Determination of Assay Cut-Off for the RadTox Test in Prostate Cancer Patients to Predict GI Radiation Toxicity

Inclusion Criteria: * Men diagnosed with adenocarcinoma of the prostate who have not received previous treatment (defined as prostatectomy, transurethral resection of the prostate \[TURP\], radiation of the pelvis, and GreenLight Laser Therapy) except for short-term (≤6 months) Androgen Deprivation Therapy (ADT) according to National Comprehensive Cancer Network (NCCN) guidelines. * Candidate for definitive prostate radiotherapy (either IMRT or proton). * Patients to be treated with IMRT should have all radiation treatment planned with IMRT, whereas patients to be treated with protons should have all radiation treatment planned with protons (including pelvic nodes if treated). * Localized prostate cancer, as confirmed by staging with Prostate-Specific Antigen (PSA), biopsy, Gleason score, and clinical stage. * Nuclear medicine bone imaging is required for confirmation of the absence of overt metastatic disease in bones if any high-risk criteria are identified (clinical stage T3a or higher; or 1-4 cores of Gleason score 8 \[NCCN Grade Group 4\] or 4+5; or PSA ≥20 ng/mL). * Diagnosed with any of the NCCN initial groups (i.e., Very-Low-Risk, Low-Risk, Intermediate-Risk \[both Favorable and Non-Favorable Intermediate-Risk\]; High-Risk; or Very-High-Risk) (see Appendix III for NCCN classifications of various risk groups). For Very-High-Risk, subjects are to have negative prostate cancer specific PET/CT imaging for confirmation of being metastasis free. * The score for Question 16 (i.e., "Overall, how big a problem have your bowel habits been for you during the last 4 weeks?") of the Bowel Habits section of Expanded Prostate Cancer Index Composite (EPIC) questionnaire must be 2 or below. * 30-85 years of age at the time of consent. * Eastern Cooperative Oncology Group (ECOG)/Zubrod Performance Status 0 - 2. Exclusion Criteria: * Findings of metastatic disease (nodal or distant, \>N1 or M1). * Prior prostatectomy, TURP, radiation of the pelvis, or GreenLight Laser Therapy. * History of invasive rectal malignancy or other pelvic malignancy, regardless of disease-free interval. * The score for Question 16 (i.e., "Overall, how big a problem have your bowel habits been for you during the last 4 weeks?") of EPIC questionnaire is 3 or above. * Active inflammatory bowel disease (i.e., patients requiring medical interventions or who are symptomatic) or documented history of inflammatory bowel disease requiring intervention. * Prior pelvic radiotherapy for any reason. * Documented lack of psychological ability or general health permitting completion of the study requirements and required follow-up. * Documented decisionally impaired persons who have a diminished capacity to understand the risks and benefits of participation in research and to autonomously provide informed consent. * Subjects who participated in a clinical trial of an investigational device, drug or biologics within the past 30 days. * Subjects who are currently undergoing any cancer drug treatment. However, patients who had received cancer drug treatment and stopped the treatment for \>4 weeks prior to the start of radiotherapy can be included. (Hormone therapy is allowed if judged appropriate and necessary by the treating physicians.)