OTHR-08. PREDICTION OF RISK OF CENTRAL NERVOUS SYSTEM METASTASIS FOR AJCC 8(TH) EDITION STAGE III MELANOMA PATIENTS

Among common solid tumors, melanoma has the highest risk of CNS metastasis. Improved understanding of the incidence, risk factors, and timing of CNS metastasis is needed to inform surveillance strategies for at-risk patients. Clinical data were extracted from two institutions for AJCC 8th edition stage III melanoma patients, diagnosed from 1998–2014 who had negative baseline CNS imaging within 4 months of diagnosis. The cumulative incidence of CNS metastasis was calculated in the presence of the competing risk of death from stage III presentation, and at benchmark time points 1-, 2-, and 5-years post-diagnosis. The cohort (N=1,918) consisted of patients from major melanoma centers in the US (50.6%) and Australia (49.4%). The first site of distant metastasis was CNS only for 3.9%, CNS and extra-cranial sites (ECS) for 1.9%, and ECS only for 31.2% of patients (N=1918); 15.5% of patients who developed distant metastases (N=708) had CNS involvement at first diagnosis of stage IV disease. Cumulative incidence of CNS metastasis from stage III diagnosis was 3.7% (95% Confidence Interval (CI): 2.9–4.6) at 1-year; 9.6% (95% CI: 8.3–11.0) at 2-years; and 15.9% (95% CI: 14.2–17.7) at 5-years. In multivariable analyses, risk of CNS metastasis was significantly higher for males; younger patients; increasing AJCC stage group; scalp primary tumor site, acral melanoma subtype, and increased primary tumor mitotic rate. Conditional analyses showed that only high primary tumor mitotic rate (>9 per mm2) was significantly associated with risk of subsequent CNS metastasis among patients who survived without CNS recurrence 1-, 2-, and 5-years after the diagnosis of stage III disease. Similar rates of CNS metastasis were observed between these two large, geographically-distinct stage III melanoma patient cohorts. These results provide a framework for developing evidence-based surveillance strategies and for evaluating the impact of contemporary adjuvant therapies on the risk of melanoma CNS metastasis.