Prostate Oncologic Therapy While Ensuring Neurovascular Conservation (POTEN-C)

Inclusion Criteria: 1. Age ≥ 18 years. 2. Appropriate staging studies identifying patient as AJCC 7th edition clinical stage T1 (a, b, or c) or T2 (a, b, or c) adenocarcinoma of the prostate gland. The patient should not have direct evidence of regional or distant metastases after appropriate staging studies. See Appendix I for details on AJCC 7th Edition staging criteria. Histologic confirmation of cancer will be required by biopsy performed within 12 months of registration. T-staging may be assessed by multi-parametric imaging alone if digital rectal examination was deferred. 3. The patient's Zubrod performance status must be 0-2 (see Appendix II for definition). 4. The Gleason summary score should be less than or equal to 7 \[Grade group 1 (Gleason 3+3=6), group 2 (Gleason 3+4=7), and group 3 (Gleason 4+3=7) are allowed\]. See Appendix III for details on definitions. While a template biopsy is recommended, it is not required in the case of MRI fusion biopsy performed on all dominant MR lesions (defined as PIRADS v2 4-5). 5. Baseline AUA symptom score ≤19 (see Appendix IV for questionnaire) without need for maximum medical therapy (specifically, not on tamsulosin 0.8mg daily). 6. EPIC sexual domain composite score 60-100 (see Appendix V). 7. Multi-parametric MRI evaluation of the prostate is required for this study within 12 months of registration. Gross radiographic disease on MRI (defined as PIRADS v2 score 3-5) must be \> 5mm at minimum distance from at least one side's neurovascular bundle, which is typically the closest of the neurovascular elements to the prostate. 8. The serum PSA should be less than or equal to 20 ng/ml within 90 days of registration. -Study entry PSA must not be obtained during the following time frames: (1) 10-day period following prostate biopsy; (2) following initiation of ADT or anti-androgen therapy; (3) within 30 days after discontinuation of finasteride; (4) within 90 days after discontinuation of dutasteride; (5) within 5 days of a digital rectal examination (which is not a required exam on the protocol). 9. Ultrasound or MRI based volume estimation of prostate gland ≤ 80 grams. Cytoreduction therapy (finasteride or dutasteride only) may be considered for those with \>60 gram size. 10. All patients must be willing and capable to provide informed consent to participate in the protocol within the 30 days prior to registration. Exclusion Criteria: 1. Subjects with clinical (digital rectal examination) evidence of extraprostatic extension (T3a) or seminal vesicle involvement (T3b). MRI evidence of equivocal/potential but not definite extraprostatic extension is allowed, as long as it is unilateral and not on the side of the gland proposed for neurovascular element sparing. In equivocal cases of potential extracapsular extension on MRI only, discretion is left to the treating physician. 2. MRI evidence of gross disease (defined as PIRADS v2 score 3-5 lesions) ≤5mm of BOTH neurovascular bundles, which are the most proximate of the neurovascular elements planned for sparing on this protocol. 3. Patients with all three intermediate risk factors (PSA \>10 and ≤ 20, Gleason 7, clinical stage T2b-T2c) who ALSO have ≥50% of the number of their template biopsy cores positive for cancer are ineligible. 4. Inability to undergo multi-parametric MRI. 5. Evidence of metastatic disease. Note bone scan is not required for this study given the low-intermediate NCCN risk cohort to be enrolled. 6. Evidence of clinical nodal involvement of the pelvis. Biopsy is required for lymph nodes over ≥1.5cm in short-axis measured size. 7. No currently active ADT or anti-androgen therapy at time of registration is allowed. Further, no more than 3 cumulative months of prior ADT or anti-androgen therapy is allowed. If either has been used by the patient, there must be a demonstration of testosterone recovery (\>50ng/dL serum blood level), EPIC sexual domain score ≥60, and at least 1 month between demonstration of testosterone recovery and study registration (any one measurement of testosterone recovery suffices). 8. Testosterone ≤ 50 ng/dL (any one measurement \>50 ng/dL suffices for inclusion) within 90 days of study entry. 9. Subjects who have had previous pelvic radiotherapy or have had chemotherapy or surgery for prostate cancer. 10. Subjects who have plans to receive other concomitant or post treatment adjuvant antineoplastic therapy while on this protocol including surgery, cryotherapy, conventionally fractionated radiotherapy, hormonal therapy, or chemotherapy given as part of the treatment of prostate cancer. 11. Subjects who have undergone previous transurethral resection of the prostate (TURP) within 1 year of enrollment or ablative procedures to the prostate for benign prostatic hyperplasia or other conditions (i.e. cryotherapy, HIFU). 12. Subjects who have baseline severe urinary symptoms, as defined by AUA symptom score \>19 (alpha-blocker medication allowed except if taking tamsulosin 0.8mg daily at baseline which indicates compensated severe symptoms and also can affect sexual function). 13. Subjects who have a history of significant psychiatric illness that would confound informed consent. 14. Severe, active co-morbidity, defined as follows: 1. Unstable angina and/or congestive heart failure requiring hospitalization within the last 6 months 2. Myocardial infarction within the last 6 months 3. Acute bacterial or fungal infection requiring intravenous antibiotics at time of registration 4. Patients with active inflammatory colitis (including Crohn's Disease and ulcerative colitis) currently requiring systemic steroids and/or systemic immunosuppression are not eligible. 15. Subjects with a known allergy to polyethylene glycol hydrogel (rectal spacer material) or contraindication to spacer products (SpaceOAR). 16. Subjects with uncontrolled coagulation disorder which cannot be controlled with anticoagulants. 17. Men active with partners of reproductive potential who do not agree that they will use an effective contraceptive method during treatment and 6 months after treatment. 18. Men who require erectile function medication or aid to achieve an erection sufficient for intercourse. Ability to achieve erection sufficient for intercourse without medication or aid at least once time in the month prior to registration is sufficient for inclusion. 19. Men who have clinically significant penile malformation (i.e. Peyronie's disease) or history of penile implantation are excluded. 20. If DRE is performed, patient may not have palpable disease on side of gland to be planned for neurovascular sparing. Given the poor accuracy of DRE, such a finding should be confirmed by MRI and/or biopsy to harbor actual disease before excluding a patient on this basis.