Evaluation of Treatment Response Using Multiparametric MRI After Prostate Radiotherapy

Inclusion Criteria: * Patients with biopsy proven (completed and/or reviewed at MSK) adenocarcinoma of the prostate, who will undergo external beam radiotherapy using conventional fractionation IGRT including moderate hypofractionated radiation, low rate brachytherapy alone, brachytherapy (either high dose or low dose) combined with supplemental image guided radiation (including IGRT, EBRT, and SBRT), SBRT, or proton radiotherapy. * Age ≥ 18 years old * Baseline MP-MRI, which is obtained at MSKCC Main Campus on the 3-Tesla Philips MRI unit as part of the standard (clinical) staging assessment demonstrates at least one dominant or visible lesion which measures \> 0.5 cm in maximum axial diameter as assessed on T2-weighted images. * No prior history of androgen deprivation therapy within the last month. However patients who will receive neoadjuvant and concurrent and adjuvant hormonal therapy will be eligible. * Willing to come to MSK Main Campus for baseline and follow-up MP-MRIs. Exclusion Criteria: * History of radical prostatectomy * Patients who are unwilling or unable to undergo MRI including patients with contraindications to MRI such as the presence of cardiac pacemakers or non-compatible intracranial vascular clips, claustrophobia, inability to lie flat for the duration of the study etc. * Patients with a metallic hip implant, metallic implant or device in the pelvis or ferromagnetic fiducial beacons (Calypso) that might distort local magnetic field and compromise quality of MP-MRI * Patients in which gadolinium contrast is contra-indicated. * Metastatic disease