Delivery of conformal radiation therapy following radical prostatectomy is limited by the inability to define the clinical target volume (CTV). This stems from a poor understanding of patterns of local recurrence and limitations of standard imaging techniques.Magnetic Resonance Imaging (MRI) is a promising modality for imaging the prostatic bed following radical prostatectomy. MRI provides more detailed postoperative anatomy to guide CTV delineation, the ability to document local recurrence patterns, and a method to assess intrafraction prostatic bed motion and deformity to define appropriate planning target volume margins. The aim of this pilot study is to develop a technique for external beam radiation therapy (EBRT) following radical prostatectomy based on MRI-delineation of the CTV. Twenty patients will be enrolled in this pilot, development phase.

Inclusion Criteria:

• Patients following radical prostatectomy for prostate adenocarcinoma with one or more of the following adverse features:

pT2 + positive surgical margins ) pT3 ) PSA ≤ 2ng/ml Persistently elevated PSA at 3 months )

• Patients with initially undetectable PSA following prostatectomy for prostate adenocarcinoma with subsequent PSA relapse
• Patients being planned for radiation therapy
• No evidence of distant metastases
• Age ≥ 18 years
• ECOG performance status 0 or 1
• Informed consent

Exclusion Criteria:

• Contraindications to MRI Patients weighing >136 kgs (weight limit for the scanner tables) Patients with pacemakers, cerebral aneurysm clips, shrapnel injury or implantable electronic devices not compatible with MRI.
• Severe claustrophobia
• Inflammatory bowel disease or collagen vascular disease
• Previous colorectal surgery
• Previous pelvic radiotherapy