Abstract only 68 Background: Nivolumab improved overall survival (OS) in the treatment of platinum-refractory recurrent and metastatic head and neck squamous cell carcinoma (R/M HNSCC) in a phase III clinical trial. However, Kaplan-Meier OS and progression-free survival (PFS) curves for the nivolumab and cytotoxic agent arms crossed at 3-6 months, suggesting that patients with initial resistance to immunotherapy may have better outcomes with cytotoxic treatment. Here, we explored suitable conditions and candidates of predictive factors for nivolumab for R/M HNSCC. Methods: We retrospectively reviewed the clinical records of 28 consecutive patients with HNSCC from 2014-2018. Tumor size was evaluated by computed tomography according to RECIST ver.1.1. Tumor growth rate (Gr) was defined as 3Log (Dt/D0)/t, where D 0 and D t are the sum of the longest diameters of the target lesions (SumTLs) at baseline and pre-baseline, and t is time, with 1 t defined as the 4-week interval between tumor evaluations. Results: After exclusion of 2 patients with unevaluable responses, 26 patients were included in the study. Median follow-up time for survivors was 9.5 months. Median SumTLs at baseline was 43.2 mm (range, 10.1-135.3). Median Gr was 0.22 (range, 0.05-0.76). Median OS and PFS was 7.9 months (95% CI, 5.8 to 34.5) and 3.7 months (95% CI, 2.1 to 12.9), respectively. Patients with progression within 3 months showed significantly worse survival (HR 6.33, 95% CI 2.08-21.67, p=0.001). Moreover, higher Gr and bigger SumTLs appeared to be associated with poorer outcomes. We therefore explored the association between prognosis and the ratio of SumTLs to Gr (SumTLs/Gr). The cut off value of SumTLs/Gr was calculated by ROC analysis. Pre-treatment SumTLs/Gr > 212 was associated with significantly worse OS (HR 8.87, 95% CI 2.33-58.29, p<0.001) and PFS (HR 2.86, 95% CI 1.10-8.33, p=0.03). Conclusions: Although retrospective with a small sample size, these results suggest that pre-treatment SumTLs/Gr > 212 was significantly associated with inferior OS and PFS in R/M HNSCC patients treated with nivolumab. R/M HNSCC patients with pre-treatment SumTLs/Gr > 212 might be unsuitable for treatment with nivolumab.