SU‐E‐T‐623: Comparison of Planned and Delivered Doses for Prostate Cancer Using a Deformable Image‐Guided Adaptive Radiation Therapy (IGART) Method

Purpose: To evaluate an IGART scheme for prostate cancer that uses deformable CBCT image registration to track daily delivered dose (accumulated over time) to targets and organs‐at‐risk and allows comparison to the original treatment plan. Methods: Six patients treated with external beam radiation therapy for high risk prostate cancer were evaluated. Daily CBCTs were obtained for setup evaluation for all 45 fractions. Contours were transferred from the planning CT to the Day 1 CBCT to serve as the surrogate planning CT for deformation. Dose was applied automatically to each CBCT based on the recorded shift for that day. The daily CBCTs were deformed back to the first CBCT to sum the dose over the course of treatment. Contours were also deformed from the Day 1 CBCT to each subsequent day to allow for daily DVH evaluation. The accumulated delivered dose (summed from each daily treatment) and the planned dose were compared in terms of min, mean, max doses to structures: bladder, rectum, and PTV. Results: The average percentage differences between planned and delivered doses were: bladder — Dmin (8.2 +/− 13.4), Dmean (7.3 +/− 12.7), Dmax (−4.2 +/− 2.7); rectum — Dmin (−16.4 +/−41.4), Dmean (0.8 +/−9.8), Dmax (−3.2 +/−2.3); PTV — Dmin (−16.7 +/− 12.9), Dmean (−3.8 +/−2.7), Dmax (−4.1 +/− 2.9). Conclusion: An IGART scheme for prostate cancer using daily CBCTs and deformable registration was evaluated and found to be feasible. Further work is planned using a larger cohort of patients to evaluate the impact of dose differences between planned and delivered doses. Aaron Nelson and Jonathon Piper are owners and employees of MIM Software Inc. Sara Pirozzi, Ian Liu, and Zhilei Shen are employees of MIM Software Inc.