SU‐E‐T‐173: Patient‐Specific IMRT QA for Large Volume CyberKnife Plans Using MatriXX

Purpose: The recent introduction of CyberKnife M6 FM and FIM models expands the field of robotic radiotherapy into the domain of large volume tumors. The increased complexity of CyberKnife plans with MLC requires at least 2D dosimetric verification in a phantom before the patient is treated. We describe a protocol for IMRT QA using an array of ionization chambers (MatriXX, IBA) for IMRT QA of large‐target CyberKnife plans. Methods: We developed post‐processing software that reads the dose frame sequence recorded by the MatriXX and applies a number of CyberKnife‐specific correction factors to each frame. Individual beams are located within the frames and angular correction factors are applied depending on beam angle. In addition, the background evolution is tracked in the “beam‐off” frames which are subsequently subtracted from the “beam‐on” frames according to a predictive model. Results: The clinical passing rates are significantly improved when the per‐frame analysis software was introduced in our IMRT QA procedure. We present a number of cases, for which the passing rates are increased from an average 80% in the raw data to 95% after correction. The MatriXX is found to have additional advantages, for it allows vertex beams, which are often part of the predefined paths in the planning software. Conclusion: The per‐beam correction strategy described in this work is found to improve the passing rates of CyberKnife plans designed for target volumes in excess of 40cc. Such large target volumes will be commonly treated with the new MLC enabled CyberKnife robot. Point dose verification remains a viable option for smaller targets, for which the spatial averaging in the MatriXX chambers hinders the performance of the device.