Adaptive Radiation Therapy (ART) Stereotactic Ablative Body Radiotherapy (SABR) for Primary Localized Prostate Cancer

PRIMARY OBJECTIVE: I. To evaluate treatment-related, patient-reported early quality of life (QOL) changes between near "margin-less" ART in 2 fractions versus standard of care 3-5mm SABR in 5 fractions, using the Expanded Prostate Cancer Index Composite (EPIC)-26 bowel and urinary irritative/obstructive domains. SECONDARY OBJECTIVES: I. To assess treatment-related, patient-reported late QOL changes after SABR using the EPIC-26 bowel and urinary irritative/obstructive domains. II. To assess and compare early physician-reported grade ≥ 2 gastrointestinal (GI) and/or genitourinary (GU) toxicities of interest within 3 months after SABR using Common Terminology Criteria for Adverse Events (CTCAE) version (v) 5.0. III. To assess and compare patient-reported financial toxicities, using the Functional Assessment of Chronic Illness Therapy-Comprehensive Score for Financial Toxicity (FACIT-COST) assessment tool. IV. To assess and compare late physician-reported grade ≥ 2 GI and/or GU toxicities of interest within 24 months after SABR using the CTCAE v5.0. V. To assess and compare change in International Index of Erectile Function (IIEF-15) and International Prostate Symptom Score (IPSS). VI. To explore the association of fiducial-free treatment on dosimetry, patient reported outcomes (PROs), and toxicity. VII. To explore the association of bladder filling on dosimetry, PROs, and toxicity. VIII. To assess and compare the cumulative incidence of biochemical failure, local progression, distant metastasis, and metastasis-free survival within 60 months after SABR. OUTLINE: Patients are randomized to 1 of 2 arms. ARM I: Patients undergo near margin-less adaptive radiation therapy (ART) for 2 treatments at least 3 days apart in the absence of disease progression or unacceptable toxicity. Patients also undergo cone beam computed tomography (CBCT) and may undergo computed tomography (CT) and/or magnetic resonance imaging (MRI) on study. ARM II: Patients undergo standard SABR for 5 treatments at least 2 days apart in the absence of disease progression or unacceptable toxicity. Patients also undergo CT and/or MRI on study. After completion of study treatment, patients are followed up at months 1, 3, and 6, and then every 6 months for up to 60 months.