Purpose/Objective(s): While dose to pharyngeal constrictors has previ- ously been associated with postchemoradiation therapy (CRT) swallowing dysfunction, there is little extant data regarding effects for other swal- lowing muscles. We sought to identify, using nonmodel-dependent statis- tical methods, specific swallowing muscle dose-response thresholds associated with chronic dysphagia. Materials/Methods: Data from sequential series of T1e4 N0e3 M0 patients who received concurrent chemoradiation with intensity modu- lated CRT were accessed from an institutional speech pathology dataset. Chronic dysphagia was coded as a binary outcome defined as any of the following occurring 12 months post-CRT: modified barium swal- lowedetected aspiration or stricture, gastrostomy tube, and/or aspiration pneumonia. Delivered CRT plan data was extracted from the treatment planning system and autosegmented using a custom region-of-interest (ROI) library. Segmented ROIs included: inferior, middle, and superior constrictors (IPC, MPC, and SPC); medial and lateral pterygoid muscles (MPM and LPM); anterior and posterior digastric muscles (ADM and PDM); intrinsic tongue muscles (ITM); mylohyoid muscle (MHM); geniohyoid muscle (GHM); genioglossus muscle (GGM); and palato- glossus muscles (PGM). ROI-specific dose-volume histograms (DVHs) were calculated. Bootstrap-resampled recursive portioning analysis (RPA) was used to identify dose-volume effects associated with chronic dysphagia, and the resultant ROI dose-volume parameters were included in a multivariate (MV) model using stepwise regression selection of clinical variables, with receiver operator characteristic curve (ROC) area under the curve (AUC) to assess potential to discriminate chronic dysphagia. Results: A total of 284 patients were available for DVH analysis; 31 had chronic dysphagia. RPA showed DVH-derived MHM V69 (i.e., the volume receiving 69 Gy), GGM V35, ADM V60, and MPC V49 were strongly associated with chronic dysphagia (all P < .01; cumulative ROC AUC 0.83). Stepwise regression demonstrated among DVH-based models that a model using binary MHM V69 and ADM V60 thresholds maximally discriminated between patients with and without chronic dysphagia (ROC AUC 0.85). A model including age in addition to MHM V69 and ADM V60 was optimal among tested MV models for discrimination of chronic dysphagia (ROC AUC 0.90). Conclusion: Identification of swallowing muscle DVH parameters related to chronic dysphagia may allow selection of patients for aggressive swallowing rehabilitation. Dose to MHM, ADM, and GGM should be monitored and constrained in addition to pharyngeal constrictors when feasible given adequate target coverage. Author Disclosure: T.M. Dale: None. K.A. Hutcheson: None. A.S. Mohamed: None. G.B. Gunn: None. D.I. Rosenthal: None. C.D. Fuller: Research Grant; GE Healthcare, Elekta AB, NIH. Serve on professional society committees; Radiological Society of North America. 1039 Contralateral Submandibular Gland Sparing in Head and Neck Squamous Cell Carcinoma Is Safe and Improves Patient-Reported Outcomes (PROs) Y. Mao, 1 S. Samuels, 2 E. Sapir, 2 and A. Eisbruch 2 ; 1 Cancer Center, Sun Yat-sen University, Guangzhou, China, 2 University of Michigan, Ann Arbor, MI Purpose/Objective(s): To determine the feasibility and potential benefits of sparing contralateral submandibular gland (cSMG) during definitive (chemo) intensity modulated radiation therapy (cIMRT) for head and neck squamous cell carcinoma (HNSCC). Materials/Methods: We reviewed treatment plans, mean doses to organs at risk, PROs, and tumor control in 265 patients with HNSCC treated with definitive cIMRT at our institution between 2005 and 2014. Patients were periodically given the validated xerostomia questionnaire (XQ; fasting and eating domains) and Head and Neck Quality of Life questionnaire (HNQOL; communication, eating, emotion, and pain domains) pretreat- ment and at follow-up. We used correlation analysis and regression models to determine predictors of XQ and HNQOL. Results: Of 265 patients, 191 had oropharynx, 16 nasopharynx, 7 hypo- pharynx, 3 oral cavity, 33 laryngeal, 2 nasal and paranasal, and 13 un- known primary cancers. Six percent (17/265) had stage I to II, and 94 % (248/265) had stage III to IV. Median follow-up was 23.1 months (1e120 months), and 662 surveys were analyzed from pretreatment to 10 years posttreatment. A total of 55.2% (137/248) of stage III to IV patients received a <40 Gy mean dose to cSMG to preserve salivary output. The locoregional recurrence rate at 2 years was 8%, and no recurrences occurred in the contralateral level IB lymph nodes. The median dose to the oral cavity (OC) was 36 Gy (3e64 Gy), contralateral parotid gland (cPG) 25 Gy (4e53 Gy), ipsilateral PG (iPG) s 36 Gy (6e69 Gy), bilateral PG (bPG) 31 Gy (6e58 Gy), ipsilateral SMG (iSMG) 62 Gy (28e72 Gy), and the cSMG 37 Gy (8e70 Gy). Predictors of the XQ summary score on univariate analysis included the OC (P < .0001), cPG (P Z .0008), iPG (P < .0001), bPG (P < .0001), cSMG (P Z .0005), and iSMG (P Z .0053) mean doses, and time from treatment (TFT; P < .0001). On multivariate analyses, mean doses to bPG (r Z 0.11, P Z .011), OC (r Z 0.12, P Z .029), cSMG (r Z 0.098, P Z .019), and TFT (r Z -0.16, P Z .0001) predicted for XQ summary scores. In the XQ domains, the bPG (r Z 0.15, P Z .0003) and OC (r Z 0.15, P Z .0002) doses predicted for worse scores in the eating domain, while iPG (r Z 0.11, P Z .0067) and cSMG (r Z 0.12, P Z .003) and TFT (r Z -0.22, P < .0001) predicted worse scores in the fasting domain. For the HNQOL summary score, bPG (P Z .0175), cSMG (P Z .0131), and TFT (P < .0001) were predictors on univariate analysis and cSMG (r Z 0.15, P Z .0003) and TFT (r Z -0.27, P < .0001) remained significant on MVA. Using 40eGy cSMG dose as a prespecified threshold based on dose-saliva relationships, regression modeling showed that patients receiving <40 Gy had improvement (P Z .0001) in their XQ score over time after RT, while patients receiving 40 Gy did not (P Z .29). Conclusion: cSMG mean dose predicts for both patient-reported xero- stomia and QOL after IMRT. cSMG sparing did not compromise disease control. We recommend keeping the cSMG to less than 40 Gy if clinically possible. Author Disclosure: Y. Mao: None. S. Samuels: None. E. Sapir: None. A. Eisbruch: None. 1040 Reaffirming Metastatic Risk Profiles in Human Papillomavirus- related Oropharyngeal Cancer Following Definitive Radiation Therapy With or Without Chemotherapy B. O’Sullivan, S.H. Huang, J. Waldron, S. Su, A. Bayley, S. Bratman, J. Cho, M.E. Giuliani, A.J. Hope, J.H. Kim, J.G. Ringash, A. Hansen, D. Goldstein, L. Tong, B. Perez-Ordonez, I. Weinreb, F.F. Liu, and W. Xu; Princess Margaret Cancer Centre / University of Toronto, Toronto, ON, Canada Purpose/Objective(s): A prior publication from our group identified profiles of patients (pts) with differing risk of distant metastasis (DM), the main form of failure for HPV-related (HPV[+]) oropharyngeal cancer (OPC). These profiles are currently influencing the design of trials in HPV(+) OPC. The aim of this study is to reaffirm DM risk groups following definitive radiation therapy (RT) +/- chemotherapy (CTx) in a substantially expanded HPV(+) OPC cohort. Materials/Methods: All p16-confirmed newly diagnosed HPV(+) OPC patients treated with RT +/- CTx between 2000 and 2012 were included. Overall survival (OS), locoregional control (LRC), distant control (DC), and grade 3 to 4 late toxicity (LT) were estimated. Multivariable analysis (MVA) identified predictors for DC and OS. Recursive partitioning International Journal of Radiation Oncology  Biology  Physics S172