Ultra-hypofractionated Carbon-ion Therapy for Prostate Cancer

STUDY DESIGN This study is a prospective, single-arm clinical study designed to evaluate the safety of an ultra-hypofractionated carbon ion radiotherapy (CIRT) regimen in patients with localized prostate cancer. A total of 20 participants will be enrolled sequentially at a single institution. An initial cohort of 10 participants will be enrolled first. If fewer than 2 participants develop Grade ≥3 treatment-related acute toxicity within 90 days of completing CIRT, an additional 10 participants will be enrolled to reach the total sample size of 20. Participants will receive CIRT according to the following procedures. 1. Carbon Ion Radiotherapy : CIRT will be administered at 7 GyE per fraction, twice weekly on alternating days, for a total of 6 fractions over 3 weeks (total prescribed dose: 42 GyE). At the investigator's discretion, a simultaneous integrated boost (SIB) of 6 GyE × 6 fractions (total 36 GyE) may be prescribed to the seminal vesicle target. Treatment planning will be performed using RayStation with consideration of dose constraints for the rectum and bladder. 2. Androgen Deprivation Therapy (ADT) : ADT will not be administered to low-risk and favorable intermediate-risk patients. Unfavorable intermediate-risk patients will receive ADT concurrently for a total of 6 months, with the first dose completed prior to CIRT initiation. Permitted agents include GnRH agonists (Leuprolide, Goserelin, Triptorelin), GnRH antagonists (Degarelix, Relugolix), and anti-androgens (Bicalutamide, Flutamide, Nilutamide). 3. Permitted Procedures : Insertion of gold fiducial markers for image-guided radiotherapy and SpaceOAR hydrogel for rectal protection are permitted as part of the institutional standard of care and are not considered part of the study intervention.