Pretreatment quality assurance of volumetric modulated arc therapy on patient CT scan using indirect 3D dosimetry system

Purpose: Aim of this study is to clinically implement the COMPASS 3D dosimetry system for pretreatment quality assurance of volumetric modulated arc therapy (VMAT-RapidArc) treatment plans. Methods: For this study, 10 head and neck (H&N) and pelvis VMAT plans dose response from Linac was measured using COMPASS system along with MatriXXEvolution and 3D dose reconstructed in the patient computed tomography (CT) scan. Dose volume histograms and 3D gamma were used to evaluate difference between the measured and calculated values. In order to validate the COMPASS system, dose response for open fields were acquired for both homogeneous and inhomogeneous phantoms. Results: The average dose difference between Eclipse treatment planning system (TPS) calculated and COMPASS measured (homogenous medium) in normalization region, inner region, penumbra region and buildup region was less than ±2%. In inhomogeneous phantom, there was a maximum difference -3.17% in lung, whereas the difference other densities was within ±2%. The systematic increase in the average 3D gamma between the TPS calculated and COMPASS measured for VMAT plans with known dose errors and multi-leaf collimator (MLC) offset errors shows that COMPASS system was sensitive enough to find clinical significant errors. The 3D dose parameters (D95, D1, and average dose) of all H&N and pelvis patients were well within the clinically acceptable tolerance level of ±5%. The average 3D gammas for planning target volumes (PTV) and organ at risks (OAR) of the patients were less than 0.6. Conclusion: The results from this study show that COMPASS along with MatriXXEvolution can be effectively used for pretreatment verification of VMAT plans in the patient anatomy.