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4054 Background: NMASCC is a rising incidence disease with up to 30% of treatment failure to achieve complete response (CR) after standard chemoradiotherapy (CRT) leading to severe morbidity and death. Stage III-TNM, p53 mutations, HPV negativity, HIV infection are linked to treatment failure. We investigated the predictive/prognostic role of TNM, CR, HPV, PDL1 positivity and CD3/CD8 densities in NM-ASCC from a single institution. Methods: All 79 eligible consecutive NMASCC pts (available FFPE pre-treatment samples) seen from October-2009 to April-2019 having completed definitive CRT (50.4 Gy Pelvic Radiotherapy with Mitomycin-C 12mg/m2/IV/d1-5 / FU 1000mg/m2/d1-4 d29-32 (28%), Mitomycin-C/Capecitabine 825 mg/m2/bid (38%), Cisplatin 60 mg/m2/IV d1-29 and 5FU (34%) were analyzed. Mean age: 59 (range 26-87), 72% female, Stage III: 59%, HPV positive: 86% (HPV-16: 80%);14% HIV positive. IHC assessed by two pathologist for PD-L1 expression (ClonSP263) and CD3-CD8+ TILS densities (Clone 2GV6, Clone SP57). HPV-DNA assessed by PCR (BSGP5+/6+ multiplexed with beta-globin). Kaplan-Meier survival, CR, DFS, OS and Univariate analyses were performed using Cox proportional hazard model. Results: CR achieved within 6 months of treatment completion was 68%( 53pts). Median follow-up after treatment completion: 35 months (range 6 –149). As of February 2020, 82% (65 pts) are alive, no evidence of disease:(57%) 46 pts, recurrence rate: 26%(22 pts), cancer death: 18% (14 pts). PDL1+ tumors ( > 1% positivity-CPS score): 56%, expression levels: 1-5% (57%,26p), > 10%-100% (43%,19p). PDL1+ had a strong association with CR (p = 0.021); higher PDL1+ levels had 8-fold of CR-likelihood than PDL1 negative.(OR 8.50 vs. 1.12). Significative Spearman correlation between PDL1 tumors with CR and CD3-CD8 TILS density was observed (R = 0.43,p = 0.0017 and R = 0.36,p = 0.00094 respectively), albeit CD3-CD8 failed to reach significance as prognostic factors for either CR, DFS or OS. Only CR and PDL1 positive were strongly significantly associated to DFS (HR 0.10 [IC 95% 0.04-0.28] p < 0.001 and HR 0.28 [IC 95% 0.11-0.73] p = 0.006) and OS (HR 0.12 [IC 95% 0.03-0.45] p < 0.001 and HR 0.15 [IC 95% 0.03-0.68] p < 0.004). Low prevalence of HPV negative, early tumors, HIV positive cases in our series probably impacted in statistical power for prognosis correlation. Conclusions: PDL1 positivity was the strongest predictive/prognostic factor in NM-ASCC. Alternative therapeutics options to standard CRT should be explored on poor-risk patients as HPV-negative, P53-mutated and PDL1 negative patients.