LAUSR

9552 Background: Melanoma is notorious for its high degree of heterogeneity with the implication that metastases in different sites react differently to immunotherapy. We aimed to explore the site-specific response pattern in anti-PD-1 monotherapy treated advanced melanoma patients. Methods: Clinical data was collected retrospectively from advanced melanoma patients treated with anti-PD-1 monotherapy at Massachusetts General Hospital (MGH) from Sept 2009 to Dec 2017. Radiological evaluations were carried out by radiologists from the MGH Tumor Imaging Metrics Core using irRECIST 1.1. Statistical analysis was carried out using ANOVA test. Results: Among 61 evaluated patients, 56 (91.8%) had at least one measurable target lesion(s) at baseline, including 35 (57.4%) patients with measurable lymph nodes (LN)/subcutaneous lesion(s), 25 (50.0%) with lung lesion(s), and 21 (34.4%) with liver lesion(s). At week-12 radiological evaluation after anti-PD-1 monotherapy initiation, lesions at different sites responded differently at marginal statistical significance (P = 0.071), namely mean percent changes compared with baseline were 3.75%, 5.12%, and -30.95% for LN/subcutaneous, liver, and lung lesions, respectively. Among patients who had disease under control (CR/PR/SD) (n = 37, 60.7%) by week-12 evaluation, the mean tumor percentage change at week-24 compared with week-12 were -8.94%, -12.18%, and -5.91% for LN/subcutaneous, liver, and lung lesions, respectively (P = 0.479). Conclusions: Although our small sample size limits definitive discrimination in organ site-specific response differences, it appears that lung lesions respond more quickly and to a greater extent compared with LN/subcutaneous and liver lesions in advanced melanoma patients treated with anti-PD-1 monotherapy. We will explore this in a larger, multicenter cohort.