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patients had tumoral PD-L1 expression !5%. Patients with tumoral PD-L1 !5% had better OS vs those with lower expression, HRZ0.29 (CI 0.10-0.78), pZ0.009; 10 years OS: 84% for PD-L1 !5% vs. 46% for PD-L1 <5%. On univariate analysis, OS was associated with PD-L1 status, as well as T stage, N stage, ECOG status and gender. On multivariate analysis, PD-L1 !5% remained statistically significant for better OS, HRZ0.35 (CI 0.12-0.99), pZ0.047. 33 of 65 (51%) of tumors had high CD8 levels (3 or 4). There was no association between CD8 status and OS; further stratifying PD-L1 high patients by CD8 did not improve the prognostic impact vs PD-L1 status alone. Conclusion: This is the first study reporting significant association of PD-L1 expression with OS in patients with anal cancer treated with CRT. PD-L1 status warrants consideration in the prognostication of patients with anal cancer. Future studies are required to determine the benefit of alternative treatment strategies based on PD-L1 status.

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