MO‐F‐108‐02: A Retrospective Study On the Potential Clinical Gain of On‐Line Adaptive Re‐Planning for Pancreatic Cancer SBRT

Purpose: Stereotactic body radiotherapy (SBRT) shows promise in unresectable pancreatic cancer with high rates of local control compared to conventionally fractionated radiotherapy. One critical limitation with pancreas SBRT arises from toxicity in the nearby duodenum, which is a dynamic and distensible organ that changes shape daily. The purpose of this pilot study was to evaluate the potential gains of daily adaptive re‐planning with SBRT in pancreatic cancer. Methods: Pancreas SBRT patients were re‐planned using our previously developed GPU‐based supercomputing online re‐planning environment (SCORE), which automatically and quickly generates plans from daily cone‐beam CT (CBCT) data with the original plan and DVH as a reference. For each patient, the tumor volume and normal structure contours were deformed from the original planning CT to each daily CBCT. The deformed contours were reviewed and edited by a physician, and then the plan was re‐optimized to account for daily changes in anatomy. The daily adaptive re‐planned dose was compared to the actual dose received for the target volume and nearby duodenum. Results: Fourteen daily CBCT scans were used to generate daily adaptive radiation plans. For all patients, the average target coverage for the PTV improved, and the cold spot decreased with daily adaptive re‐planning. The cold spot (minimum dose to 1% of PTV) decreased on average from 25.1% to 4.6% (p=0.002, paired t‐test). With daily adaptive re‐planning the hot spot in the duodenum (volume that received 100% of dose) decreased from 2.8% to 1.7% (p=0.09). Doses to other organs were clinically insignificant, and did not significantly change with daily adaptive radiotherapy. Conclusion: This pilot study shows that daily adaptive re‐planning with pancreas SBRT has the potential to improve target coverage, and reduce dose to normal organs. Further study into the technical implementation of daily adaptive re‐planning is ongoing. Varian Medical Systems through a Master Research Agreement.