clinical feasibility of choosing optimal virtual monochromatic images for improving OAR delineation in the head-and-neck radiotherapy planning. This could help us enable a stepwise clinical implementation of optimized “twin-beam” protocol for head-and-neck cancer patients, and has the po- tential to improve head-and-neck radiotherapy. Author Disclosure: X. Yang: None. T. Wang: None. B. Ghavidel: None. J.J. Beitler: None. X. Tang: None. W.J. Curran: ASCO. T. Liu: None. 215 Erectile Function after High-Dose-Rate Brachytherapy-like Stereotactic Body Radiation Therapy for Organ-Confined Prostate Cancer D.B. Fuller, 1 B.L. Kane, 2 K. Underhill, Jr, 3 J.R. Gray, 4 A.V. Peddada, 5 and R.C. Chen 6 ; 1 Genesis Healthcare Partners, San Diego, CA, 2 Oncology Care Providers, Fresno, CA, United States, 3 Benefis Healthcare, Great Falls, MT, United States, 4 Tennessee Oncology, Dickson, TN, United States, 5 Penrose Cancer Center, Colorado Springs, CO, 6 UNC Lineberger Comprehensive Cancer Center, University of North Carolina School of Medicine, Chapel Hill, NC Purpose/Objective(s): Given the higher dose of radiation per fraction using stereotactic body radiation therapy (SBRT) there is concern about erectile function after prostate SBRT. This endpoint was measured in a prospective multi-institutional study of high-dose-rate brachytherapy-like (HDR-like) SBRT for prostate cancer. Materials/Methods: Between 2007 and 2012, 259 men with clinically localized prostate cancer were treated with SBRT monotherapy using a heterogeneous, HDR-like approach. Patients were treated with robotic SBRT to a total dose of 38 Gy in 4 fractions. Patients did not receive androgen deprivation therapy (ADT). Potency was defined as a score of 4 or 5 on question 8.a of the patient-reported quality of life (PR-QOL) survey EPIC-26. In addition, the Sexual Health Inventory for Men (SHIM) was collected. PR-QOL was assessed at baseline and 6, 12, 24, 36, 48, and 60 months after treatment. Erectile function was correlated with multiple patient and dosimetric variables. Results: There were 107 men potent at baseline, including 46 low- and 61 intermediate-risk patients, with a median age of 65 years (range, 45-80 years). The proportion of patients reporting potency decreased with time post-treatment. The median time post-treatment to the first report of “non- potent” was 12 months. The 24- and 60-month proportion potent was 43% and 36%, respectively. None of the dosimetric variables (dose to prostate, penile bulb, and neurovascular bundles) were found to correlate with po- tency at any time point. Likewise, neither baseline testosterone levels nor age at treatment predicted potency. Baseline SHIM score was correlated with higher potency rates at 6 and 60 months post-treatment (median baseline SHIM 21.6 for potent versus 16.2 for not potent at 60 months, pZ0.004). Smaller prostate volume was correlated with a higher potency rate at 60 months (median prostate volume 33.8 cc for potent versus 46.1 cc for not potent at 60 months, PZ0.008). Conclusion: Only higher baseline sexual QOL and a smaller pre-SBRT prostate volume were correlated with improved long-term post-treatment erectile function post-SBRT, while no other prostate, penile bulb or neu- rovascular bundle dosimetry variable had any significant measurable po- tency outcome correlation. This suggests that “patient factors” are more significant than dosimetry factors in predicting sexual QOL domain outcome post-SBRT. Author Disclosure: D.B. Fuller: Honoraria; Accuray Incorporated. Stock; Accuray Incorporated, Varian, ViewRay. B.L. Kane: California Medical Association, Sante Oncology Associates Foundation. Radiation Oncology; Fresno Community Medical Centers. K. Underhill: Honoraria; Accuray Incorporated. J.R. Gray: Honoraria; Accuray Incorporated. A.V. Ped- dada: Honoraria; Accuray Incorporated. R.C. Chen: Research Grant; Accuray Inc. Consultant; Accuray Inc. 216 Stereotactic Body Radiation Therapy for Unfavorable Intermediate- and High-Risk Prostate Cancer: 3-Year Outcomes of a Phase II Trial V.A. Macias-Hernandez, 1 I. Barrera-Mellado, 2 C. Marti, 1 A. Pont, 3 A. Fernandez-Lara, 1 and P. Soria 1 ; 1 Salamanca University Hospital, Salamanca, Spain, 2 Faculty of Medicine, Salamanca, Spain, 3 Institut Municipal d’Investigacions Mediques, Health Services Research Unit, Barcelona, Spain Purpose/Objective(s): SBRT as monotherapy is being increasingly used for the management of low- and favorable intermediate-risk prostate cancer as a growing body of literature has shown SBRT to be safe and efficacious. However, the evidence for high-risk is based on observational studies with relatively few patients and short follow-up. Here we present 3- year CTCAE urinary (GU) and rectal (GI) toxicity, quality of life (QOL), PSA profile and biochemical/clinical progression-free survival of a phase II study focused on intermediate and high-risk patients. Materials/Methods: cT1-3a N0 M0 prostate cancer patients without recent acute urinary retention or IPSS score >20 were eligible. Extreme hypo- fractionation was used to increase dose in prostate cancer to 92.3 Gy1.5 while late responding tissues received 78.2 Gy3 over at least 2 weeks. Therefore, 8 fractions of 5.65 Gy (intermediate and high risk) or 5.48 Gy (low risk), 2-3 fractions/week, were delivered with helical tomotherapy and on-line MVCT guidance. Dose was prescribed to 95% PTV and ho- mogeneous dose distribution into PTV (95-103%) was required. CTV Z (prostate and seminal vesicles) + 0-5 mm margin. PTVZCTV + 3-6 mm margin. Endorectal balloon and bladder catheterization were introduced in 8/2015. 72% received ADT (73% neoadjuvant and 27% also adjuvant). The threshold for clinically significant worsening in the incontinence, irritative-obstructive, bowel, sexual and hormonal EPIC-26 domains was defined as ½ standard deviation below the baseline. Results: Since 2012, 154 patients were treated. Unfavorable intermediate-, high- and very high-risk accounted for 58% of the series (90 men). Mean PSA: 13.5 ng/ml (1.2-214). Gleason 8-10: 18.1%, cT3a: 19.7%. The me- dian follow-up was 36 months (5-72). Crude PSA relapse-free survival (2 ng/ml + nadir): 99.3% (1 unfavorable intermediate-risk patient relapsed at 29 months follow-up). Cancer-specific survival: 100%. Benign PSA bounce was seen in 2 patients. Without ADT, median PSA levels at baseline, 12, 24 and 36 months were 7.30, 0.77, 0.45 and 0.31 ng/ml, respectively. No acute or late GI Grade 3+ toxicities were observed. One late GU Grade 3 (prostatic urethra ulcer, managed conservatively) was found at 12 months. Clinically significant QOL worsening was observed in the irritative-obstructive and bowel domains, returning to baseline levels within the first year. Median age: 71.5 years (50-81). 10 men, out of 32 before SBRT, preserved erectile function at 12 months, according to the questionnaire responses. Conclusion: Although longer follow-up is required, this slightly protracted 8-fraction regime has shown good tolerance, little impact on quality of life, and promising disease control outcomes in an unfavorable group of prostate cancer patients. Abstract 216; Table CTCAE Toxicity (%) Grade Acute 2 mo. 6 mo. 12 mo. 24 mo. 36 mo. 48 mo. 60 mo. GU 1 46.4 17.7 17 17.8 14.2 11.9 5.4 0 2 19.6 5.2 2.7 1.6 2 1.4 5.4 7.6 3 0 0 0 0.8 0 0 0 0 GI 1 22.2 9.2 8.8 12.1 10.2 10.4 2.7 7.6 2 9.1 0 0.6 0.8 2 0 0 0 3 0 0 0 0 0 0 0 0 N 153 152 147 123 98 67 37 13 Volume 102  Number 3S  Supplement 2018 Oral Scientific Sessions S105