This phase II trial studies the side effects and how well hypofractionated proton beam radiation therapy works in treating patients with prostate cancer that has not spread to nearby lymph nodes or to other parts of the body. Specialized radiation therapy, such as proton beam radiation therapy, that delivers a high dose of radiation directly to the tumor may kill more tumor cells and cause less damage to normal tissue.

PRIMARY OBJECTIVES:

I. Estimate the incidence of grade 2 gastrointestinal toxicity following the proposed treatment regimens at 2 years post-treatment.

SECONDARY OBJECTIVES:

I. Estimate the change in health related quality of life (HRQOL) following the proposed treatment regimen compared to pre-treatment assessment as defined by Expanded Prostate Cancer Index Composite (EPIC), Utilization of Sexual Medications/Devices, and Medical history/conditions questionnaire.

II. Estimate the rates of acute toxicity of the treatment regimens. III. Estimate the rates of late toxicity at 3, 4, and 5 years post-treatment. IV. Assess the efficacy of hypo-fractionated proton beam therapy, defined by the incidence of a rising prostate-specific antigen (PSA) at 5 years.

V. Determine the rate of local failure by biopsy of the prostate when objective tests, prostate-specific antigen (PSA), magnetic resonance imaging (MRI), digital rectal exam (DRE), suggest relapse.

OUTLINE:

Patients undergo proton beam radiation therapy in 15 fractions over 5-6 weeks.

After completion of study treatment, patients are followed up every 6-12 months for 24 months and then annually until month 60.

Inclusion Criteria:

• Histologically confirmed diagnosis of adenocarcinoma of the prostate within one year of study entry; evaluation can happen outside of MD Anderson as long as histological confirmation takes place at MD Anderson
• History/physical examination with digital rectal examination of the prostate within 90 days prior to registration
• Histological evaluation of prostate biopsy with assignment of a Gleason score to the biopsy material demonstrating Gleason score 2-7 within 365 days of registration
• Clinical stage T1-2b (American Joint Committee on Cancer [AJCC] 7th edition) and PSA < 20 ng/mL within 90 days prior to registration; PSA should not be obtained within 10 days after prostate biopsy
• Zubrod performance status 0-1 within 90 days prior to registration
• Patient must be able to provide study-specific informed consent prior to study entry
• Willingness and ability to complete the EPIC questionnaire

Exclusion Criteria:

• Prior or concurrent invasive malignancy (except non-melanomatous skin cancer or lymphomatous/hematogenous malignancy) unless continually disease free for a minimum of 5 years
• Evidence of distant metastases
• Regional lymph node involvement
• Previous prostatectomy, cryosurgery, or high intensity focused ultrasound (HIFU) for prostate cancer
• Previous pelvic radiation or prostate brachytherapy
• Active and severe medical co-morbidity defined as follows: unstable angina and/or congestive heart failure requiring hospitalization within the last 6 months, transmural myocardial infarction within the last 6 months, acute bacterial or fungal infection requiring intravenous antibiotics at the time of registration, chronic obstructive pulmonary disease exacerbation or other respiratory illness requiring hospitalization or precluding study therapy at the time of registration, hepatic insufficiency resulting in clinical jaundice, active inflammatory bowel disease (Crohn's disease or ulcerative colitis), diagnosed connective tissue disorder, or congenital coagulation defects (patients on medical therapy with Coumadin or other blood thinning agents are eligible for participation)