of Radiation Oncology, Stanford Cancer Institute, Stanford University, Stanford, CA, 3 Department of Radiation Oncology, Stanford, CA Purpose/Objective(s): A retrospective analysis of patients treated with Stereotactic Ablative Radiotherapy (SABR) for oligometastatic colo- rectal cancer (CRC) to characterize local failure (LF) and overall survival (OS). Materials/Methods: This IRB-approved, single-institution retrospective analysis included patients with metastatic CRC treated with SABR, from 2004-16, to the liver, lung, spine, or brain. In total 108 patients were treated to 204 lesions, with 4 patients treated to 2 sites. Patient de- mographics, primary tumor characteristics, prior treatment, radiation details, and outcomes were collected from medical records. Primary CRC arising from the splenic flexure, descending colon, or recto-sigmoid colon were defined as left-sided, while those arising in the cecum, ascending colon, hepatic flexure, or transverse colon were defined as right-sided. Results: There were 61 males (56.5%) and 47 females (43.5%) with a median age of 58 years included in this study with a median follow-up of 16 months (range: 0-127 months). Of the 204 treated lesions, 70 (34%) were in the brain, 57 (28%) in the liver, 28 (14%) in the lung, and 49 (24%) in the spine. A total of 180 (88%) lesions were from left- sided CRC (53 liver, 35 spine, 70 brain, and 22 lung) and 24 (12%) from right-sided CRC (4 liver, 14 spine, 0 brain, and 6 lung). The median biologically effective dose (BED) by site was 100 Gy for liver, 50 Gy for spine, 60 Gy for brain, 112 Gy for lung, and 70 Gy overall (range: 34-300 Gy). The median OS for the overall group was 26.3 months, 31.4 months for liver, 13.6 months for spine, 38.8 months for brain, and not reached for lung. On multivariable analysis for OS, metastatic site (pZ0.0025) and CRC sidedness (pZ0.0498) were sig- nificant, while gender, age, and BED were not. Table 1 shows the rate of 1- and 2-year cumulative incidence of recurrence (CIR). On multi- variable analysis for LF, metastatic site (pZ0.0061), sidedness (pZ0.0230), and BED (pZ0.0088) were significant predictors, whereas gender and age were not. Compared to left-sided CRC, patients with right-sided CRC were more at risk for LF (hazard ratio, 2.511 per Gy). Increasing BED was associated with decreased LF (hazard ratio, 0.979 per Gy). With analysis by metastasis site, BED  median was associ- ated with lower LF for the liver lesions (0.0072) but not for brain, spine, and lung lesions. Conclusion: OS for SABR for oligometastatic CRC is favorable, justifying the use of aggressive local therapy, although there is heterogeneity based on metastatic disease site. LF is associated with metastatic site, primary tumor sidedness, and BED. BED in particular is associated with LF for liver. Further investigation is needed to better understand these differences in radiosensitivity. Author Disclosure: K. Benson: None. N. Sandhu: None. C. Zhang: None. R.B. Ko: None. D.A. Toesca: None. R. Von Eyben: None. M. Diehn: Employee; Kaiser Permanente. Consultant; Roche, AstraZeneca, Bio- NTech. Stock; CiberMed. K. Bush: None. P.G. Maxim: None. M.F. Gensheimer: None. S.G. Soltys: None. B.W. Loo: Research Grant; Varian Medical Systems Inc. Stock; TibaRay, Inc.; National Comprehensive Cancer Network, American College of Radiology, TibaRay, Inc. E. Pol- lom: None. D.T. Chang: Research Grant; Varian Medical Systems. Hon- oraria; Varian Medical Systems. Travel Expenses; Varian Medical Systems. Stock; ViewRay. 2369 Racial Disparities and Standard Treatment of Squamous Cell Carcinoma of the Anus: An Analysis of the National Cancer Database D.H. Chen, 1 E. Lin, 2 S.K. Yoo, 2 and S.X. Bian 2 ; 1 Keck School of Medicine of USC, Los Angeles, CA, 2 University of Southern California Keck School of Medicine, Department of Radiation Oncology, Los Angeles, CA Purpose/Objective(s): Standard treatment for nonmetastatic anal squa- mous cell carcinoma (SCC) consists of concurrent chemoradiation (CRT). Due to the stigma and health disparities associated with anal SCC, concern exists over whether certain populations are receiving the standard of care. We aim to evaluate racial differences in the receipt of standard treatment and its impact on survival. Materials/Methods: Using data from the National Cancer Database, we identified patients diagnosed with nonmetastatic anal SCC (Stages 1-3) between 2004 and 2015. We excluded patients who received an abdomi- noperineal resection prior to CRT. We studied the association between race and standard CRT, defined as chemotherapy and radiation initiated within 7 days of each other, using multivariable logistic regression. We then used a multivariable Cox proportional hazards model to estimate differences in survival associated with standard CRT and race/ethnicity. Results: We identified 26,935 patients for our study. Patients receiving standard CRT had better survival than patients receiving non-standard treatment on multivariable analysis (hazard ratio [HR] 0.70; 95% confi- dence interval [CI] 0.67-0.74; p<0.001). Compared to white non-Hispanic patients, black patients were less likely to receive standard treatment (odds ratio 0.82; 95% CI 0.74-0.90; p<0.001). There was no increase in mor- tality risk in black patients overall (HR, 1.04; 95% CI, 0.96-1.12; pZ0.375). However, within the subgroup of patients receiving non-stan- dard treatment, black patients had an increased mortality risk compared to white non-Hispanic patients (HR, 1.14; 95% CI, 1.02-1.28; pZ0.023). This disparity in black patients was no longer significant in the subgroup of patients receiving standard treatment (HR 1.03, 95% CI 0.91-1.16; pZ0.644). Conclusion: Standard treatment in anal SCC was associated with better overall survival. Racial disparities exist as black patients were less likely to receive standard treatment than their white non-Hispanic counterparts. Although black patients receiving substandard therapy had higher rates of mortality than white non-Hispanic patients, this difference was ameliorated when receiving standard therapy. Increasing physician awareness of and adherence to standard treatment recommendations could potentially improve racial disparities. Author Disclosure: D.H. Chen: None. E. Lin: None. S.K. Yoo: None. S.X. Bian: None. 2370 Routine Pathologic Assessment of Local Response to Definitive Chemoradiation for Localized Anal Cancer: An Institutional Report K.A. D’Rummo, C.E. Lominska, M.J. Tennapel, and A. Hoover; Department of Radiation Oncology, University of Kansas School of Medicine, Kansas City, KS Purpose/Objective(s): The optimal post-treatment surveillance regimen after definitive chemoradiation (CRT) for localized anal squamous cell carcinoma (SCC) is controversial. The utility of a repeat biopsy for assessment of tumor response in the early post-treatment period has come into question based on the results from a post-hoc analysis of the ACT II trial. Here, we aimed to study the outcomes of patients from a single institution who were followed with routine post-treatment anal biopsies, compared to those receiving only clinically-indicated post-treatment bi- opsies after definitive CRT for anal SCC. Abstract 2368; Table 1 Local Failure by Metastatic Site CIR (percent, [95% CI]) Liver Spine Brain Lung Overall 1-year 26.3 [15.7, 38.3] 23.0 [12.2, 35.8] 13.4 [6.5, 22.7] 17.9 [4.1, 39.4] 20.0 [14.6, 26.0] 2-year 37.5 [24.9, 50.1] 28.9 [16.1, 43.0] 25.6 [14.9, 37.7] 17.9 [4.1, 39.4] 29.0 [22.4, 35.9] International Journal of Radiation Oncology  Biology  Physics E162