This phase III trial compares the effects of radiation therapy using RapidPlan, trademark, knowledge-based planning to human-driven planning in treating patients with prostate cancer. Radiation therapy uses high energy x-rays, particles, or radioactive seeds to kill tumor cells and shrink tumors. Successful delivery of radiation requires planning to develop a treatment plan for how and where the radiation is to be delivered. RapidPlan is a knowledge-based treatment planning tool that automatically creates an optimal treatment plan based on identified targets and organs at risk for radiation exposure. Human-driven treatment planning by a dosimetrist, the current standard of care, requires significant resources and time and may vary within and among radiation centers. Giving radiation therapy with RapidPlan knowledge-based planning may have similar or less side effects compared to human-driven planning in treating patients with prostate cancer.

PRIMARY OBJECTIVE:

I. To determine if RapidPlan knowledge-based planning is non-inferior to human-driven planning regarding treatment-related rates of grade 3 or higher genitourinary (GU) and gastrointestinal (GI) adverse events at 3 months post-radiotherapy for prostate cancer.

EXPLORATORY OBJECTIVES:

I. After completion of radiation therapy, determine the incidence of:

Ia. Grade 2 or greater GU and GI toxicity at 3 months (Common Terminology Criteria for Adverse Events [CTCAE] version 5); Ib. Quality of life 3 months post-radiotherapy; Ic. Rate of achieving dose-volume constraints. II. Determine if there are any statistical differences in dose-volumes results with RapidPlan knowledge-based planning.

OUTLINE: Patients are randomized to 1 of 2 arms.

ARM A (HUMAN-DRIVEN): Patients receive human-driven treatment planning and undergo radiation therapy over 15-30 minutes on 5-44 fractions per standard of care. Additionally, patients undergo magnetic resonance imaging (MRI) pre-treatment and blood sample collection and positron emission tomography (PET) throughout study.

ARM B (KNOWLEDGE-BASED): Patients receive RapidPlan treatment planning and undergo radiation therapy over 15-30 minutes on 5-44 fractions per standard of care. Additionally, patients undergo MRI pre-treatment and blood sample collection and PET throughout study.

After completion of study treatment, patients are followed up at 3 and 12 months then yearly for up to year 5.

Inclusion Criteria:

• Patients must be at least 18 years old
• Histologically confirmed prostate cancer
• Clinical or pathologic stages T1c-T3b, Nx or N0-1, M0-1 (American Joint Committee on Cancer [AJCC] criteria 8th edition [Ed])
• Planned definitive dose radiotherapy to the prostate or prostate bed
• Eastern Cooperative Oncology Group (ECOG) performance status 0-2 assessed within 90 days of enrollment
• Patients must sign Institutional Review Board (IRB) approved study specific informed consent
• Patients must complete all required pre-entry tests within the specified time frames
• Patients must be able to start treatment (radiation) within 180 days of study registration
• In case of confusion about the eligibility or ineligibility of an individual patient, the principal investigator (PI) can be used as the arbiter and a note to file will be added to the subject's regulatory binder to document outcome

Exclusion Criteria:

• Previous pelvic radiation > 5 Gy
• Planned delivery of radiotherapy to pelvic lymph nodes
• Planned delivery of brachytherapy of the prostate
• Active rectal diverticulitis, Crohn's disease affecting the rectum, or ulcerative colitis (non-active diverticulitis and Crohn's disease not affecting the rectum are allowed)
• Prior hip replacement or penile implant
• Major medical, addictive, or psychiatric illness which in the investigator's opinion, will prevent the consent process, completion of the treatment and/or interfere with follow-up. (consent by legal authorized representative is not permitted for this study)
• Indwelling or intermittent urinary catheter use