Development of the Independent Dose Verification Method for the Ring Gantry PET/CT Linac

Abstract

The RefleXion Xi is a new external beam radiotherapy delivery modality combining kilovoltage fan-beam CT and on-board PET for Biology-guided Radiotherapy. Although the machine shares similar components with the Tomotherapy machine, it has unique hardware features such as stationary beam delivery system with couch increments. The goal of this study is to develop an in-house independent secondary dose calculation method for this ring gantry PET/CT Linac. The method aggregates the beam intensities at each discrete firing gantry angle and couch moving position. The non-uniform intensity map is decomposed into a series of segments of uniform beam intensities, and then coordinates of beam segments were modified based on the relative distance from the dose calculation point to keep the calculation point in the same spot along the gantry rotation. The dose to the calculation point of each segment is determined by using measured tissue-maximum-ratio, output factors, and off-axis ratio by independent binary collimators. Two-dimensional convolution method is applied to integrate all dose contribution components in segment areas for efficient calculation. The final dose is obtained by summing all dose to the calculation point of segments for all firing gantry angle and couch position. Twenty patients with different treatment sites including head and neck, prostate, and lung regions were analyzed. Comparison of the point dose calculated by the independent program to that calculated by the planning system has shown reasonable agreement within ±5%. The independent dose verification program has been developed as an initial patient specific QA for improving patient safety. Further study will be performed to enhance the accuracy and reliability by including additional leakage and scatter models.