The primary objective of this study is to demonstrate that ultra-hypofractionation of prostate cancer does not increase urinary toxicity as defined by the EPIC-26 GU domain patient reported outcome.

This is a pilot clinical trial looking at 2 fraction SBRT radiation therapy as an alternative to standard of care. Data does not yet exist for the safety and efficacy of this regimen.

However, the feasibility of ultra-short radiation therapy treatments has already been demonstrated in an analogous treatment using high-dose rate (HDR) brachytherapy. HDR brachytherapy has been adopted at high volume cancers centers as a standard treatment for prostate cancer. Typical doses have been 26 - 27 Gy over 2 fractions (13 or 13.5 Gy per fraction). Overall, toxicity and efficacy of HDR brachytherapy have compared favorably to other treatment modalities.

Dosimetric planning models between SBRT and HDR brachytherapy suggest minor differences. HDR brachytherapy was able to achieve higher intraprostatic maximum doses and lower rectal doses, but target volume coverage and urethral dose was not significantly different. These data suggest that reducing SBRT treatments from 5 fractions to 2 fractions may be feasible, efficacious and tolerable.

Eligible patients include all patients who are otherwise eligible for standard 5 fraction SBRT prostate. Study population will be low and intermediate patients with good urinary function (as defined by small prostate volume and low IPSS score). SBRT treatment will be delivered to the prostate to 12.5 Gy x 2 fractions.

Hormonal therapy is permitted on this study. Permitted agents include: leuprolide (Lupron/Eligard), biclutamide (Casodex), and degarelix (Firmagon).

Rectal sparing with hydrogel spacer (SpaceOAR) will be encouraged.

All patients will be enrolled with interim safety analyses after every occurrence of a grade 3 acute or late toxicity. Interval safety analysis will also be performed for recurrence and decrease in EPIC GU domain quality of life. Biospecimen and financial toxicity data will also be collected.

Inclusion Criteria:

• Ability of participant to sign a written informed consent.
• Diagnosed with prostate cancer, T1-T2bN0M0 GS6-7, PSA < 20
• IPSS score < 15 (and < 10 if on medication for benign prostatic hypertrophy such as tamsulosin) at time of enrollment (Appendix 21.4)
• Prostate volume (by US, CT or MRI measurement) < 50 cc at time of enrollment
• Androgen deprivation therapy based on clinician judgment is permitted on study
• Life expectancy > 10 years based on clinician's judgment
• No other active malignancy
• Age ≥ 18 years
• Performance Status Eastern Cooperative Oncology Group (ECOG) 0-1 (Appendix 21.5).
• Other study-specific criteria:
• Men of child-bearing potential must not donate sperm while on this study and for 90 days after their last study treatment.

NOTE: Acceptable forms of birth control are listed below:

• One Barrier method (cervical cap with spermicide plus male condom; diaphragm with spermicide plus male condom) PLUS
• Hormonal method (oral contraceptives, implants, or injections) or an intrauterine device (e.g., Copper-T).

Exclusion Criteria:

• Current or anticipated use of other investigational agents while participating in this study.
• Psychiatric illness/social situations that would limit compliance with study requirements
• Prior pelvic radiation therapy
• Prior prostatectomy
• Inflammatory bowel disease or connective tissue disease requiring medical management