Purpose/Objective(s): RTOG 9811 reported relapse rates of 49% in locally advanced anal squamous cell carcinoma, following radical che- moradiation therapy (CRT). As the majority of patients relapse at the site of gross tumor, there may be a role for dose escalation to the primary tumor, however the dose-response relationship for anal cancer is not currently well known. Intensity modulated radiation therapy (IMRT) has been widely adopted; allowing delivery of a simultaneous integrated boost (SIB) to the gross tumor over the same number of fractions as the pro- phylactic dose to the uninvolved nodes. The use of SIB can reduce overall treatment time both by employing larger fraction sizes to the gross tumor and by causing fewer interruptions due to reduced toxicity. The aim is to fit a tumor control probability (TCP) model to the published outcome IMRT data. Materials/Methods: We performed a systematic review of PubMed and Embase databases to identify original papers published in English, from 2005 to the present which report the use of IMRT CRT including out- comes. Thirteen papers including a total of 625 patients were identified and predefined data fields collected; including number of patients (range 17 to 148) , median (54Gy) and range (45-67.6Gy) of doses delivered , frac- tionations used, overall treatment time (range 38-56 days) and local control at 2 to 3 years. A standard linear quadratic tumor control probability (TCP) model, which included repopulation, was fit by least squares minimization. Accelerated repopulation was assumed to commence after 20 days and contribute an additional 0.7 Gy per day; an a/b ratio of 14 was assumed, in keeping with published data for cervical cancer. Results: The fitted TCP curve demonstrated a dose response relationship with a Z 0.15. The estimated local control at EQD2a/b Z 14 50 Gy is 85% (95% confidence interval 57% to >100%). Calculations using an a/b of 10 demonstrated an a Z 0.14 therefore resulted in minimal difference. The adjustment for overall treatment time resulted in significant changes to the TCP curve (up to 10 Gy equivalent for prolonged or heavily interrupted schedules). In the IMRT series, specifically looking at locally advanced patients, the 3 year local control was 79%; relatively better than in RTOG 9811. The sigmoid nature of the TCP curve, suggests a 5 Gy EQD2 increase in dose, in locally advanced tumors may result in 10% improvement in local control. Conclusions: The published data are broadly consistent with a linear quadratic dose response model. The IMRT series appear to have better outcomes in locally advanced patients. IMRT based dose escalation using an SIB, in locally advanced anal cancer, should be further investigated in the context of clinical trials. Author Disclosure: R. Muirhead: None. M. Partridge: None. M.A. Hawkins: None. 2471 Patterns of Failure in Squamous Cell Carcinoma of the Anal Canal: A Comparison Between Intensity Modulated Radiation Therapy and 3- Dimensional Conformal Radiation Therapy C.J. Baden, R.B. Taylor, J. Lopez-Araujo, and R. Jacob; University of Alabama Medical Center, Birmingham, AL Purpose/Objective(s): IMRT has emerged as an important modality for the treatment of SCC of the anal canal. When compared with 3D-CRT, the technique’s increased dose conformality allows for improved sparing of organs at risk thereby decreasing toxicity and associated treatment breaks. With increased average weekly dose (AWD) the intensity of local treat- ment associated with IMRT is higher, potentially leading to improved local control; however, more-conformal treatment volumes also increase the possibility for marginal recurrence. We completed this study in order to assess for differences in patterns of recurrence following chemoradiation with either IMRT or 3D-CRT. Materials/Methods: We completed a retrospective review of all patients with SCC of the anal canal treated with definitive chemoradiation at our institution between May 2002 and February 2010. Patient characteristics and disease-related outcome data were compared between IMRT and 3D- CRT groups using appropriate statistical tests. First sites of failure were coded as local (involving the anal canal or original site of gross disease), regional (involving the inguinal or pelvic lymph nodes or true pelvis), or distant, and frequencies compared between groups. Results: Median follow-up for the cohort was 61.7 months. Of 44 patients treated with definitive chemoradiation, 25 (57%) were treated with IMRT and 19 (43%) with 3D-CRT. There were no statistically significant dif- ferences between IMRTand 3D-CRT groups with respect to sex, age, HIV- positivity, T-stage, or N-stage. Median radiation dose was 5400 cGy for both groups, but average weekly radiation dose was higher in patients treated with IMRT (879 cGy vs 786 cGy, p Z 0.002) due primarily to fewer treatment breaks >3 days (8 vs 15, p Z 0.002). Comparison of IMRT and 3D-CRT with respect to five-year LC (95.7% vs 82.0%, p Z 0.29), CFS (83.3% vs 65.6%, p Z 0.26), and OS (87.5% vs 77.0%, p Z 0.30) revealed non-significant trends in favor of IMRT. Of the 4 patients with recurrence in the IMRT group, first site was local and regional in 2 cases and distant in 2 cases. For the 3D-CRT group, 6 patients experienced recurrence; 1 local, 2 local and regional, 1 local and distant, and 2 distant. Comparing IMRT and 3D-CRT, the frequencies of local (8.0% vs 21.1%, p Z 0.38), regional (8.0% vs 10.5%, p Z 1.0), and distant (8.0% vs 15.8%, p Z 0.64) recurrence as a component of first recurrence were not significantly different. Conclusions: In this study we observed no apparent significant differences in patterns of failure between IMRT and 3D-CRT, but trends in 5y LC, CFS, and OS favored IMRT. Larger studies with adequate follow-up would be required in order to corroborate these findings related to patterns of failure and disease-related outcomes. Author Disclosure: C.J. Baden: None. R.B. Taylor: None. J. Lopez- Araujo: None. R. Jacob: None. 2472 Outcomes After IMRT Compared to 3DCRT for Squamous Cell Carcinoma of the Anus C.R. Spencer, M. Cardenas, T.A. DeWees, R. Jain, P. Grigsby, B. Tan, M. Silviera, S. Hunt, R.J. Myerson, P.J. Parikh, and J.R. Olsen; Washington University, Saint Louis, MO Purpose/Objective(s): IMRT is routinely used to reduce toxicity in squamous cell carcinoma of the anus (SCCA). We report our clinical outcomes after both IMRT and 3DCRT in a cohort of patients staged by FDG-PET/CT. Materials/Methods: The records of 110 patients that received definitive radiation therapy from 2003 to 2013 with SCCA were retrospectively reviewed with IRB-approval. The staging evaluation included FDG-PET/ CT for all patients. Of these, 28 received 3DCRTand 82 underwent IMRT. Median radiation therapy dose was 50.6 Gy (Range 30-73 Gy) in the 3DCRT group and 52.3 Gy (Range 40-60 Gy) in the IMRT cohort deliv- ered in 1.8-2.0 Gy per fraction. Within the IMRT group 28 patients received a sequential boost technique and 54 underwent a simultaneous integrated boost (dose-painted IMRT). Overall, 109 patients received chemotherapy and 94 received Nigro regimen chemotherapy consisting of Mitomycin C and 5-FU (79% of the 3DCRT cohort and 89% of the IMRT cohort; p Z 0.23). There was no difference between groups in terms of age, sex, race, HIV status, T stage, N stage, AJCC stage, radiation therapy dose or chemotherapy delivery. Survival outcomes for groups were compared using the Kaplan Meier method with log rank analysis. Results: Median follow-up for the 3DCRT group was 36 months versus 24 months for the IMRT group. There were 8 locoregional recurrences in each group. The 3DCRT group had 8 local and 0 regional recurrences with a median time to recurrence of 10 months and the IMRT group had 4 local and 4 regional recurrences also with a median time to recurrence of 10 months. Two year local recurrence free survival was 72% for the 3DCRT and 95% for the IMRT group (p < 0.01). Recurrence free survival was 86% for the IMRT group and 72% for the 3DCRT group at 2 years (p Z 0.09). Overall Survival was 88% for both groups at 2 years (p Z 0.4). Within the IMRT group there were 2 local and 2 regional recurrences each in both the simultaneous integrated boost and the sequential boost groups. Conclusions: Survival outcomes are similar when comparing IMRT to 3DCRT. There does not appear to be a difference in locoregional International Journal of Radiation Oncology  Biology  Physics S400