Pre-PROTO-PRIME: stereotactic body proton therapy to the prostate and pelvic nodes for high-risk and node-positive prostate cancer - feasibility, acute toxicity and dosimetric insights.

[PURPOSE] This study aimed to assess the clinical feasibility, acute toxicity, and dosimetric advantages of stereotactic body proton therapy (SBPT) to the prostate and pelvic lymph nodes in high-risk and node-positive prostate cancer.

[METHODS] This multicentric analysis included 26 patients treated from February 2024 to February 2025 with SBPT to the prostate and pelvis delivered in five fractions. The doses prescribed were 36.25 CGE to the prostate, 35 CGE to the gross nodes and 25 CGE to the pelvic nodal CTV. Acute genitourinary (GU) and gastrointestinal (GI) toxicities were evaluated up to three months post-treatment using CTCAE v5.0 criteria. Dosimetric parameters of SBPT plans were compared with matched photon-based SBRT plans, generated using identical datasets. Statistical analyses were performed using SPSS (IBM v30) and Python (v3.10).

[RESULTS] No grade 3 toxicities were observed. Grade 2 GU toxicity occurred in 27 % of patients, predominantly characterized by dysuria and urinary frequency. Grade 2 GI toxicity was observed in 3.8 % of patients. SBPT demonstrated comparable target coverage to SBRT, both for the prostate (median D97%: 98.4 % vs 98.8 %) and the nodal CTV (median D97%: 97.9 % vs 98.9 %), while significantly improving sparing of organs-at-risk (OAR), notably the bowel bag (median V14: 123 cc vs 422 cc; p < 0.001), rectum (median V20: 11 % vs 26 %; p < 0.0001), and bladder (median V20: 16 % vs 21 %; p = 0.004).

[CONCLUSION] SBPT to the prostate and pelvic lymph nodes is clinically feasible, with low acute toxicity and superior OAR sparing compared to photon-based SBRT. The dosimetric data from this study were used to propose volumetric dose constraints for SBPT planning of the prostate and pelvis. These preliminary results substantiate the rationale for the prospective PROTO-PRIME trial, which will investigate the clinical benefits of SBPT relative to SBRT in this patient population.