3-D Quality Assurance in CyberKnife Radiotherapy Using a Novel N-(3-methoxypropyl) Acrylamide Polymer Gel Dosimeter and Optical CT

In radiotherapy, dosimetry tools are needed to record and verify complex 3-D dose distributions. A novel composition of polymer gel dosimeter based on radiation-induced polymerization of N-(3-methoxypropyl) acrylamide (NMPA) is presented and studied as a 3-D dose verification quality assurance tool (QA) in radiotherapy treatment planning. A 160 mm diameter crystal glass round bottom flask (CrystalBall) containing NMPA gel was used as a phantom. The gel was irradiated by a CyberKnife robotic radiotherapy system and scanned optically with a red light source using an optical computed tomography OCT scanner with sub-millimeter spatial resolution. Additionally, a commercial polymer gel dosimeter of BANG-GEL-QA™ was irradiated and scanned to compare its performance with that of the novel gel dosimeter. The dose distributions of a CyberKnife robotic SRS/SBRT treatment planning system (TPS) for a patient with brain metastasis were compared to the measurements of both polymers using 2-D and 3-D gamma analysis with a criteria of 5% dose difference and 2 mm distance-to-agreement. A gamma pass rate of 95% was calculated within a diameter of at least 32 mm for the 2-D and 3-D gamma analysis of both polymer gels. This suggests that the novel NMPA polymer dosimeter can provide an accurate 3-D verification of complex dose distribution in SRS/SBRT radiotherapy treatments. This study demonstrates that the newly developed CrystalBall NMPA phantom can provide a promising 3-D quality assurance tool for radiation therapy.