Purpose: The Radixact is a helical tomotherapy device capable of delivering treatments with intrafraction target tracking, called Synchrony. The system dynamically collimates the radiation to follow target motion during treatment, addressing the effects of the motion by maintaining target dose. However, the radiation is directed without considering location changes of organs at risk (OARs). This study computed the dose to OARs from these motion-synchronized treatments.
Methods: We developed a calculation algorithm that uses the patient’s 4DCT scan as input and accounts for tissue deformations. The patient’s respiratory amplitude throughout treatment was simulated by extracting the amplitude data from the 4DCT scan or assuming a generalized motion pattern (e.g. sin^2). Based on the amplitude at each projection in the helical treatment, MLC leaf shifts and jaw corrections were applied, mimicking the corrections Synchrony applies during treatment. The helical treatment was broken up into 153 sub-projections per gantry rotation, which were each assigned to a corresponding discrete phase of the 4DCT. Dose was calculated on each phase separately and accumulated using a 4D deformable registration algorithm.
Results: The algorithm has been used to predict dose for 4 lung SBRT treatments (10 Gy x 5 fx). Target motion ranged from 14 to 30 mm and PTV volume ranges from 12 to 130 cc. Dose to OARs was observed to deviate by up to 4 Gy between the planned dose and the synchrony-accumulated dose. In future work, the number of cases will be expanded, including upper abdominal cases.
Conclusion: An algorithm for calculating dose from motion-synchronized treatments on Radixact has been developed and demonstrated. The algorithm can be used to predict dose to OARs and identify plans likely to deviate from planned dose when target tracking is activated.