Assessment Of Daily Dose Accumulation For Robustly Optimized Intensity Modulated Proton Therapy Treatment Of Prostate Cancer

Purpose: To implement a daily CBCT based dose accumulation technique in order to assess ideal robust optimization (RO) parameters for IMPT treatment of prostate cancer.Methods: Ten prostate cancer patients previously treated with VMAT and having daily CBCT were included. First, RO-IMPT plans were created with +/- 3 mm and +/- 5 mm patient setup and +/- 3% proton range uncertainties, respectively. Second, the planning CT (pCT) was deformably registered to the CBCT to create a synthetic CT (sCT). Both daily and weekly sampling strategies were employed to determine optimal dose accumulation frequency. Doses were recalculated on sCTs for both +/- 3 mm/+/- 3% and +/- 5 mm/+/- 3% uncertainties and were accumulated back to the pCT. Accumulated doses generated from +/- 3 mm/+/- 3% and +/- 5 mm/+/- 3% RO-IMPT plans were evaluated using the clinical dose volume constraints for CTV, bladder, and rectum.Results: Daily accumulated dose based on both +/- 3mm/+/- 3% and +/- 5 mm/+/- 3% uncertainties for RO-IMPT plans resulted in satisfactory CTV coverage (RO IMPT3mm/3% CTVv95 = 99.01 +/- 0.87% vs. RO IMPT5mm/3% CTVV95 = 99.81 +/- 0.2%, P = 0.002). However, the accumulated dose based on +/- 3 mm/3% RO-IMPT plans consistently provided greater OAR sparing than +/- 5 mm/+/- 3% RO-IMPT plans (RO-IMPT3mm/3% rectum(V65Gy) = 2.93 +/- 2.39% vs. RO-IMPT5mm/3% rectum(V65Gy )= 4.38 +/- 3%, P < 0.01; RO-IMPT3mm/3% bladder(V65Gy) = 5.2 +/- 7.12% vs. RO-IMPT5mm/3% bladder(V65Gy) = 7.12 +/- 9.59%, P < 0.01). The gamma analysis showed high dosimetric agreement between weekly and daily accumulated dose distributions.Conclusions: This study demonstrated that for RO-IMPT optimization, +/- 3mm/+/- 3% uncertainty is sufficient to create plans that meet desired CTV coverage while achieving superior sparing to OARs when compared with +/- 5 mm/+/- 3% uncertainty. Furthermore, weekly dose accumulation can accurately estimate the overall dose delivered to prostate cancer patients.