A Phase I/II Dose-escalation Study Evaluating the Safety and Efficacy of 21 Gy, 23 Gy and 25 Gy for High Dose Rate (HDR) Prostate Brachytherapy

Inclusion Criteria: * Histologically or cytologically confirmed diagnosis of early stage prostate cancer. * Must be considered either low-risk (T1-T2a, Gleason ≤ 6, PSA \< 10 ng/mL) or favorable intermediate-risk (Gleason 3 +4 = 7, percentage of positive biopsy cores \< 50%, no more than one NCCN intermediate risk factor). * Prior androgen deprivation therapy is allowed and may have been initiated up to 6 months prior to the date of the HDR implant. The complete duration of androgen deprivation therapy can range from 4 months to 36 months provided it has been initiated no more than 6 months prior to the date of the HDR implant. * At least 18 years of age. * ECOG performance status ≤ 2 * Able to understand and willing to sign an IRB approved written informed consent document (or that of legally authorized representative, if applicable). Exclusion Criteria: * Prior radiation therapy to the prostate or lower pelvis encompassing the prostate. * A history of other malignancy ≤ 5 years previous with the exception of basal cell or squamous cell carcinoma of the skin which were treated with local resection only. * Currently receiving any other investigational agents. * Uncontrolled intercurrent illness including, but not limited to, ongoing or active infection, symptomatic congestive heart failure, unstable angina pectoris, or cardiac arrhythmia. * Unable to undergo general, spinal or local anesthesia. * Prior TURP with a sufficiently large defect that would compromise the integrity of the implant per clinician's assessment.