SU‐E‐CAMPUS‐T‐04: A Delivery Method of Tumor Tracking Radiation Beam Using a Beam Switching Interface for Irregular Breathing

Purpose: Dose rate regulated Tracking (DRRT) is known to be an efficient method which adaptively deliver tracking treatments when patient breathing is irregular. The interface for the dose rate controlling, however, is not commercially available. The Motion Management Interface (MMI, Varian Medical System, CA), which provides beam on/off switching during treatment is available for clinic. This study is to test if delivering the adaptive tumor tracking is feasible for irregular breathing using beam switching with MMI. Methods: Fifty‐five free breathing RPM traces acquired from lung cancer patients are used. The first day RPM traces of the patients are utilized to design preprogrammed tracking MLC patterns, of which periods are intentionally reduced by 20% in order to catch up the variation of patient breathing irregularity in the treatment day. Eligibility criteria for this technique is variation of amplitude and period less than 20%. An algorithm which determines beam on/off every 100 ms by considering the preprogrammed (MLC) positions and current breathing positions is developed. Tracking error and delivery efficiency is calculated by simulating the beam‐switching adaptive tracking from the RPM traces. Results: Breathing patterns of 38 patients (70%) met the eligibility criteria. Tracking errors of all of the cases who meet the criteria are less than 2mm (average 1.4mm) and the average delivery efficiency was 71%. Those of rest of the cases are 1.9 mm and 48%. Conclusions: Beam switching is effectively equivalent to the dose rate regulation for DRRT. Adaptive tracking with beam switching is feasible if patient selection is based on the eligibility criteria. NIH R01CA133539