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Pancreatic cancer is one of the deadliest cancer types with less than a 10% 3-year survival rate (SR). Radiotherapy (RT) is critical for achieving local control and enabling dose escalation, which can double and triple the 2-year and 3-year SR. However, intrafraction organs at risk (OAR) and target motion cause considerable uncertainty in dose delivery. Motion management, such as via beam gating triggered by ultrasound (US) guidance, can mitigate these uncertainties without the need for large safety margins that increase OAR dose and toxicity. We propose a novel motion management technique for pancreatic cancer radiotherapy called FLEX-RT, which uses real-time US images acquired by the flexible array transducer (FLEX). We created a motion phantom for simulating the pancreas' motion. A realistic model of the pancreas and duodenum was floated in a water tank over two straps that were connected to a respiratory motion platform with alternating motion. To evaluate the head of the pancreas, we compared the measured motion from the generic L7-4 probe (reference) with the FLEX. We then virtually placed FLEX on the patient's body in the CT scans of ten pancreatic cancer patients treated at our institution. To evaluate the feasibility of FLEX for motion tracking, we compared the dose distribution for (i) without FLEX, (ii) with FLEX and beam avoidance, and (iii) with FLEX and without beam avoidance. Both L7-4 and FLEX could successfully track the induced motion, with similar amplitude and cross-correlation of 0.76. The dosimetric analysis showed that FLEX can be used directly during treatment, with minimal impact on dose distribution. Not accounting for the probe caused a minor PTV coverage drop ranging from 1% to 3%. FLEX-RT, RT with FLEX-enabled motion management, is a novel and feasible solution for the longstanding problem of high motion uncertainty of pancreatic cancer RT.
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