Prospective Study of Ultrahypofractionated Proton Beam Therapy for Localized Prostate Cancer

The goal of this prospective observational cohort study is to evaluate the long-term efficacy and safety of ultrahypofractionated proton beam therapy in patients with localized prostate cancer and to identify clinical and treatment-related factors associated with oncologic outcomes and treatment-related toxicity. The main questions it aims to answer are: What are the long-term biochemical recurrence-free survival, clinical progression-free survival, overall survival, and acute and late treatment-related toxicity outcomes following ultrahypofractionated proton beam therapy? Which clinical characteristics, treatment planning parameters, and patient-reported outcomes are predictive of long-term oncologic outcomes and treatment-related toxicity? Participants will: Receive protocol-based ultrahypofractionated proton beam therapy (35-40 Gy\[RBE\] in 5 fractions). Undergo routine clinical evaluation, laboratory testing including prostate-specific antigen (PSA), and imaging studies according to the institutional follow-up schedule. Complete patient-reported outcome questionnaires (EPIC-CP and Visual Analog Scale \[VAS\]) before treatment and during follow-up. Be followed prospectively to assess biochemical recurrence, clinical progression, survival outcomes, and acute and late treatment-related toxicities.

Inclusion Criteria: * Histologically confirmed localized adenocarcinoma of the prostate. * No evidence of distant metastasis (M0). * Eastern Cooperative Oncology Group (ECOG) performance status of 0-2. * Eligible to receive definitive image-guided ultrahypofractionated pencil beam scanning proton beam therapy according to the study protocol. * Ability to complete study questionnaires and comply with scheduled follow-up assessments. * Provision of written informed consent before study enrollment. Exclusion Criteria: * Residual or recurrent prostate cancer following prior radical prostatectomy. * Previous pelvic radiotherapy. * Evidence of distant metastatic disease. * Active malignancy within the previous 1 year, except adequately treated basal cell carcinoma or squamous cell carcinoma of the skin, carcinoma in situ of the cervix, or completely resected T1N0M0 malignancies. * Uncontrolled psychiatric illness, severe traumatic brain injury, chronic alcohol or substance dependence, or other medical conditions that, in the investigator's judgment, would preclude safe participation or adequate follow-up. * Inability or unwillingness to provide written informed consent or comply with study procedures.