A Dosimetric Evaluation of the Feasibility and Robustness of Ultra-hypofractionated Carbon Ion Radiotherapy Planning for the Treatment of Prostate Cancer

Background/Aim: This study investigated the dosimetric feasibility and robustness of treatment planning for ultra-hypofractionated carbon ion radiotherapy (CIRT) for prostate cancer, in which the dose to urethra was reduced. Patients and Methods: The prescribed dose of 40 Gy (relative biological effectiveness: RBE) was given in 4 fractions using dose constraints based on X-ray stereotactic body radiotherapy studies: urethra Dmax <38 Gy (RBE), bladder V40 <10 cc, and rectum D2cc <38 Gy (RBE). The treatment planning method for ultra-hypofractionated CIRT was developed using the simulation computed tomography (simCT) of one patient. In three of the four irradiations, the prostate and proximal seminal-vesicle were irradiated. In the remaining irradiation, a patched-field technique was used to spare the dose to the urethra. The feasibility of the treatment planning method was evaluated using the simCTs of eight additional patients. Robustness against inter-fractional motion was also verified by comparing the dose distributions of the simCTs with composite dose distributions from the daily treatment CTs of nine patients. Results: A urethra-sparing method for ultra-hypofractionated CIRT was developed using one patient. The treatment plans created for the eight other patients met the dose constraints. The median maximum dose to urethra was 35.31 Gy (RBE), the V40 of bladder was 2.6 cc, and the D2cc of the rectum was 33.67 Gy (RBE). In the robustness assessment, the dose distributions calculated using the treatment-day CTs met the dose constraints for eight of nine patients, with the other patient having a rectum D2cc of 38.85 Gy (RBE). When a hydrogel spacer was assumed, all the dose constraints were satisfied. Conclusion: Ultra-hypofractionated CIRT treatment plans for prostate cancer can be generated to meet dose constraints and have sufficient robustness against inter-fractional motion.