Machine learning for tracking planned versus delivered dose in pancreas SBRT.

Abstract

561 Background: Cone-beam CT (CBCT) used for daily image guidance in pancreatic stereotactic body radiotherapy (SBRT) is fraught with imaging artifacts leading to poor visualization of target and avoidance structures. Machine learning methods can be used to generate synthetic CT (sCT) images from CBCT to eliminate imaging artifacts. We sought to compare planned versus delivered radiation dose to luminal GI organs at risk (OARs) during pancreas SBRT using sCT. Methods: Previously treated patients from a single institution were eligible if treated with 5 fractions of pancreas SBRT using daily CBCT and inspiration breath hold. CBCT-based sCT was generated for each treatment fraction using our previously reported cycleGAN technique. Physicians manually contoured GI OARs (stomach, duodenum, and bowel [remaining small bowel and large bowel]) on sCTs. Rigid online registration from treatment was used to re-calculate the delivered dose on each sCT. Fractional delivered dose to OARs was compared to planned dose and dose constraints with descriptive statistics. Max dose to 1cc or 20cc of an OAR was defined as D1cc and D20cc respectively. Results: 7 patients and 35 cumulative SBRT CBCT datasets were included. The median SBRT cumulative dose was 8 Gy x 5 fractions (40 Gy total). Each GI OAR was constrained per fraction to D1cc < 7 Gy (cumulative D1cc < 35 Gy) and D20cc < 4 Gy (cumulative D20cc < 20 Gy) during SBRT planning. Table shows the estimated OAR delivered doses on each sCT compared to planned doses and dose constraints. Each patient had at least 1 fraction that exceeded a planned GI OAR dose; 2 patients had delivered dose to 1cc of duodenum exceeding constraint for all 5 fractions. After a median follow-up of 9 months (range 5-34), there were 2 CTCAE Grade 1 and no Grade 2+ GI toxicity events possibly or likely related to SBRT. Conclusions: Physician contouring and dose calculation using CBCT-based sCT was feasible. Estimated delivered doses of pancreas SBRT generally exceed planned doses. Further study of sCT in pancreas SBRT is warranted for improved localization, dose calculation, and adaptive radiotherapy.[Table: see text]