Machine log file-based dose verification using novel iterative CBCT reconstruction algorithm in commercial software during volumetric modulated arc therapy for prostate cancer patients

PurposeTo evaluate the utility of the use of iterative cone-beam computed tomography (CBCT) for machine log file-based dose verification during volumetric modulated arc therapy (VMAT) for prostate cancer patients.MethodsAll CBCT acquisition data were used to reconstruct images with the Feldkamp-Davis-Kress algorithm (FDK-CBCT) and the novel iterative algorithm (iCBCT). The Hounsfield unit (HU)-electron density curves for CBCT images were created using the Advanced Electron Density Phantom. The I’mRT and anthropomorphic phantoms were irradiated with VMAT after CBCT registration. Subsequently, fourteen prostate cancer patients received VMAT after CBCT registration. Machine log files and both CBCT images were exported to the PerFRACTION software, and a 3D patient dose was reconstructed. Mean dose for planning target volume (PTV), the bladder, and rectum and the 3D gamma analysis were evaluated.ResultsFor the phantom studies, the variation of HU values was observed at the central position surrounding the bones in FDK-CBCT. There were almost no changes in the difference of doses at the isocenter between measurement and reconstructed dose for planning CT (pCT), FDK-CBCT, and iCBCT. Mean dose differences of PTV, rectum, and bladder between iCBCT and pCT were approximately 2% lower than those between FDK-CBCT and pCT. For the clinical study, average gamma analysis for 2%/2 mm was 98.22% ± 1.07 and 98.81% ± 1.25% in FDK-CBCT and iCBCT, respectively.ConclusionsA similar machine log file-based dose verification accuracy is obtained for FDK-CBCT and iCBCT during VMAT for prostate cancer patients.     

Detailed evaluation of Mobius3D dose calculation accuracy for volumetric-modulated arc therapy

PurposeTo perform a detailed evaluation of dose calculation accuracy and clinical feasibility of Mobius3D. Of particular importance, multileaf collimator (MLC) modeling accuracy in the Mobius3D dose calculation algorithm was investigated.MethodsMobius3D was fully commissioned by following the vendor-suggested procedures, including dosimetric leaf gap (DLG) optimization. The DLG optimization determined an optimal DLG correction factor which minimized the average difference between calculated and measured doses for 13 patient volumetric-modulated arc therapy (VMAT) plans. Two sets of step-and-shoot plans were created to examine MLC and off-axis open fields modeling accuracy of the Mobius3D dose calculation algorithm: MLC test set and off-axis open field test set. The test plans were delivered to MapCHECK for the MLC tests and an ionization chamber for the off-axis open field test, and these measured doses were compared to Mobius3D-calculated doses.ResultsThe mean difference between the calculated and measured doses across the 13 VMAT plans was 0.6% with an optimal DLG correction factor of 1.0. The mean percentage of pixels passing gamma from a 3%/1 mm gamma analysis for the MLC test set was 43.5% across the MLC tests. For the off-axis open field tests, the Mobius3D-calculated dose for 1.5 cm square field was −4.6% lower than the chamber-measured dose.ConclusionsIt was demonstrated that Mobius3D has dose calculation uncertainties for small fields and MLC tongue-and-groove design is not adequately taken into consideration in Mobius3D. Careful consideration of DLG correction factor, which affects the resulting dose distributions, is required when commissioning Mobius3D for patient-specific QA.     

Comparison of two different EPID-based solutions performing pretreatment quality assurance: 2D portal dosimetry versus 3D forward projection method

PurposeThe aim of this paper is to characterize two different EPID-based solutions for pre-treatment VMAT quality assurance, the 2D portal dosimetry and the 3D projection technique. Their ability to catch the main critical delivery errors was studied.MethodsMeasurements were performed with a linac accelerator equipped with EPID aSi1000, Portal Dose Image Prediction (PDIP), and PerFRACTION softwares. Their performances were studied simulating perturbations of a reference plan through systematic variations in dose values and micromultileaf collimator position. The performance of PDIP, based on 2D forward method, was evaluated calculating gamma passing rate (%GP) between no-error and error-simulated measurements. The impact of errors with PerFRACTION, based on 3D projection technique, was analyzed by calculating the difference between reference and perturbed DVH (%ΔD). Subsequently pre-treatment verification with PerFRACTION was done for 27 patients of different pathologies.ResultsThe sensitivity of PerFRACTION was slightly higher than sensitivity of PDIP, reaching a maximum of 0.9. Specificity was 1 for PerFRACTION and 0.6 for PDIP. The analysis of patients’ DVHs indicated that the mean %ΔD was (1.2 ± 1.9)% for D2%, (0.6 ± 1.7)% for D95% and (−0.0 ± 1.2)% for Dmean of PTV. Regarding OARs, we observed important discrepancies on DVH but that the higher dose variations were in low dose area (<10 Gy).ConclusionsThis study supports the introduction of the new 3D forward projection method for pretreatment QA raising the claim that the visualization of the delivered dose distribution on patient anatomy has major advantages over traditional portal dosimetry QA systems.     

An assessment of a 3D EPID-based dosimetry system using conventional two- and three-dimensional detectors for VMAT

PurposeTo provide a 3D dosimetric evaluation of a commercial portal dosimetry system using 2D/3D detectors under ideal conditions using VMAT.MethodsA 2D ion chamber array, radiochromic film and gel dosimeter were utilised to provide a dosimetric evaluation of transit phantom and pre-treatment ‘fluence’ EPID back-projected dose distributions for a standard VMAT plan. In-house 2D and 3D gamma methods compared pass statistics relative to each dosimeter and TPS dose distributions.ResultsFluence mode and transit EPID dose distributions back-projected onto phantom geometry produced 2D gamma pass rates in excess of 97% relative to other tested detectors and exported TPS dose planes when a 3%, 3 mm global gamma criterion was applied. Use of a gel dosimeter within a glass vial allowed comparison of measured 3D dose distributions versus EPID 3D dose and TPS calculated distributions. 3D gamma comparisons between modalities at 3%, 3 mm gave pass rates in excess of 92%. Use of fluence mode was indicative of transit results under ideal conditions with slightly reduced dose definition.Conclusions3D EPID back projected dose distributions were validated against detectors in both 2D and 3D. Cross validation of transit dose delivered to a patient is limited due to reasons of practicality and the tests presented are recommended as a guideline for 3D EPID dosimetry commissioning; allowing direct comparison between detector, TPS, fluence and transit modes. The results indicate achievable gamma scores for a complex VMAT plan in a homogenous phantom geometry and contributes to growing experience of 3D EPID dosimetry.     

Evaluation of two-dimensional electronic portal imaging device using integrated images during volumetric modulated arc therapy for prostate cancer

BackgroundThe aim of the study was to evaluate analysis criteria for the identification of the presence of rectal gas during volumetric modulated arc therapy (VMAT) for prostate cancer patients by using electronic portal imaging device (EPID)-based in vivo dosimetry (IVD).Materials and methodsAll measurements were performed by determining the cumulative EPID images in an integrated acquisition mode and analyzed using PerFRACTION commercial software. Systematic setup errors were simulated by moving the anthropomorphic phantom in each translational and rotational direction. The inhomogeneity regions were also simulated by the I’mRT phantom attached to the Quasar phantom. The presence of small and large air cavities (12 and 48 cm³) was controlled by moving the Quasar phantom in several timings during VMAT. Sixteen prostate cancer patients received EPID-based IVD during VMAT.ResultsIn the phantom study, no systematic setup error was detected in the range that can happen in clinical (< 5-mm and < 3 degree). The pass rate of 2% dose difference (DD2%) in small and large air cavities was 98.74% and 79.05%, respectively, in the appearance of the air cavity after irradiation three quarter times. In the clinical study, some fractions caused a sharp decline in the DD2% pass rate. The proportion for DD2% < 90% was 13.4% of all fractions. Rectal gas was confirmed in 11.0% of fractions by acquiring kilo-voltage X-ray images after the treatment.ConclusionsOur results suggest that analysis criteria of 2% dose difference in EPID-based IVD was a suitable method for identification of rectal gas during VMAT for prostate cancer patients.     

A Deep Learning-based correction to EPID dosimetry for attenuation and scatter in the Unity MR-Linac system

PurposeEPID dosimetry in the Unity MR-Linac system allows for reconstruction of absolute dose distributions within the patient geometry. Dose reconstruction is accurate for the parts of the beam arriving at the EPID through the MRI central unattenuated region, free of gradient coils, resulting in a maximum field size of ~10 × 22 cm² at isocentre. The purpose of this study is to develop a Deep Learning-based method to improve the accuracy of 2D EPID reconstructed dose distributions outside this central region, accounting for the effects of the extra attenuation and scatter.MethodsA U-Net was trained to correct EPID dose images calculated at the isocenter inside a cylindrical phantom using the corresponding TPS dose images as ground truth for training. The model was evaluated using a 5-fold cross validation procedure. The clinical validity of the U-Net corrected dose images (the so-called DEEPID dose images) was assessed with in vivo verification data of 45 large rectum IMRT fields. The sensitivity of DEEPID to leaf bank position errors (±1.5 mm) and ±5% MU delivery errors was also tested.ResultsCompared to the TPS, in vivo 2D DEEPID dose images showed an average γ-pass rate of 90.2% (72.6%–99.4%) outside the central unattenuated region. Without DEEPID correction, this number was 44.5% (4.0%–78.4%). DEEPID correctly detected the introduced delivery errors.ConclusionsDEEPID allows for accurate dose reconstruction using the entire EPID image, thus enabling dosimetric verification for field sizes up to ~19 × 22 cm² at isocentre. The method can be used to detect clinically relevant errors.     

Clinical impact of dosimetric changes for volumetric modulated arc therapy in log file-based patient dose calculations

PurposeA log file-based method cannot detect dosimetric changes due to linac component miscalibration because log files are insensitive to miscalibration. Herein, clinical impacts of dosimetric changes on a log file-based method were determined.Methods and materialsFive head-and-neck and five prostate plans were applied. Miscalibration-simulated log files were generated by inducing a linac component miscalibration into the log file. Miscalibration magnitudes for leaf, gantry, and collimator at the general tolerance level were ±0.5 mm, ±1°, and ±1°, respectively, and at a tighter tolerance level achievable on current linac were ±0.3 mm, ±0.5°, and ±0.5°, respectively. Re-calculations were performed on patient anatomy using log file data.ResultsChanges in tumor control probability/normal tissue complication probability from treatment planning system dose to re-calculated dose at the general tolerance level was 1.8% on planning target volume (PTV) and 2.4% on organs at risk (OARs) in both plans. These changes at the tighter tolerance level were improved to 1.0% on PTV and to 1.5% on OARs, with a statistically significant difference.ConclusionsWe determined the clinical impacts of dosimetric changes on a log file-based method using a general tolerance level and a tighter tolerance level for linac miscalibration and found that a tighter tolerance level significantly improved the accuracy of the log file-based method.     

Commissioning and clinical implementation of Mobius3D and MobiusFX: Experience on multiple linear accelerators

PurposeTo provide practical guidelines for Mobius3D commissioning based on experiences of commissioning/clinical implementation of Mobius3D and MobiusFX as patient-specific quality assurance tools on multiple linear accelerators.MethodsThe vendor-suggested Mobius3D commissioning procedures, including beam model adjustment and dosimetric leaf gap (DLG) optimization, were performed for 6 MV X-ray beams of six Elekta linear accelerators. For the beam model adjustment, beam data, such as the percentage depth dose, off-axis ratio (OAR), and output factor (OF), were measured using a water phantom and compared to the vendor-provided reference values. DLG optimization was performed to determine an optimal DLG correction factor to minimize the mean difference between Mobius3D-calculated and measured doses for multiple volumetric modulated arc therapy (VMAT) plans. Small-field VMAT plans, in which Mobius3D has dose calculate uncertainties, were initially included in the DLG optimization, but excluded later.ResultsThe measured beam data were consistent across the six linear accelerators. Relatively large differences between the reference and measured values were observed for the OAR at large off-axis distances (>5 cm) and for the OF for small fields (<3 × 3 cm²). The optimal DLG correction factor was 0.6 ± 0.3 (range: 0.3–1.0) with small-field plans and 0.2 ± 0.2 (0.0–0.5) without them.ConclusionsA reasonable agreement was found between the vendor-provided reference and measured beam models. DLG optimization results were dependent on the selection of the VMAT plans, requiring careful attention to the known dose calculation uncertainties of Mobius3D when determining a DLG correction factor.     

Dosimetric and localization accuracy of Elekta high definition dynamic radiosurgery

Background and PurposeWith the increasingly prominent role of stereotactic radiosurgery in radiation therapy, there is a clinical need for robust, efficient, and accurate solutions for targeting multiple sites with one patient setup. The end-to-end accuracy of high definition dynamic radiosurgery with Elekta treatment planning and delivery systems was investigated in this study.Materials and MethodsA patient-derived CT scan was used to create a radiosurgery plan to seven targets in the brain. Monaco was used for treatment planning using 5 VMAT non-coplanar arcs. Prior to delivery, 3D-printed phantoms from RTsafe were ordered including a gel phantom for 3D dosimetry, phantom with 2D film insert, and an ion chamber phantom for point dose measurement. Delivery was performed using the Elekta VersaHD, XVI cone-beam CT, and HexaPOD six degree of freedom tabletop.ResultsAbsolute dose accuracy was verified within 2%. 3D global gamma analysis in the film measurement revealed 3%/2 mm passing rates >95%. Gel dosimetry 3D global gamma analysis (3%/2 mm) were above 90% for all targets with the exception of one. Results were indicative of typical end-to-end accuracies (<1 mm spatial uncertainty, 2% dose accuracy) within 4 cm of isocenter. Beyond 4 cm, 2 mm accuracy was found.ConclusionsHigh definition dynamic radiosurgery expands clinically acceptable stereotactic accuracy to a sphere around isocenter allowing for radiosurgery of several targets with one setup with a high degree of dosimetric precision. Gel dosimetry proved to be an essential tool for the validation of the 3D dose distributions in this technique.     

Clinical results of an EPID-based in-vivo dosimetry method for pelvic cancers treated by intensity-modulated radiation therapy

The purpose of our work was to investigate the feasibility of using an EPID-based in-vivo dosimetry method initially designed for conformal fields on pelvic dynamic IMRT fields. The method enables a point dose delivered to the patient to be calculated from the transit signal acquired with an electronic portal imaging device (EPID). After defining a set of correction factors allowing EPID pixel values to be converted into absolute doses, several tests on homogeneous water-equivalent phantoms were performed to estimate the validity of the method in reference conditions. The effects of different treatment parameters, such as delivered dose, field size dependence and patient thickness were also studied. The model was first evaluated on a group of 53 patients treated by 3D conformal radiotherapy (3DCRT) and then on 92 patients treated by IMRT, both for pelvic cancers. For each measurement, the dose was reconstructed at the isocenter (D_REC) and compared with the dose calculated by our treatment planning system (D_TPS). Excellent agreement was found between D_REC and D_TPS for both techniques. For 3DCRT treatments, the mean deviation between D_REC and D_TPS for the 211 in-vivo dose verifications was equal to −1.0  ±  2.2% (1SD). Concerning IMRT treatments, the averaged deviation for the 418 fields verified was equal to −0.3 ± 2.6% (1SD) proving that the method is able to reconstruct a dose for dynamic IMRT pelvic fields. Based on these results, tolerance criteria and action levels were established before its implementation in clinical routine.     

Gamma-index method sensitivity for gauging plan delivery accuracy of volumetric modulated arc therapy

PurposeThe aim of this study was to investigate the sensitivity of the gamma-index method according to various gamma criteria for volumetric modulated arc therapy (VMAT).MethodsTwenty head and neck (HN) and twenty prostate VMAT plans were retrospectively selected for this study. Both global and local 2D gamma evaluations were performed with criteria of 3%/3 mm, 2%/2 mm, 1%/2 mm and 2%/1 mm. In this study, the global and local gamma-index calculated the differences in doses relative to the maximum dose and the dose at the current measurement point, respectively. Using log files acquired during delivery, the differences in parameters at every control point between the VMAT plans and the log files were acquired. The differences in dose–volumetric parameters between reconstructed VMAT plans using the log files and the original VMAT plans were calculated. The Spearman's rank correlation coefficients (r_s) were calculated between the passing rates and those differences.ResultsConsiderable correlations with statistical significances were observed between global 1%/2 mm, local 1%/2 mm and local 2%/1 mm and the MLC position differences (r_s = −0.712, −0.628 and −0.581). The numbers of r_s values with statistical significance between the passing rates and the changes in dose–volumetric parameters were largest in global 2%/2 mm (n = 16), global 2%/1 mm (n = 15) and local 2%/1 mm (n = 13) criteria.ConclusionLocal gamma-index method with 2%/1 mm generally showed higher sensitivity to detect deviations between a VMAT plan and the delivery of the VMAT plan.     

The effect of beam interruption during VMAT delivery on the delivered dose distribution

PurposeThe aim of this study is to investigate the effect of beam interruptions during delivery of volumetric modulated arc therapy (VMAT) on delivered dose distributions.MethodsTen prostate and ten head and neck (H&N) VMAT plans were retrospectively selected. Each VMAT plan was delivered using Trilogy™ without beam interruption, and with 4 and 8 intentional beam interruptions per a single arc. Two-dimensional global and local gamma evaluations with a diode array were performed with gamma criteria of 3%/3 mm, 2%/2 mm, 1%/2 mm and 2%/1 mm for each VMAT plan with and without beam interruptions. The VMAT plans were reconstructed with log files recorded during delivery and the dose-volumetric parameters were calculated for each reconstructed plan. The differences among dose-volumetric parameters due to the beam interruptions were calculated.ResultsThe changes in global gamma passing rates with various gamma criteria were less than 1.6% on average, while the changes in local gamma passing rates were less than 5.3% on average. The dose-volumetric parameter changes for the target volumes of prostate and H&N VMAT plans due to beam interruptions were less than 0.72% and 1.5% on average, respectively.ConclusionThe delivered dose distributions with up to 8 beam interruptions per an arc were clinically acceptable, showing minimal changes in both gamma passing rates and dose-volumetric parameters.     

Accuracy of dose calculation on iterative CBCT for head and neck radiotherapy

PurposeTo evaluate the feasibility of the use of iterative cone-beam computed tomography (CBCT) for dose calculation in the head and neck region.MethodsThis study includes phantom and clinical studies. All acquired CBCT images were reconstructed with Feldkamp–Davis–Kress algorithm-based CBCT (FDK-CBCT) and iterative CBCT (iCBCT) algorithm. The Hounsfield unit (HU) consistency between the head and body phantoms was determined in both reconstruction techniques. Volumetric modulated arc therapy (VMAT) plans were generated for 16 head and neck patients on a planning CT scan, and the doses were recalculated on FDK-CBCT and iCBCT with Anisotropic Analytical Algorithm (AAA) and Acuros XB (AXB). As a comparison of the accuracy of dose calculations, the absolute dosimetric difference and 1%/1 mm gamma passing rate analysis were analyzed.ResultsThe difference in the mean HU values between the head and body phantoms was larger for FDK-CBCT (max value: 449.1 HU) than iCBCT (260.0 HU). The median dosimetric difference from the planning CT were <1.0% for both FDK-CBCT and iCBCT but smaller differences were found with iCBCT (planning target volume D_50%: 0.38% (0.15–0.59%) for FDK-CBCT, 0.28% (0.13–0.49%) for iCBCT, AAA; 0.14% (0.04–0.19%) for FDK-CBCT, 0.07% (0.02–0.20%) for iCBCT). The mean gamma passing rate was significantly better in iCBCT than FDK-CBCT (AAA: 98.7% for FDK-CBCT, 99.4% for iCBCT; AXB: 96.8% for FDK_CBCT, 97.5% for iCBCT).ConclusionThe iCBCT-based dose calculation in VMAT for head and neck cancer was accurate compared to FDK-CBCT.     

Initial clinical experience with Epid-based in-vivo dosimetry for VMAT treatments of head-and-neck tumors

We evaluated an EPID-based in-vivo dosimetry algorithm (IVD) for complex VMAT treatments in clinical routine. 19 consecutive patients with head-and-neck tumors and treated with Elekta VMAT technique using Simultaneous Integrated Boost strategy were enrolled. In-vivo tests were evaluated by means of (i) ratio R between daily in-vivo isocenter dose and planned dose and (ii) γ-analysis between EPID integral portal images in terms of percentage of points with γ-value smaller than one (γ_%) and mean γ-values (γ_mean), using a global 3%–3 mm criteria. Alert criteria of ±5% for R ratio, γ_% < 90% and γ_mean > 0.67 were chosen. A total of 350 transit EPID images were acquired during the treatment fractions. The overall mean R ratio was equal to 1.002 ± 0.019 (1 SD), with 95.9% of tests within ±5%. The 2D portal images of γ-analysis showed an overall γ_mean of 0.42 ± 0.16 with 93.3% of tests within alert criteria, and a mean γ% equal to 92.9 ± 5.1% with 85.9% of tests within alert criteria. Relevant discrepancies were observed in three patients: a set-up error was detected for one patient and two patients showed major anatomical variations (weight loss/tumor shrinkage) in the second half of treatment. The results are supplied in quasi real-time, with IVD tests displayed after only 1 minute from the end of arc delivery. This procedure was able to detect when delivery was inconsistent with the original plans, allowing physics and medical staff to promptly act in case of major deviations between measured and planned dose.     

An in-vivo dosimetry procedure for Elekta step and shoot IMRT

PurposeThe aim of this work was to extend an in-vivo dosimetry (IVD) method, previously developed by the authors for 3D-conformal radiotherapy, to step and shoot IMRT treatments for pelvic tumors delivered by Elekta linacs.Materials and methodsThe algorithm is based on correlation functions to convert EPID transit signals into in-vivo dose values at the isocenter point, D_iso. The EPID images were obtained by the so-called “IMRT Dosimetric Weighting” mode as a superposition of many segment fields. This way each integral dosimetric image could be acquired in about 10 s after the end of beam delivery and could be processed while delivering the successive IMRT beams. A specific algorithm for D_iso reconstruction especially featured for step and shoot IMRT was implemented using a fluence inhomogeneity index, FI, introduced to describe the degree of beam modulation with respect to open beams. A γ-analysis of 2D-EPID images obtained day to day, resulted rapid enough to verify the plan delivery reproducibility.ResultsFifty clinical IMRT beams, planned for patients undergoing radiotherapy of pelvic tumors, were used to irradiate a homogeneous phantom. For each beam the agreement between the reconstructed dose, D_iso, and the TPS computed dose, D_iso,TPS, was well within 5%, while the mean ratio R = D_iso/D_iso,TPS resulted for 250 tests equal to 1.006 ± 0.036. The same beams were checked in vivo, i.e. during patient treatment delivery, obtaining 500 tests whose average R ratio resulted equal to 1.011 ± 0.042. The γ-analysis of the EPID images with 5% 3 mm criteria supplied 85% of the tests with pass rates γ_mean ≤ 0.5 and P_γ<1 ≥ 90%.     

Incorrect dosimetric leaf separation in IMRT and VMAT treatment planning: Clinical impact and correlation with pretreatment quality assurance

PurposeDynamic treatment planning algorithms use a dosimetric leaf separation (DLS) parameter to model the multi-leaf collimator (MLC) characteristics. Here, we quantify the dosimetric impact of an incorrect DLS parameter and investigate whether common pretreatment quality assurance (QA) methods can detect this effect.Methods16 treatment plans with intensity modulated radiation therapy (IMRT) or volumetric modulated arc therapy (VMAT) technique for multiple treatment sites were calculated with a correct and incorrect setting of the DLS, corresponding to a MLC gap difference of 0.5 mm. Pretreatment verification QA was performed with a bi-planar diode array phantom and the electronic portal imaging device (EPID). Measurements were compared to the correct and incorrect planned doses using gamma evaluation with both global (G) and local (L) normalization. Correlation, specificity and sensitivity between the dose volume histogram (DVH) points for the planning target volume (PTV) and the gamma passing rates were calculated.ResultsThe change in PTV and organs at risk DVH parameters were 0.4–4.1%. Good correlation (>0.83) between the PTV_mean dose deviation and measured gamma passing rates was observed. Optimal gamma settings with 3%L/3 mm (per beam and composite plan) and 3%G/2 mm (composite plan) for the diode array phantom and 2%G/2 mm (composite plan) for the EPID system were found. Global normalization and per beam ROC analysis of the diode array phantom showed an area under the curve <0.6.ConclusionsA DLS error can worsen pretreatment QA using gamma analysis with reasonable credibility for the composite plan. A low detectability was demonstrated for a 3%G/3 mm per beam gamma setting.     

Trajectory log file sensitivity: A critical analysis using DVH and EPID

Aim

The aim of this study was to investigate the sensitivity of the trajectory log file based quality assurance to detect potential errors such as MLC positioning and gantry positioning by comparing it with EPID measurement using the most commonly used criteria of 3%/3?mm.

EPID in vivo dosimetry in RapidArc technique

Aim

The aim of the study was to estimate the dose at the reference point applying an aSi-EPID device in the course of patient treatment.

Materials and methods

The method assumes direct proportionality between EPID signal and dose delivered to the patient reference point during the treatment session. The procedure consists of treatment plan calculation for the actual patient in the arc technique. The plan was realized with an elliptic water-equivalent phantom. An ionization chamber inside the phantom measured the dose delivered to the reference point. Simultaneously, the EPID matrix measured the CU distribution. EPID signal was also registered during patient irradiation with the same treatment plan. The formula for in vivo dose calculation was based on the CU(g) function, EPID signal registered during therapy and the relation between the dose and EPID signal level measured for the phantom. In vivo dose was compared with dose planned with the treatment planning system.Irradiation was performed with a Clinac accelerator by Varian Medical Systems in the RapidArc technique. The Clinac was equipped with an EPID matrix (electronic portal image device) of aSi-1000. Treatment plans were calculated with the Eclipse/Helios system. The phantom was a Scanditronix/Wellhöfer Slab phantom, and the ionization chamber was a 0.6 ccm PTW chamber.

Results

In vivo dose calculations were performed for five patients. Planned dose at the reference point was 2 Gy for each treatment plan. Mean in vivo dose was in the range of 1.96–2.09.

Conclusions

Our method was shown to be appropriate for in vivo dose evaluation in the RapidArc technique.     

Monte Carlo calculations of radiotherapy dose in “homogeneous” anatomy

Given the substantial literature on the use of Monte Carlo (MC) simulations to verify treatment planning system (TPS) calculations of radiotherapy dose in heterogeneous regions, such as head and neck and lung, this study investigated the potential value of running MC simulations of radiotherapy treatments of nominally homogeneous pelvic anatomy. A pre-existing in-house MC job submission and analysis system, built around BEAMnrc and DOSXYZnrc, was used to evaluate the dosimetric accuracy of a sample of 12 pelvic volumetric arc therapy (VMAT) treatments, planned using the Varian Eclipse TPS, where dose was calculated with both the Analytical Anisotropic Algorithm (AAA) and the Acuros (AXB) algorithm. In-house TADA (Treatment And Dose Assessor) software was used to evaluate treatment plan complexity, in terms of the small aperture score (SAS), modulation index (MI) and a novel exposed leaf score (ELS/ELA). Results showed that the TPS generally achieved closer agreement with the MC dose distribution when treatments were planned for smaller (single-organ) targets rather than larger targets that included nodes or metastases. Analysis of these MC results with reference to the complexity metrics indicated that while AXB was useful for reducing dosimetric uncertainties associated with density heterogeneity, the residual TPS dose calculation uncertainties resulted from treatment plan complexity and TPS model simplicity. The results of this study demonstrate the value of using MC methods to recalculate and check the dose calculations provided by commercial radiotherapy TPSs, even when the treated anatomy is assumed to be comparatively homogeneous, such as in the pelvic region.     

Three-dimensional printer-generated patient-specific phantom for artificial in vivo dosimetry in radiotherapy quality assurance

Pretreatment intensity-modulated radiotherapy quality assurance is performed using simple rectangular or cylindrical phantoms; thus, the dosimetric errors caused by complex patient-specific anatomy are absent in the evaluation objects. In this study, we construct a system for generating patient-specific three-dimensional (3D)-printed phantoms for radiotherapy dosimetry. An anthropomorphic head phantom containing the bone and hollow of the paranasal sinus is scanned by computed tomography (CT). Based on surface rendering data, a patient-specific phantom is formed using a fused-deposition-modeling-based 3D printer, with a polylactic acid filament as the printing material. Radiophotoluminescence glass dosimeters can be inserted in the 3D-printed phantom. The phantom shape, CT value, and absorbed doses are compared between the actual and 3D-printed phantoms. The shape difference between the actual and printed phantoms is less than 1 mm except in the bottom surface region. The average CT value of the infill region in the 3D-printed phantom is −6 ± 18 Hounsfield units (HU) and that of the vertical shell region is 126 ± 18 HU. When the same plans were irradiated, the dose differences were generally less than 2%. These results demonstrate the feasibility of the 3D-printed phantom for artificial in vivo dosimetry in radiotherapy quality assurance.