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.

Localized prostate cancer can be effectively treated with external beam radiotherapy (EBRT). Ultrahypofractionated radiotherapy has become an established treatment option because of its favorable radiobiological characteristics, shorter treatment duration, and improved patient convenience. Owing to the low alpha/beta ratio of prostate cancer, delivery of larger doses per fraction may provide radiobiological advantages while maintaining excellent tumor control.

Proton beam therapy (PBT) offers unique dosimetric advantages over photon radiotherapy by reducing unnecessary radiation exposure to surrounding normal tissues through the Bragg peak effect. These characteristics may be particularly beneficial for ultrahypofractionated treatment, especially in patients with intermediate- or high-risk disease who require treatment of larger target volumes, including the seminal vesicles or pelvic lymph nodes. However, prospective data regarding the long-term efficacy, safety, patient-reported outcomes, and predictive factors associated with ultrahypofractionated proton beam therapy remain limited.

This is a prospective, single-center observational cohort study conducted at the National Cancer Center, Republic of Korea. Approximately 50 adult patients with histologically confirmed localized prostate adenocarcinoma who are scheduled to undergo definitive ultrahypofractionated proton beam therapy will be prospectively enrolled after providing written informed consent.

All participants will receive protocol-based ultrahypofractionated proton beam therapy consisting of 35-40 Gy(RBE) delivered in five fractions using image-guided proton therapy. Androgen deprivation therapy will be administered according to contemporary clinical guidelines and physician discretion when indicated.

The primary objective of this study is to evaluate the long-term efficacy and safety of ultrahypofractionated proton beam therapy for localized prostate cancer. Secondary objectives include evaluation of biochemical recurrence-free survival, clinical progression-free survival, overall survival, acute and late treatment-related toxicity, and patient-reported quality of life. In addition, the study will prospectively investigate clinical characteristics, treatment planning parameters, dosimetric variables, and patient-reported outcome measures to identify factors associated with oncologic outcomes and treatment-related toxicity.

Clinical information, treatment planning data, physician-reported adverse events, laboratory findings including prostate-specific antigen (PSA), imaging studies, and patient-reported outcome measures using the Expanded Prostate Cancer Index Composite for Clinical Practice (EPIC-CP) and the Visual Analog Scale (VAS) will be prospectively collected. Patients will be followed every three months during the first two years, every six months during years three and four, and annually thereafter according to institutional follow-up practice.

The findings from this prospective cohort study are expected to provide high-quality evidence regarding the clinical effectiveness and safety of ultrahypofractionated proton beam therapy and to facilitate the development of individualized treatment strategies for patients with localized prostate cancer by identifying predictors of long-term oncologic outcomes and treatment-related toxicity.

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.

Only individuals who are biologically male are eligible because prostate cancer occurs in the prostate gland, a male-specific organ.