The purpose of this study is to determine if a non-invasive cancer treatment, high-dose stereotactic body radiotherapy (SBRT), is associated with changes in circulating tumor cells counts in patients with low- and intermediate-risk prostate cancer.

SBRT (stereotactic body radiotherapy) may decrease or actually increase CTC levels in patients with cancer, and knowledge about the effects of different therapies, including SBRT, on CTC levels will be of general interest to the oncology community. Patients will undergo blood draws at defined times as indicated below.

1. Prior to CT-simulation, but more than 2 days after digital rectal examination
2. Within 24 hours following CT simulation (an endorectal balloon is used at the time of simulation)
3. Within 24 hours following the first treatment
4. Within 24 hours following the third treatment
5. Within 1 week following the fifth treatment

Inclusion Criteria:

Men who satisfy all of the following conditions will be eligible for this study:

• Willing and capable to provide informed consent
• Signed study specific informed consent form.
• PSA ≤ 20 prior to hormone therapy (if given) for patients with Gleason 2-6. For those with Gleason score of 7. PSA should be less than or equal to 15 ng/ml prior to hormonal therapy (if given). Thus, risk of pelvic lymph node involvement according to Roach formula would be under 20%.
• Gleason score ≤ 7
• Appropriate staging studies identifying as AJCC stage T1a, T1b, T1c, T2a, or T2b
• No direct evidence of regional or distant metastases after appropriate staging studies
• Histologic confirmation of cancer by biopsy
• Adenocarcinoma of the prostate
• Age ≥ 18
• Zubrod Performance Status 0-2
• Up to 9 months of previous hormonal therapy is allowed (but not required)
• AUA score must be ≤ 15 (alpha blockers allowed)
• CT or Ultrasound-based volume estimation of prostate gland ≤ 60 grams
• Agreement to use effective contraceptive methods such as condom/diaphragm and spermicidal foam, intrauterine device, or prescription birth control pills.

Exclusion Criteria:

Women are not eligible for this study. Men with one or more of the following conditions also are ineligible for this study:

• Positive lymph nodes or metastatic disease from prostate cancer
• Prior invasive malignancy unless disease free for a minimum of 3 years (carcinoma in situ of the breast, oral cavity, or cervix, or non-melanomatous skin cancer are all permissible)
• T2c, T3, or T4 tumors
• Previous pelvic radiotherapy
• Previous surgery or chemotherapy for prostate cancer
• Previous transuretheral resection of the prostate (TURP) or cryotherapy to the prostate
• Previous hormonal therapy given for more than 9 months prior to therapy
• Concomitant antineoplastic therapy (including surgery, cryotherapy, conventionally fractionated radiotherapy, and chemotherapy) while on this protocol.
• History of Crohn's Disease or Ulcerative Colitis.
• Previous significant obstructive symptoms; AUA score must be ≤ 15 (alpha blockers allowed)
• Significant psychiatric illness
• Men of reproductive potential may not participate unless they agree to use an effective contraceptive method.
• Ultrasound or CT estimate of prostate volume > 60 grams

Low- and Intermediate-Risk Prostate Cancer wo undergo stereotactic body radiation therapy