The purpose of this study is to determine the effects of CyberKnife radiosurgery in patients with early stage organ-confined prostate cancer.

The CyberKnife Robotic Radiosurgery System is a unique radiosurgical system capable of treating tumors anywhere in the body noninvasively and with sub-millimeter accuracy. The CyberKnife System delivers radiation using a precise targeting methodology allowing a focal treatment margin around the target, thus limiting the volume of adjacent tissue receiving high doses radiation. This in turn allows the delivery of high doses of radiation to the prostate over a short series of treatments.

Inclusion Criteria:

• Patient must be at least 18 years of age
• Histologically proven prostate adenocarcinoma
• Patients belonging in one of the following risk groups:

Low: CS T1b-T2a and Gleason 2-6 and PSA ≤ 10, or Intermediate: CS T2b and Gleason 2-6 and PSA ≤ 10 or CS T1b-T2b, and Gleason 2-6 and PSA ≤ 20 ng/ml or Gleason 7 and PSA ≤ 10 ng/ml

• Prostate volume: ≤ 100 cc
• ECOG performance status 0-1

Exclusion Criteria:

• Prior prostatectomy or cryotherapy of the prostate
• Prior radiotherapy to the prostate or lower pelvis
• Implanted hardware or other material that would prohibit appropriate treatment planning or treatment delivery, in the investigator's opinion
• Chemotherapy for a malignancy in the last 5 years
• History of an invasive malignancy (other than this prostate cancer, or basal or squamous skin cancers) in the last 5 years.
• Hormone ablation for two months prior to enrollment, or during treatment.