This research study is evaluating whether a standard prostate MRI examination can improve radiation therapy planning for prostate cancer.

In this research study, the investigators want to determine if prostate radiotherapy treatment planning can be improved by using an MRI scan to help doctors more accurately target the prostate with radiation and decrease radiation dose to the rectum, which is just behind the prostate.

To do this, the investigators will create a treatment plan based on the MRI scan and compare it to the standard treatment, which currently uses a CT scan rather than an MRI. The results of this study will help inform doctors whether it is beneficial to routinely use an MRI scan for prostate radiotherapy treatment planning.

Inclusion Criteria:

• Participants must have histologically confirmed prostate cancer.
• PSA level and prostate biopsy must be reviewed at Brigham and Women's Hospital or the Dana Farber Cancer Institute and should support a diagnosis of stage I-III prostate adenocarcinoma.
• Candidates with PSA greater than 20, digital rectal exam consistent with disease outside the prostate (clinical T3/T4 disease), or Gleason score 8 or greater, should have a bone scan and diagnostic pelvic CT or MRI to exclude metastatic disease.
• Prostate biopsy, serum PSA measurement, and any indicated diagnostic imaging studies must be performed within 60 days of the date of registration.
• Participants should not have had prior curative local treatment for prostate cancer, including no radiotherapy or prostatectomy. A maximum 90 days of systemic androgen deprivation therapy prior to registration is allowed.
• Participation is limited to adult patients, age 18 years or older.
• ECOG performance status ≥2 (Karnofsky ≥60%)
• Life expectancy of greater than 10 years.
• Able to tolerate an MRI examination.
• Ability to understand and the willingness to sign a written informed consent document.

Exclusion Criteria:

• Participants with known or suspected metastatic (stage IV) prostate cancer.
• Participants with an implanted device, prosthesis, or any other foreign body with ferromagnetic properties that would make them ineligible to undergo MRI examination.
• Uncontrolled intercurrent illness including, but not limited to, ongoing or active infection, symptomatic congestive heart failure, unstable angina pectoris, cardiac arrhythmia, or psychiatric illness/social situations that would limit compliance with study requirements.

1 year