Physical dosimetry of volumetric modulated arc therapy (VMAT) using EPID and 2D array for quality assurance

Outline

To address the correspondence of measured and predicted doses for different malignant tumours utilizing various gamma criteria and QA for confirmation of VMAT with an EPID and 2D array detector.

Establishing an optimized patient-specific verification program for volumetric modulated arc therapy

Quality assurance (QA) of volumetric modulated arc therapy (VMAT) increases the workload significantly. We compared the results from 4 verification methods to establish an efficient VMAT QA. Planning for VMAT treatments was carried out for 40 consecutive patients. Pretreatment verifications were carried out with ion chamber array Physikalish-Technische Werkstätten (PTW729), electronic portal dosimetry (EPID), ion chamber measurements, and independent dose calculation with Diamond program. 2D analyses were made using the gamma analysis (3 mm distance to agreement and 3% dose difference relative to maximum, 10% dose threshold). Average point dose difference calculated by Eclipse relative to ion chamber measurements and Diamond were 0.1%±0.9% and 0.6%±2.2%, respectively. Average pass rate for PTW729 was 99.2%±1.9% and 98.3%±1.3% for EPID. The total required time (linac occupancy time given in parentheses) for each QA method was: PTW729 43.5 minutes (26.5 minutes), EPID 14.5 minutes (2.5 minutes), ion chamber 34.5 minutes (26.5 minutes), and Diamond 12.0 minutes (0 minute). The results were consistent and allowed us to establish an optimized protocol, considering safety and accuracy as well as workload, consisting of 2 verification methods: EPID 2D analysis and independent dose calculation.     

A retrospective analysis for patient-specific quality assurance of volumetric-modulated arc therapy plans

Volumetric-modulated arc therapy (VMAT) is now widely used clinically, as it is capable of delivering a highly conformal dose distribution in a short time interval. We retrospectively analyzed patient-specific quality assurance (QA) of VMAT and examined the relationships between the planning parameters and the QA results. A total of 118 clinical VMAT cases underwent pretreatment QA. All plans had 3-dimensional diode array measurements, and 69 also had ion chamber measurements. Dose distribution and isocenter point dose were evaluated by comparing the measurements and the treatment planning system (TPS) calculations. In addition, the relationship between QA results and several planning parameters, such as dose level, control points (CPs), monitor units (MUs), average field width, and average leaf travel, were also analyzed. For delivered dose distribution, a gamma analysis passing rate greater than 90% was obtained for all plans and greater than 95% for 100 of 118 plans with the 3%/3-mm criteria. The difference (mean ± standard deviation) between the point doses measured by the ion chamber and those calculated by TPS was 0.9% ± 2.0% for all plans. For all cancer sites, nasopharyngeal carcinoma and gastric cancer have the lowest and highest average passing rates, respectively. From multivariate linear regression analysis, the dose level (p = 0.001) and the average leaf travel (p < 0.001) showed negative correlations with the passing rate, and the average field width (p = 0.003) showed a positive correlation with the passing rate, all indicating a correlation between the passing rate and the plan complexity. No statistically significant correlation was found between MU or CP and the passing rate. Analysis of the results of dosimetric pretreatment measurements as a function of VMAT plan parameters can provide important information to guide the plan parameter setting and optimization in TPS.     

Dosimetric comparison of helical tomotherapy and VMAT for anal cancer: A single institutional experience

To compare the dosimetric results of helical tomotherapy (HT) and volumetric arc therapy (VMAT) in the treatment of anal cancer. Plans were created for 20 (n = 20) patients treated for anal cancer using HT and 2 arc VMAT. Dosimetric comparison was assessed for doses to targets and organs at risk (small bowel, bladder, external genitalia, and femoral heads). Delivery time and dosimetric verification results were also compared. HT showed a higher V95% for both primary and nodal targets (V95% increase by 0.5% to 1.3%; p = ≤0.05). No differences were seen in V105%, V107%, or V110 % between techniques. HT provided better sparing of the small bowel for dose levels V30, V35, and V40 (p = 0.005, 0.001, and 0.030), but was similar at higher doses. Similarly HT provided better bladder dose at V35 only (p = 0.020). Doses to femoral heads and genitalia were similar. Delivery time was higher for the HT plans (4.58 ± 1.1 min) than VMAT (3.13 ± 0.2 minutes) (p = 0.011). Dose verification results were 99.5 ± 0.9% and 100 ± 0% (HT, n = 6) vs 95.0 ± 3.1% and 99.2 ± 0.8% (VMAT, n = 20) for global gamma criteria 3%/3 mm and 4%/4 mm, respectively. Both HT and VMAT produced high quality plans that frequently met most of the dose objectives apart from genitalia V20, V40, bladder V35, and V50. Although absolute dose differences were small, the PTV V95%, small bowel V30, V35, and V40 and bladder V35 were statistically better in the HT plans. VMAT provided a shorter delivery time by 1.45 minutes; however, our HT plans were more likely to pass tighter plan dose verification criteria than VMAT.     

Comparison of dose accuracy between 2D array detectors for pre-treatment IMRT QA

The dosimetric properties between various 2D array detectors were compared and were evaluated with regard to the accuracy in absolute dose and dose distributions for clinical treatment fields. We used to check the dose accuracy: 2D array detectors; MapCHECK (Sun Nuclear), EPID (Varian Medical Systems), EPID-based dosimetry (EPIDose, Sun Nuclear), COMPASS (IBA) and conventional system; EDR2 film (Eastman Kodak), Exradin A-14SL ion chamber (0.016 cc, Standard Imaging). First, we compared the dose linearity, dose rate dependence, and output factor between the 2D array detectors. Next, the accuracy of the absolute dose and dose distributions were evaluated for clinical fields. All detector responses for the dose linear were in agreement within 1%, and the dose rate dependence and output factor agreed within a standard deviation of ±1.2%, except for EPID. This is because EPID is fluence distributions. In all the 2D array detectors, the point dose agreed within 5% with treatment planning system (TPS). Pass rates of each detector for TPS were more than 97% in the gamma analysis (3 mm/3%). EPIDose was in a good agreement with TPS. All 2D array detectors used in this study showed almost the same accuracy for clinical fields. EPIDose has better resolution than other 2D array detectors and thus this is expected for dose distributions with a small field.     

Dosimetric comparison of analytical anisotropic algorithm and the two dose reporting modes of Acuros XB dose calculation algorithm in volumetric modulated arc therapy of carcinoma lung and carcinoma prostate

Volumetric Modulated Arc Therapy (VMAT) is an important modality for radical radiotherapy of all major treatment sites. This study aims to compare Analytical Anisotropic Algorithm (AAA) and the two dose-reporting modes of Acuros XB (AXB) algorithm -the dose to medium option (D_m) and the dose to water option (D_w) in Volumetric Modulated Arc Therapy (VMAT) of carcinoma lung and carcinoma prostate. We also compared the measured dose with Treatment Planning System calculated dose for AAA and the two dose reporting options of Acuros XB using Electronic Portal Imaging Device (EPID) and ArcCHECK phantom. Treatment plans of twenty patients each who have already undergone radiotherapy for cancer of lung and cancer of prostate were selected for the study. Three sets of VMAT plans were generated in Eclipse Treatment Planning System (TPS), one with AAA and two plans with Acuros-D_m and Acuros-D_w options. The Dose Volume Histograms (DVHs) were compared and analyzed for Planning Target Volume (PTV) and critical structures for all the plans. Verification plans were created for each plan and measured doses were compared with TPS calculated doses using EPID and ArcCHECK phantom for all the three algorithms. For lung plans, the mean dose to PTV in the AXB-D_w plans was higher by 1.7% and in the AXB-D_m plans by 0.66% when compared to AAA plans. For prostate plans, the mean dose to PTV in the AXB-D_w plans was higher by 3.0% and in the AXB-D_m plans by 1.6% when compared to AAA plans. There was no difference in the Conformity Index (CI) between AAA and AXB-D_m and between AAA and AXB-D_w plans for both sites. But the homogeneity worsened in AXB-D_w and AXB-D_m plans when compared to AAA plans for both sites. AXB-D_w calculated higher dose values for PTV and all the critical structures with significant differences with one or two exceptions. Point dose measurements in ArcCHECK phantom showed that AXB-D_m and AXB-D_w options showed very small deviations with measured dose distributions than AAA for both sites. Results of EPID QA also showed better pass rates for AXB-D_w and AXB-D_m than AAA for both sites when gamma analysis was done for 3%/3 mm and 2%/2 mm criteria. With reference to the results, it is always better to choose Acuros algorithm for dose calculations if it is available in the TPS. AXB-D_w plans showed very high dose values in the PTV when compared to AAA and AXB-D_m in both sites studied. Also, the volume of PTV receiving 107% dose was significantly high in AXB-D_w plans compared to AXB-D_m plans in sites involving high density bones. Considering the results of dosimetric comparison and QA measurements, it is always better to choose AXB-D_m algorithm for dose calculations for all treatment sites especially when high density bony structures and complex treatment techniques are involved. For patient specific QA purposes, choosing AXB-D_m or AXB-D_w does not make any significant difference between calculated and measured dose distributions.     

Commissioning of a grid-based Boltzmann solver for cervical cancer brachytherapy treatment planning with shielded colpostats

PurposeWe sought to commission a gynecologic shielded colpostat analytic model provided from a treatment planning system (TPS) library. We have reported retrospectively the dosimetric impact of this applicator model in a cohort of patients.Methods and MaterialsA commercial TPS with a grid-based Boltzmann solver (GBBS) was commissioned for ¹⁹²Ir high-dose-rate (HDR) brachytherapy for cervical cancer with stainless steel–shielded colpostats. Verification of the colpostat analytic model was verified using a radiograph and vendor schematics. MCNPX v2.6 Monte Carlo simulations were performed to compare dose distributions around the applicator in water with the TPS GBBS dose predictions. Retrospectively, the dosimetric impact was assessed over 24 cervical cancer patients’ HDR plans.ResultsApplicator (TPS ID #AL13122005) shield dimensions were within 0.4 mm of the independent shield dimensions verification. GBBS profiles in planes bisecting the cap around the applicator agreed with Monte Carlo simulations within 2% at most locations; differing screw representations resulted in differences of up to 9%. For the retrospective study, the GBBS doses differed from TG-43 as follows (mean value ± standard deviation [min, max]): International Commission on Radiation units [ICRU]rectum (?8.4 ± 2.5% [?14.1, ?4.1%]), ICRUbladder (?7.2 ± 3.6% [?15.7, ?2.1%]), D_2cc-rectum (?6.2 ± 2.6% [?11.9, ?0.8%]), D_2cc-sigmoid (?5.6 ± 2.6% [?9.3, ?2.0%]), and D_2cc-bladder (?3.4 ± 1.9% [?7.2, ?1.1%]).ConclusionsAs brachytherapy TPSs implement advanced model-based dose calculations, the analytic applicator models stored in TPSs should be independently validated before clinical use. For this cohort, clinically meaningful differences (>5%) from TG-43 were observed. Accurate dosimetric modeling of shielded applicators may help to refine organ toxicity studies.     

An empirical calibration method for an a-Si portal imaging device: applications in pretreatment verification of IMRT

PURPOSE: A new calibration method for an amorphoussilicon (a-Si) electronic portal imaging device (EPID) used for dose measurements in pretreatment verification (field-related) of intensity-modulated radiation therapy (IMRT) with sliding-window technique. The method is independent of data contained in the multileaf collimator (MLC) leaf-motion files and of any calculations made by the treatment planning system (TPS). MATERIALS AND METHODS: Sensitivity of the EPID is dependent on radiation energy. For fluence-modulated fields, different dose/reading calibration factors are associated with each pixel of the image acquired by calculating equivalent areas representing the exact ratio between primary and scatter components. The dose measured in the detector plane was compared with that calculated with TPS by using gamma-analysis. Each calibration factor was compared with that calculated by considering the individual contributions of primary and secondary radiation obtained using the convolution method with analytical kernel for homogeneous media. RESULTS: In 837/854 (98%) of the clinical fields analysed, the proportion of irradiated area in which the gamma-index was <1.0 exceeded 95%. The overall average gamma-index was 0.39. There was good agreement between the dose/reading calibration factors obtained with the empirical algorithm and with the convolution method. CONCLUSIONS: The proposed calibration method is suitable for routine clinical pretreatment verification in IMRT.     

Characterization and use of a 2D-array of ion chambers for brachytherapy dosimetric quality assurance

The two-dimensional (2D) ionization chamber array MatriXX Evolution is one of the 2D ionization chamber arrays developed by IBA Dosimetry (IBA Dosimetry, Germany) for megavoltage real-time absolute 2D dosimetry and verification of intensity-modulated radiation therapy (IMRT). The purpose of this study was to (1) evaluate the performance of ion chamber array for submegavoltage range brachytherapy beam dose verification and quality assurance (QA) and (2) use the end-to-end dosimetric evaluation that mimics a patient treatment procedure and confirm the primary source strength calibration agrees in both the treatment planning system (TPS) and treatment delivery console computers. The dose linearity and energy dependence of the 2D ion chamber array was studied using kilovoltage X-ray beams (100, 180 and 300 kVp). The detector calibration factor was determined using 300 kVp X-ray beams so that we can use the same calibration factor for dosimetric verification of high-dose-rate (HDR) brachytherapy. The phantom used for this measurement consists of multiple catheters, the IBA MatriXX detector, and water-equivalent slab of RW3 to provide full scattering conditions. The treatment planning system (TPS) (Oncentra brachy version 3.3, Nucletron BV, Veenendaal, the Netherlands) dose distribution was calculated on the computed tomography (CT) scan of this phantom. The measured and TPS calculated distributions were compared in IBA Dosimetry OmniPro-I‘mRT software. The quality of agreement was quantified by the gamma (γ) index (with 3% delta dose and distance criterion of 2 mm) for 9 sets of plans. Using a dedicated phantom capable of receiving 5 brachytherapy intralumenal catheters a QA procedure was developed for end-to-end dosimetric evaluation for routine QA checks. The 2D ion chamber array dose dependence was found to be linear for 100–300 kVp and the detector response (k_user) showed strong energy dependence for 100–300 kVp energy range. For the Ir-192 brachytherapy HDR source, dosimetric evaluation k_user factor determined by photon beam of energy of 300 kVp was used. The maximum mean difference between ion chamber array measured and TPS calculated was 3.7%. Comparisons of dose distribution for different test plans have shown agreement with >94.5% for γ ≤1. Dosimetric QA can be performed with the 2D ion chamber array to confirm primary source strength calibration is properly updated in both the TPS and treatment delivery console computers. The MatriXX Evolution ionization chamber array has been found to be reliable for measurement of both absolute dose and relative dose distributions for the Ir-192 brachytherapy HDR source.     

Commissioning and validation of fluence-based 3D VMAT dose reconstruction system using new transmission detector

In this study, we evaluated the basic performance of the three-dimensional dose verification system COMPASS (IBA Dosimetry). This system is capable of reconstructing 3D dose distributions on the patient anatomy based on the fluence measured using a new transmission detector (Dolphin, IBA Dosimetry) during treatment. The stability of the absolute dose and geometric calibrations of the COMPASS system with the Dolphin detector were investigated for fundamental validation. Furthermore, multileaf collimator (MLC) test patterns and a complicated volumetric modulated arc therapy (VMAT) plan were used to evaluate the accuracy of the reconstructed dose distributions determined by the COMPASS. The results from the COMPASS were compared with those of a Monte Carlo simulation (MC), EDR2 film measurement, and a treatment planning system (TPS). The maximum errors for the absolute dose and geometrical position were ? 0.28% and 1.0 mm for 3 months, respectively. The Dolphin detector, which consists of ionization chamber detectors, was firmly mounted on the linear accelerator and was very stable. For the MLC test patterns, the TPS showed a?>?5% difference at small fields, while the COMPASS showed good agreement with the MC simulation at small fields. However, the COMPASS produced a large error for complex small fields. For a clinical VMAT plan, COMPASS was more accurate than TPS. COMPASS showed real delivered-dose distributions because it uses the measured fluence, a high-resolution detector, and accurate beam modeling. We confirm here that the accuracy and detectability of the delivered dose of the COMPASS system are sufficient for clinical practice.     

Weekly Verification of Dosimetric Data for Virtual Wedge Using a 2D Diode Detector Array

A linac manufacturer has recommended that users measure virtual wedge (VW) angle and VW factor as a weekly quality assurance (QA) procedure. The purpose of this study was to investigate whether a 2D diode detector array (MapCHECK™) is a useful tool for the verification of dosimetric data for VW. Measurements were performed on 2 linear accelerators (4, 6, and 10 MV) at 10-cm depth for a field size of 10 × 10 cm² and with wedge angles of 15, 30, 45, and 60°. To verify the VW dose distributions generated by the treatment planning system (TPS), we confirmed that agreement between TPS data and measurements were ≤2% dose difference or 2-mm distance-to-agreement based on American Association of Physicists in Medicine Task Group Report 53 (AAPM TG-53). We present here the results of a 1-year evaluation of VW by means of a 2D diode detector array. The maximum 2-fold standard deviation of the measured wedge angle turned out to be within 1.0, and all measured VW factors to be 1.00 ± 0.03. Although >95% of the points measured for 6 and 10 MV were generally within the tolerance of the dose distribution as mentioned above, the percentage of agreement between the measured data for 4 MV and TPS data were somewhat below 90%. We also verified generally good reproducibility for the dose distribution. The 2-D diode detector array was thus found to be useful as a tool for weekly VW QA.     

胸段食管癌患者三种放疗计划心脏和肺的剂量学比较

目的 比较胸中下段食管癌3种放疗技术心脏和肺的剂量分布。方法 搜集2015年1月至2016年2月在浙江省肿瘤医院接受治疗的15例胸中下段食管鳞癌患者资料。患者均接受胸部放射治疗,每位患者共制作3套放疗计划。调强放疗（IMRT）和容积旋转调强放疗（VMAT）在RayStation 4.0v系统制作,螺旋断层放疗（TOMO）在TomoH^TM Version 2.0.5系统制作。处方剂量60 Gy/30次。比较计划体积（PTV）、心脏、心脏亚单位以及肺剂量参数。结果 PTV、心脏和肺的平均体积为（399±355）、（671±274）和（3 907±1 717） cm³。与IMRT和VMAT相比,TOMO可以降低PTV、心脏、左心房及肺的最大剂量（H=10.889、7.433、12.080、11.401,P<0.05）。3种放疗技术的适形指数和均匀性指数差异无统计学意义（P>0.05）。结论 相较于IMRT和VMAT,TOMO可以降低PTV、心脏、左心房和肺的最大剂量,但均匀性及适形性差异无统计学意义。放疗过程中心脏与肺存在相互影响,TOMO技术可能带来的临床优势尚待进一步研究证实。     

A study on correlation between 2D and 3D gamma evaluation metrics in patient-specific quality assurance for VMAT

In this study, we investigated the correlation between 2-dimensional (2D) and 3D gamma analysis using the new PTW OCTAVIUS 4D system for various parameters. For this study, we selected 150 clinically approved volumetric-modulated arc therapy (VMAT) plans of head and neck (50), thoracic (esophagus) (50), and pelvic (cervix) (50) sites. Individual verification plans were created and delivered to the OCTAVIUS 4D phantom. Measured and calculated dose distributions were compared using the 2D and 3D gamma analysis by global (maximum), local and selected (isocenter) dose methods. The average gamma passing rate for 2D global gamma analysis in coronal and sagittal plane was 94.81% ± 2.12% and 95.19% ± 1.76%, respectively, for commonly used 3-mm/3% criteria with 10% low-dose threshold. Correspondingly, for the same criteria, the average gamma passing rate for 3D planar global gamma analysis was 95.90% ± 1.57% and 95.61% ± 1.65%. The volumetric 3D gamma passing rate for 3-mm/3% (10% low-dose threshold) global gamma was 96.49% ± 1.49%. Applying stringent gamma criteria resulted in higher differences between 2D planar and 3D planar gamma analysis across all the global, local, and selected dose gamma evaluation methods. The average gamma passing rate for volumetric 3D gamma analysis was 1.49%, 1.36%, and 2.16% higher when compared with 2D planar analyses (coronal and sagittal combined average) for 3 mm/3% global, local, and selected dose gamma analysis, respectively. On the basis of the wide range of analysis and correlation study, we conclude that there is no assured correlation or notable pattern that could provide relation between planar 2D and volumetric 3D gamma analysis. Owing to higher passing rates, higher action limits can be set while performing 3D quality assurance. Site-wise action limits may be considered for patient-specific QA in VMAT.     

A dose verification tool for high-dose-rate interstitial brachytherapy treatment planning in accelerated partial breast irradiation

PurposeTo develop a dose verification tool for high-dose-rate interstitial brachytherapy treatment planning in accelerated partial breast irradiation.Methods and MaterialsWe have developed a software tool for interstitial brachytherapy treatment planning assessment. The software contains a database of seven ¹⁹²Ir source models and is able to estimate the dose distribution using the Task Group 43 and the Sievert integral algorithms. Dose–volume histogram analysis and dose quality assurance (QA) criteria including conformity (COnformal INdex [COIN] and conformation number [CN]), homogeneity (homogeneity index [HI]) parameters were implemented in the software to evaluate and to compare between the doses estimated by the two algorithms and a dose extracted from an external treatment planning system (TPS).ResultsThe tool was evaluated and validated on four clinical cases treated by high-dose-rate interstitial brachytherapy. The doses provided by the Task Group 43 and the Sievert integral algorithms were evaluated by establishing the dose–volume histogram analysis and then by calculating the QA criteria. The algorithms were validated by comparing the dose at different anatomic points with their corresponding dose points provided from TPS. The differences were considered in good agreement (within 5%).ConclusionsPretreatment dose verification is an important step in the QA of brachytherapy accelerated partial breast irradiation. A simple, fast, and accurate method of dose verification is therefore needed. The software proposed in this study could fulfill these requirements. In addition, it is freely available for using by anyone wishing to do a QA on any TPS.     

Initial experience of ArcCHECK and 3DVH software for RapidArc treatment plan verification

The purpose of this study was to perform delivery quality assurance with ArcCHECK and 3DVH system (Sun Nuclear, FL) and to evaluate the suitability of this system for volumetric-modulated arc therapy (VMAT) (RapidArc [RA]) verification. This software calculates the delivered dose distributions in patients by perturbing the calculated dose using errors detected in fluence or planar dose measurements. The device is tested to correlate the gamma passing rate (%GP) and the composite dose predicted by 3DVH software. A total of 28 patients with prostate cancer who were treated with RA were analyzed. RA treatments were delivered to a diode array phantom (ArcCHECK), which was used to create a planned dose perturbation (PDP) file. The 3DVH analysis used the dose differences derived from comparing the measured dose with the treatment planning system (TPS)-calculated doses to perturb the initial TPS-calculated dose. The 3DVH then overlays the resultant dose on the patient׳s structures using the resultant “PDP” beams. Measured dose distributions were compared with the calculated ones using the gamma index (GI) method by applying the global (Van Dyk) normalization and acceptance criteria, i.e., 3%/3 mm. Paired differences tests were used to estimate statistical significance of the differences between the composite dose calculated using 3DVH and %GP. Also, statistical correlation by means of logistic regression analysis has been analyzed. Dose-volume histogram (DVH) analysis for patient plans revealed small differences between treatment plan calculations and 3DVH results for organ at risk (OAR), whereas planning target volume (PTV) of the measured plan was systematically higher than that predicted by the TPS. The t-test results between the planned and the estimated DVH values showed that mean values were incomparable (p < 0.05). The quality assurance (QA) gamma analysis 3%/3 mm showed that in all cases there were only weak-to-moderate correlations (Pearson r: 0.12 to 0.74). Moreover, clinically relevant differences increased with increasing QA passing rate, indicating that some of the largest dose differences occurred in the cases of high QA passing rates, which may be called “false negatives.” The clinical importance of any disagreement between the measured and the calculated dose is often difficult to interpret; however, beam errors (either in delivery or in TPS calculation) can affect the effectiveness of the patient dose. Further research is needed to determinate the role of a PDP-type algorithm to accurately estimate patient dose effect.     

Evaluation of a new GPU-enabled VMAT multi-criteria optimisation plan generation algorithm

To evaluate the new Varian, graphical processing unit (GPU)-enabled, volumetric-modulated arc therapy (VMAT) multi-criteria optimisation (MCO) tool for both its dosimetric accuracy and calculation time. This is a new capability within V16.0 and greater of the Varian Eclipse treatment planning system that allows VMAT optimisation and dose calculation using the GPU (termed GPU-VMAT). In versions prior to V16.0 VMAT multi-criteria optimisation calculations were only possible using central processing unit (CPU) (termed CPU-VMAT) and Hybrid-VMAT (H-VMAT). The H-VMAT method breaks down the VMAT plan into IMRT fields which utilised GPU calculations. The study consisted of a cohort of 50 patients representing a range of anatomical treatment sites; bladder (5), brain (5), gynae (5), head & neck (5), lung (7), mediastinum (7) prostate (4), oesophagus (7) and rectum (5). Each case was planned to that of a clinical standard (Base) which was compared to a CPU-VMAT, GPU-VMAT and H-VMAT approaches. The study analysed dose to organ at risk (OAR) and target coverage, plan calculation time data and plan complexity through monitor unit (MU) for each approach. Negligible dosimetric differences were found between the CPU-VMAT, GPU-VMAT and H-VMAT approaches for the cohort of patients evaluated. The largest dosimetric change were observed in the lacrimal gland for a head and neck case, where the GPU-VMAT and H-VMAT achieved a max dose of +2.8 ± 0.0 Gy and −4.6 ± 0.0 Gy, respectively, when compared to CPU-VMAT. The majority of organ at risk’s (OAR) provided indistinguishable dosimetric outcomes, namely: heart, kidneys, femur, lens, oral cavity and oesophagus. Large time savings were found using the GPU-VMAT technique compared to CPU-VMAT, a mean decrease in calculation time across all sites of 60.2% ± 15.6%. Negligible dosimetric change between the 2 techniques and large time saving were observed with the GPU-VMAT and H-VMAT approaches when compared to the CPU-VMAT. We have shown that the GPU-VMAT technique has been safely implemented with minimal differences from CPU-VMAT, but with significant optimisation and calculation times savings.     

容积调强旋转放疗的计划验证通过率对多叶准直器位置误差的灵敏度

目的 针对容积调强旋转放疗技术(VMAT),分析患者计划验证通过率对多叶准直器(MLC)位置误差的灵敏度。方法 选取6例双弧VMAT计划,引入MLC位置误差 (±0.5 mm、±1 mm和±2 mm),模拟VMAT执行过程中MLC可能出现的系统误差,包括MLC射野宽度误差和MLC整体偏向一侧的误差。每个病例有10个计划,1个原计划和9个带有误差的新计划。利用螺旋形半导体探测阵列(ArcCheck)进行验证测量,得到每个病例原计划和新计划的剂量分布。采用绝对剂量结合等剂量距离差别的计算方法,以原计划计算剂量分布为参考,分别得到每个计划的通过率。结果 当评价指标为3%/3 mm时,6例原计划的平均通过率为96%,带有+1 mm、+2 mm、-2 mm射野宽度误差的计划和2 mm MLC整体偏向一侧误差的计划平均通过率下降8.8%、15.5%、6.1%和7.9%,这些MLC位置误差通过计划验证可以检测到,其他MLC位置误差对计划通过率影响小,无法检测到。2%/2 mm评价指标较3%/3 mm对MLC位置误差更敏感。结论 对于1 mm以内的MLC位置误差,VMAT计划的验证对其不敏感。为保证VMAT计划执行的准确性,需要针对MLC做专门的质量控制。     

Evaluation of a new hybrid VMAT-IMRT multi-criteria optimization plan generation algorithm

To evaluate the Varian ‘Fast hybrid multi-criteria optimization (MCO) volumetric modulated arc therapy (VMAT)’ (H-VMAT) tool for both its dosimetric accuracy and calculation time. This is a new function within V15.6 of the Varian Eclipse treatment planning system that allows VMAT optimization and dose calculation using the graphical processing unit (GPU). In versions prior to V15.6 VMAT MCO calculations were only possible using central processing unit (CPU) not GPU. We termed this approach as native VMAT (N-VMAT). The study consisted of a cohort of 53 patients representing a range of anatomical treatment sites; bladder (5), brain (6), gynaecological (5), head & neck (5), lung (7), mediastinum (7) prostate (6), oesophagus (7), and rectum (5). Each case was planned to that of a clinical standard (Base) which was compared to a H-VMAT and N-VMAT approach. The study analyzed plan calculation time data, dose to organ at risk (OAR) and target coverage for each approach. Negligible dosimetric differences were found between the H-VMAT and N-VMAT approach for the cohort of patients evaluated. The largest dosimetric changes where observed in the optic chiasm and lacrimal gland where the H-VMAT achieved a max dose of 50.9 ± 7.7 Gy and 8.0 ± 0.5 Gy in comparison to the N-VMAT 53.1 ± 6.3 Gy and 10.2 ± 2.9 Gy, respectively. Several OAR's provided indistinguishable dose outcomes, namely; brainstem, heart, kidney's, lens, parotid, and spinal cord. Large time savings were found using the H-VMAT technique when compared to N-VMAT, being 5 to 40 times faster or up to 75 minutes time saving (average of 25 minutes). Negligible dosimetric change between the 2 techniques and large time savings were observed with the GPU enabled H-VMAT approach. We have shown that the H-VMAT technique has been safely implemented and is ready for clinical use.