SU‐C‐201‐02: Dosimetric Verification of SBRT with FFF‐VMAT Using a 3‐D Radiochromic/Optical‐CT Dosimetry System

Abstract

Purpose:With an increasing use of small field size and high dose rate irradiation in the advances of radiotherapy techniques, such as stereotactic body radiotherapy (SBRT) and stereotactic radiosurgery (SRS), an in‐depth quality assurance (QA) system is required. The purpose of this study is to investigate a high resolution optical CT‐based 3D radiochromic dosimetry system for SBRT with intensity modulated radiotherapy (IMRT) and flattening filter free (FFF) volumetric modulated arc therapy (VMAT).Methods:Cylindrical PRESAGE radiochromic dosimeters of 10cm height and 11cm diameter were used to validate SBRT. Four external landmarks were placed on the surface of each dosimeter to define the isocenter of target. SBRT plans were delivered using a Varian TrueBeam™ linear accelerator (LINAC). Three validation plans, SBRT with IMRT (6MV 600MU/min), FFF‐VMAT (10MV 2400MU/min), and mixed FFF‐VMAT (6MV 1400MU/min, 10MV 2400MU/min), were delivered to the PRESAGE dosimeters. Each irradiated PRESAGE dosimeter was scanned using a single laser beam optical CT scanner and reconstructed with a 1mm × 1mm high spatial resolution. The comparison of measured dose distributions of irradiated PRESAGE dosimeters to those calculated by Pinnacle3 treatment planning system (TPS) were performed with a 10% dose threshold, 3% dose difference (DD), and 3mm distance‐to‐agreement (DTA) Gamma criteria.Results:The average pass rates for the gamma comparisons between PRESAGE and Pinnacle3 in the transverse, sagittal, coronal planes were 94.6%, 95.9%, and 96.4% for SBRT with IMRT, FFF‐VMAT, and mixed FFF‐VMAT plans, respectively. A good agreement of the isodose distributions of those comparisons were shown at the isodose lines 50%, 70%, 80%, 90% and 98%.Conclusion:This study demonstrates the feasibility of the high resolution optical CT‐based 3D radiochromic dosimetry system for validation of SBRT with IMRT and FFF‐VMAT. This dosimetry system offers higher precision QA with 3D dose information for small beams compared to what is currently available.