D.J.L. Wauben

Reconstruction of high-resolution 3D dose from matrix measurements: error detection capability of the COMPASS correction kernel method

The COMPASS system (IBA Dosimetry) is a quality assurance (QA) tool which reconstructs 3D doses inside a phantom or a patient CT. The dose is predicted according to the RT plan with a correction derived from 2D measurements of a matrix detector. This correction method is necessary since a direct reconstruction of the fluence with a high resolution is not possible because of the limited resolution of the matrix used, but it comes with a blurring of the dose which creates inaccuracies in the dose reconstruction. This paper describes the method and verifies its capability to detect errors in the positioning of a MLC with 10 mm leaf width in a phantom geometry. Dose reconstruction was performed for MLC position errors of various sizes at various locations for both rectangular and intensity-modulated radiotherapy (IMRT) fields and compared to a reference dose. It was found that the accuracy with which an error in MLC position is detected depends on the location of the error relative to the detectors in the matrix. The reconstructed dose in an individual rectangular field for leaf positioning errors up to 5 mm was correct within 5% in 50% of the locations. At the remaining locations, the reconstruction of leaf position errors larger than 3 mm can show inaccuracies, even though these errors were detectable in the dose reconstruction. Errors larger than 9 mm created inaccuracies up to 17% in a small area close to the penumbra. The QA capability of the system was tested through gamma evaluation. Our results indicate that the mean gamma provided by the system is slightly increased and that the number of points above gamma 1 ensures error detection for QA purposes. Overall, the correction kernel method used by the COMPASS system is adequate to perform QA of IMRT treatment plans with a regular MLC, despite local inaccuracies in the dose reconstruction.

Clinical introduction of a linac head-mounted 2D detector array based quality assurance system in head and neck IMRT

BACKGROUND AND PURPOSE IMRT QA is commonly performed in a phantom geometry but the clinical interpretation of the results in a 2D phantom plane is difficult. The main objective of our work is to move from film measurement based QA to 3D dose reconstruction in a patient CT scan. In principle, this could be achieved using a dose reconstruction method from 2D detector array measurements as available in the COMPASS system (IBA Dosimetry). The first step in the clinical introduction of this system instead of the currently used film QA procedures is to test the reliability of the dose reconstruction. In this paper we investigated the validation of the method in a homogeneous phantom with the film QA procedure as a reference. We tested whether COMPASS QA results correctly identified treatment plans that did or did not fulfil QA requirements in head and neck (H&N) IMRT. MATERIALS AND METHODS A total number of 24 treatments were selected from an existing database with more than 100 film based H&N IMRT QA results. The QA results were classified as either good, just acceptable or clinically rejected (mean gamma index <0.4, 0.4-0.5 or >0.5, respectively with 3%/3mm criteria). Film QA was repeated and compared to COMPASS QA with a MatriXX detector measurement performed on the same day. RESULTS Good agreement was found between COMPASS reconstructed dose and film measured dose in a phantom (mean gamma 0.83±0.09, 1SD with 1%/1mm criteria, 0.33±0.04 with 3%/3mm criteria). COMPASS QA results correlated well with film QA, identifying the same patients with less good QA results. Repeated measurements with film and COMPASS showed changes in delivery after a modified MLC calibration, also visible in a standard MLC check in COMPASS. The time required for QA reduced by half by using COMPASS instead of film. CONCLUSIONS Agreement of COMPASS QA results with film based QA supports its clinical introduction for a phantom geometry. A standard MLC calibration check is sensitive to <1mm changes that could be significant in H&N IMRT. These findings offer opportunities to further investigate the method based on a 2D detector array to 3D dose reconstruction in a patient anatomy.

Efficient and reliable 3D dose quality assurance for IMRT by combining independent dose calculations with measurements

PURPOSE Advanced radiotherapy treatments require appropriate quality assurance (QA) to verify 3D dose distributions. Moreover, increase in patient numbers demand efficient QA-methods. In this study, a time efficient method that combines model-based QA and measurement-based QA was developed; i.e., the hybrid-QA. The purpose of this study was to determine the reliability of the model-based QA and to evaluate time efficiency of the hybrid-QA method. METHODS Accuracy of the model-based QA was determined by comparison of COMPASS calculated dose with Monte Carlo calculations for heterogeneous media. In total, 330 intensity modulated radiation therapy (IMRT) treatment plans were evaluated based on the mean gamma index (GI) with criteria of 3%∕3mm and classification of PASS (GI ≤ 0.4), EVAL (0.4 < GI > 0.6), and FAIL (GI ≥ 0.6). Agreement between model-based QA and measurement-based QA was determined for 48 treatment plans, and linac stability was verified for 15 months. Finally, time efficiency improvement of the hybrid-QA was quantified for four representative treatment plans. RESULTS COMPASS calculated dose was in agreement with Monte Carlo dose, with a maximum error of 3.2% in heterogeneous media with high density (2.4 g∕cm(3)). Hybrid-QA results for IMRT treatment plans showed an excellent PASS rate of 98% for all cases. Model-based QA was in agreement with measurement-based QA, as shown by a minimal difference in GI of 0.03 ± 0.08. Linac stability was high with an average GI of 0.28 ± 0.04. The hybrid-QA method resulted in a time efficiency improvement of 15 min per treatment plan QA compared to measurement-based QA. CONCLUSIONS The hybrid-QA method is adequate for efficient and accurate 3D dose verification. It combines time efficiency of model-based QA with reliability of measurement-based QA and is suitable for implementation within any radiotherapy department.

Evaluation of DVH-based treatment plan verification in addition to gamma passing rates for head and neck IMRT.

BACKGROUND AND PURPOSE Treatment plan verification of intensity modulated radiotherapy (IMRT) is generally performed with the gamma index (GI) evaluation method, which is difficult to extrapolate to clinical implications. Incorporating Dose Volume Histogram (DVH) information can compensate for this. The aim of this study was to evaluate DVH-based treatment plan verification in addition to the GI evaluation method for head and neck IMRT. MATERIALS AND METHODS Dose verifications of 700 subsequent head and neck cancer IMRT treatment plans were categorised according to gamma and DVH-based action levels. Fractionation dependent absolute dose limits were chosen. The results of the gamma- and DVH-based evaluations were compared to the decision of the medical physicist and/or radiation oncologist for plan acceptance. RESULTS Nearly all treatment plans (99.7%) were accepted for treatment according to the GI evaluation combined with DVH-based verification. Two treatment plans were re-planned according to DVH-based verification, which would have been accepted using the evaluation alone. DVH-based verification increased insight into dose delivery to patient specific structures increasing confidence that the treatment plans were clinically acceptable. Moreover, DVH-based action levels clearly distinguished the role of the medical physicist and radiation oncologist within the Quality Assurance (QA) procedure. CONCLUSIONS DVH-based treatment plan verification complements the GI evaluation method improving head and neck IMRT-QA.

SU‐E‐T‐522: Modeling the Agility MLC for Monte Carlo IMRT and VMAT Calculations

PURPOSE The Agility multileaf collimator (MLC) mounted on an Elekta Synergy linear accelerator for 6 MV was modeled for IMRT and VMAT calculations using the BEAMnrc Monte Carlo (MC) code and verified versus measurements. METHODS To describe the Agility MLC in BEAMnrc, the available Component Module code was modified to include its characteristics; 5 mm leaf width, flat leaf sides with a focus point shifted from the radiation source. The MLC model was verified by comparison of the calculated interleaf leakage and tongue-and-groove effect for a closed MLC field and an irregular field to measurements with EBT2 film in a solid water phantom and diode measurements in a water phantom, respectively. We have developed a time dependent phase space data (PSD), which include a parameter based on MU index. Because leaf, jaw, collimator and gantry positions of each segment are controlled by MU index, this PSD enabled to simulate dynamic motions by interpolating positions between each segment. IMRT and VMAT calculations were compared with film measurements in a solid water phantom to validate the accuracy of the overall MLC model. MC statistical uncertainty was below 2% for all simulations. RESULTS We found a good agreement with our measurements on interleaf leakage. Agreement between mean calculated and measured leaf transmissions with fully opened jaws normalized to the center of a 10×10 cm2 field at the same depth was within 0.1%. Discrepancy between MC calculation and measurement for the irregular field was below 2%/2 mm. The gamma analysis of the comparison of MC and EBT2 film measurements in IMRT and VMAT fields showed 99.1%, 99.5% pass rates with 3%/3 mm criteria, respectively. CONCLUSION The Agility MLC produced by Elekta could be accurately MC modeled with an adaptation in BEAMnrc. The MC model proved to be applicable for IMRT and VMAT calculations. Japan Society for the Promotion of Science (JSPS) Core-to-Core Program.

429 poster AN EFFICIENT AND INDEPENDENT 3D DOSE CALCULATION TOOL FOR QUALITY ASSURANCE OF COMPLEX RADIOTHERAPY TREATMENT PLANS

To validate the model-based 3D dose calculation performed by the COMPASS system, and to evaluate the use of the combination of model-based and measurement-based 3D dose determination for clinical QA purposes.