It is a phase I study of radical hypofractionation delivering one single fraction of SBRT in patients with low- and favorable intermediate-risk prostate cancer that will undergo placement of the SpaceOAR hydrogel prior to treatment.

Our hypothesis is that treatments can be safely delivered in one single fraction using SBRT provided the separation between the prostate and rectum is increased using the hydrogel

There is a pre-treatment visit to the radiation oncology department. A gel is injected between the prostate and the rectum. This procedure is done with the use of a transrectal ultrasound, similarly to the prostate biopsy the patient had. A local anesthetic will be applied to numb the skin and to the underneath tissue where the injection will be performed. The procedure itself will take approximately 20 minutes and after a short observation time, the patient returns home.

The next visit (approximately after 7 days after the insertion of the gel), a CT scan and MRI-scan will be done. Prior to the scans, a urinary catheter will be inserted in the bladder through the penis, and removed once the scans are done. Using these images, the doctor and the team involved will perform an individualized planning study to establish the safest way the radiation will enter your body. When the treatment plan is ready, patient is called to receive the single treatment.

A urinary catheter again will be inserted in the bladder and the patient will be directed to the room where your treatment will be delivered. The treatment should last approximately 30 minutes. After the treatment, the urinary catheter will be removed.

Inclusion Criteria:

Histologically proven adenocarcinoma of the prostate. Tl-2b (AJCC 7th edition) Gleason score 6 or 7 (3+4)) or Gleason 7(4+3) and recent PSA < 10 (less than 30 days; must obtained >90 days from stopping dutasteride or >30 days from stopping finasteride)

Recent PSA under 15 ng/dL (less than 30 days; must obtained >90 days from stopping dutasteride or >30 days from stopping finasteride) OR Gleason 7(4+3) and recent PSA < 10 (less than 30 days; must obtained >90 days from stopping dutasteride or >30 days from stopping finasteride)

International Prostate Symptom Score <16 Prostate gland volume< 80cc

Zubrod Performance Status 0-1 within 60 days prior to registration

Age >: 18

Patient must be able to provide study-specific informed consent prior to study entry.

Exclusion Criteria:

Patients who opt to receive another treatment modality, such as surgery, or undergo active surveillance.

Prior or concurrent invasive malignancy (except non-melanomatous skin cancer) or lymphomatous/hematogenous malignancy unless continually disease free for a minimum of 5 years. All patients with in situ carcinoma are eligible for this study (for example, carcinoma in situ of the oral cavity) except patients with carcinoma of the bladder (including in situ bladder cancer or superficial bladder cancer).

Evidence of distant metastases

Regional lymph node involvement

Previous radical surgery (prostatectomy), cryosurgery, or HIFU for prostate cancer Previous pelvic irradiation, prostate brachytherapy, or bilateral orchiectomy Previous hormonal therapy, such as LHRH agonists or antagonists, anti-androgens, estrogens, or surgical castration (orchiectomy)

Use of finasteride within 30 days prior to registration. PSA should not be obtained prior to 30 days after stopping finasteride.

Use of dutasteride within 90 days prior to registration. PSA should not be obtained prior to 90 days after stopping dutasteride.

Previous or concurrent cytotoxic chemotherapy for prostate cancer Severe, active co-morbidity, defined as follows:

Unstable angina and/or congestive heart failure requiring hospitalization within the last 6 months

Transmural myocardial infarction within the last 6 months

Acute bacterial or fungal infection requiring intravenous antibiotics at the time of registration

Chronic obstructive pulmonary disease exacerbation or other respiratory illness requiring hospitalization or precluding study therapy at the time of registration

Hepatic insufficiency resulting in clinical jaundice and/or coagulation defects; note, however, that laboratory tests for liver function and coagulation parameters are not required for entry into this protocol. (Patients on Coumadin or other blood thinning agents are eligible for this study.)

Acquired Immune Deficiency Syndrome (AIDS) based upon current CDC definition; note, however, that HIV testing is not required for entry into this protocol. The need to exclude patients with AIDS from this protocol is necessary because the treatments involved in this protocol may be significantly immunosuppressive. Protocol-specific requirements may also exclude immuno-compromised patients.