION: The proposed model for angular dependent MatriXX response is necessary and effective. Dose verification using MatriXX with angular-model-based approach is acceptable for true composite beam IMRT plans with required accuracy to simplify patient specific QA.

Performance optimization of a tri-hybrid method for estimation of patient scatter into the EPID.

On-treatment EPID images are contaminated with patient-generated scattered photons. If this component can be accurately estimated, its effect can be removed, and therefore a corresponding in vivo patient dose estimate will be more accurate. Our group previously developed a "tri-hybrid" (TH) algorithm to provide fast but accurate estimates of patient-generated photon scatter. The algorithm uses an analytical method to solve for singly-scattered photon fluence, a modified Monte Carlo hybrid method to solve for multiply-scattered photon fluence, and a pencil beam scatter kernel method to solve for electron interaction generated scattered photon fluence. However, for efficient clinical implementation, spatial and energy sampling must be optimized for speed while maintaining overall accuracy. In this work, the most significant sampling issues were examined, including spatial sampling settings for the patient voxel size, the number of Monte Carlo histories used in the modified hybrid MC method, scatter order sampling for the hybrid method, and also a range of energy spectrum sampling (i.e., energy bin sizes). The total predicted patient-scattered photon fluence entering the EPID was compared with full MC simulation (EGSnrc) for validation. Three phantoms were tested with 6 and 18 MV beam energies, field sizes of 4 × 4, 10 × 10, and 20 × 20 cm2 , and source-to-imager distance of 140 cm to develop a set of optimal sampling settings. With the recommended sampling, accuracy and precision of the total-scattered energy fluence of the TH patient scatter prediction method are within 0.9% and 1.2%, respectively, for all test cases compared with full MC simulation results. For the mean energy spectrum across the imaging plane, comparison of TH with full MC simulation showed 95% overlap. This study has optimized sampling settings so that they have minimal impact on patient scatter prediction accuracy while maintaining maximum execution speed, a critical step for future clinical implementation.

Dosimetric verification of stereotactic body radiotherapy treatment plan via ArcCHECK-3DVH system. / ArcCHECK-3DVH系统在体部立体定向放射治疗剂量验证中的应用.

OBJECTIVES: To study the feasibility of ArcCHECK-3DVH system in dosimetric verification for stereotactic body radiaotherapy (SBRT) with flattening filter free (FFF) model. METHODS: SBRT treatment plans for 57 patients were introduced into ArcCHECK phantom and recalculated. The calculated dose distribution of treatment planning system and the measured dose distribution of ArcCHECK phantom were compared by γ analysis. Then the 3 dimensional dose distribution of target and organs at risk was reconstructed by 3DVH software. The reconstructed dose and calculated dose with treatment planning system (TPS) were compared, and the dose volume γ pass rate and deviation of dose volume parameters to the target and organs at risk were quantitatively valuated. RESULTS: Based on the threshold criteria (3%, 3 mm, 10%), namely the deviation of measuring points between the planned value and the measured value was less than 3%, and the proportion of points with similar values in the plane or sphere with the center of the point and the radius of 3 mm was 10%, the relative and absolute dose pass rates of SBRT treatment plans in ArcCHECK system via γ analysis were greater than 95%. Based on the stricter threshold criteria (2%, 2 mm, 10%), the relative and absolute dose pass rates of SBRT treatment plan in ArcCHECK system via γ analysis were about 93%. In 3DVH dose verification, the γ pass rate of target and organs at risk was exceed 97%, and the deviations in 3DVH of the target and organs at risk were less than ±5%. CONCLUSIONS: The ArcCHECK-3DVH system in dose verification can provide more comprehensive dose distribution information to reasonably evaluate the SBRT plan, with more significance for guiding clinical treatment.

[Research on Individualized Phantom Based on 3D Printing for Radiotherapy Dose Verification].

Dose verification is carried out on the individualized three-dimensional phantom based on 3D printing technology, which simulates the anatomical structure of human body, contour shape, tumor anatomical structure and other dangerous organs to the greatest extent, and produces a reasonable and effective dose validation phantom. According to the need to obtain effective patient data, import Mimics software to reconstruct the parts of the body and its surrounding tissues and organs that need to be measured, and make them into three-dimensional shell components. The 3D printing is used to assemble and fill the equivalent tissue, and then the body phantom is made. The phantom was scanned by CT and the data was transmitted to TPS system. The previously completed treatment plan was transplanted to the phantom. The phantom was placed according to the patient's location information, irradiated and measured data. The three-dimensional shell assembly is completely reconstructed according to the patient's data, and the contour difference is not significant. The shell is filled with tissue radiation equivalent material whose CT value is the same as the average CT value of the shell volume. The CT image data show that the radiation equivalence of the phantom is similar to the actual tissue of the patient, and the equivalent dose distribution conforms to the conventional treatment range. It can provide a reliable means of dose verification for the accurate design of intensity modulated radiation therapy.

Survey results of 3D-CRT and IMRT quality assurance practice.

PURPOSE: To create a snapshot of common practices for 3D-CRT and intensity-modulated radiation therapy (IMRT) QA through a large-scale survey and compare to TG-218 recommendations. METHODS: A survey of 3D-CRT and IMRT QA was constructed at and distributed by the IROC-Houston QA center to all institutions monitored by IROC (n = 2,861). The first part of the survey asked about methods to check dose delivery for 3D-CRT. The bulk of the survey focused on IMRT QA, inquiring about treatment modalities, standard tools used to verify planned dose, how assessment of agreement is calculated and the comparison criteria used, and the strategies taken if QA fails. RESULTS: The most common tools for dose verification were a 2D diode array (52.8%), point(s) measurement (39.0%), EPID (27.4%), and 2D ion chamber array (23.9%). When IMRT QA failed, the highest average rank strategy utilized was to remeasure with the same setup, which had an average position ranking of 1.1 with 90.4% of facilities employing this strategy. The second highest average ranked strategy was to move to a new calculation point and remeasure (54.9%); this had an average ranking of 2.1. CONCLUSION: The survey provided a snapshot of the current state of dose verification for IMRT radiotherapy. The results showed variability in approaches and that work is still needed to unify and tighten criteria in the medical physics community, especially in reference to TG-218's recommendations.

[Estimation of Dynamic MLC Parameters Using Dynamic MLC Position Accuracy Analysis Software in Prostate IMRT].

It is necessary to verify an intensity-modulated radiation therapy (IMRT) plan and to confirm dose error within the tolerance, in order to perform it securely and precisely. IMRT with dynamic multi-leaf collimator (DMLC) requires high DMLC position accuracy. The DMLC position accuracy analysis software DynaLog File Viewer (DFV; Varian Medical Systems, Palo Alto, CA, USA) is used to analyze position errors of DMLC for IMRT plans. We analyzed correlation between DMLC parameters and position error of DMLC obtained from DFV in prostate IMRT. A regression analysis of the position error and the DMLC parameters was performed. As a result, a strong correlation was found between MLC position error and each of the DMLC parameters: leaf speed, gap width, and segment monitor unit (MU). We found the factors for the DMLC position error in this study. DMLC position error could be estimated from leaf speed, gap width, and segment MU when we analyze IMRT cases in the further study.

[Dose Verification for Respiratory-gated VMAT-SBRT Using Real-time Tumor Tracking System].

Recently, the introduction of various novel technologies in clinical settings has improved the accuracy of radiation therapy. Stereotactic body radiation therapy (SBRT) involves the delivery of an accurate radiation dose to the tumor with a minimal impact on normal tissues using various measures to address changes in the tumor position due to respiratory displacement. The SyncTraX FX4 real-time tumor tracking system (Shimadzu Corporation) introduced in our hospital tracks the actual tumor location by radioscopically monitoring a metallic marker that is placed in the vicinity of the tumor. However, there have been no reports yet on respiratory-gated volumetric modulated arc therapy (VMAT)-SBRT using a real-time tumor tracking system. This study aimed to develop an irradiation procedure for respiratory-gated VMAT-SBRT using a real-time tumor tracking system and to evaluate radiation doses therein. In this study, we found that absolute doses with respiratory gating did not deviate by more than ±1.0% from those without respiratory gating. In addition, the pass rate in gamma analysis using GAFCHROMIC EBT3 was ³95% with the pass criteria in dose difference, distance to agreement, and threshold being 2%, 2 mm, and 10%, respectively. Furthermore, a trajectory log file analysis did not reveal any significant error causes. Thus, these data indicate that respiratory-gated VMAT-SBRT can be applied clinically.

Remote dosimetric auditing of clinical trials: The need for vendor specific models to convert images to dose.

INTRODUCTION: A previous pilot study has demonstrated the feasibility of a novel image-based approach for remote dosimetric auditing of clinical trials. The approach uses a model to convert in-air acquired intensity modulated radiotherapy (IMRT) images to delivered dose inside a virtual phantom. The model was developed using images from an electronic portal imaging device (EPID) on a Varian linear accelerator. It was tuned using beam profiles and field size factors (FSFs) of a series of square fields measured in water tank. This work investigates the need for vendor specific conversion models for image-based auditing. The EPID measured profile and FSF data for Varian (vendor 1) and Elekta (vendor 2) systems are compared along with the performance of the existing Varian model (VM) and a new Elekta model (EM) for a series of audit IMRT fields measured on vendor 2 systems. MATERIALS AND METHODS: The EPID measured beam profile and FSF data were studied for the two vendors to quantify and understand their relevant dosimetric differences. Then, an EM was developed converting EPID to dose in the virtual water phantom using a vendor 2 water tank data and images from corresponding EPID. The VM and EM were compared for predicting vendor 2 measured dose in water tank. Then, the performance of the new EM was compared to the VM for auditing of 54 IMRT fields from four vendor 2 facilities. Statistical significance of using vendor specific models was determined. RESULTS: Observed dosimetry differences between the two vendors suggested developing an EM would be beneficial. The EM performed better than VM for vendor 2 square and IMRT fields. The IMRT audit gamma pass rates were (99.8 ± 0.5)%, (98.6 ± 2.3)% and (97.0 ± 3.0)% at respectively 3%/3 mm, 3%/2 mm and 2%/2 mm with improvements at most fields compared with using the VM. For the pilot audit, the difference between gamma results of the two vendors was reduced when using vendor specific models (VM: P < 0.0001, vendor specific models: P = 0.0025). CONCLUSION: A new model was derived to convert images from vendor 2 EPIDs to dose for remote auditing vendor 2 deliveries. Using vendor specific models is recommended to remotely audit systems from different vendors, however, the improvements found were not major.

EPID-based in vivo dosimetry using Dosimetry Check™: Overview and clinical experience in a 5-yr study including breast, lung, prostate, and head and neck cancer patients.

BACKGROUND: Independent verification of the dose delivered by complex radiotherapy can be performed by electronic portal imaging device (EPID) dosimetry. This paper presents 5-yr EPID in vivo dosimetry (IVD) data obtained using the Dosimetry Check (DC) software on a large cohort including breast, lung, prostate, and head and neck (H&N) cancer patients. MATERIAL AND METHODS: The difference between in vivo dose measurements obtained by DC and point doses calculated by the Eclipse treatment planning system was obtained on 3795 radiotherapy patients treated with volumetric modulated arc therapy (VMAT) (n = 842) and three-dimensional conformal radiotherapy (3DCRT) (n = 2953) at 6, 10, and 15 MV. In cases where the dose difference exceeded ±10% further inspection and additional phantom measurements were performed. RESULTS: The mean and standard deviation ( µ ± σ ) of the percentage difference in dose obtained by DC and calculated by Eclipse in VMAT was: 0.19 ± 3.89 % in brain, 1.54 ± 4.87 % in H&N, and 1.23 ± 4.61 % in prostate cancer. In 3DCRT, this was 1.79 ± 3.51 % in brain, - 2.95 ± 5.67 % in breast, - 1.43 ± 4.38 % in bladder, 1.66 ± 4.77 % in H&N, 2.60 ± 5.35% in lung and - 3.62 ± 4.00 % in prostate cancer. A total of 153 plans exceeded the ±10% alert criteria, which included: 88 breast plans accounting for 7.9% of all breast treatments; 28 H&N plans accounting for 4.4% of all H&N treatments; and 12 prostate plans accounting for 3.5% of all prostate treatments. All deviations were found to be as a result of patient-related anatomical deviations and not from procedural errors. CONCLUSIONS: This preliminary data shows that EPID-based IVD with DC may not only be useful in detecting errors but has the potential to be used to establish site-specific dose action levels. The approach is straightforward and has been implemented as a radiographer-led service with no disruption to the patient and no impact on treatment time.