1545 Circulating tumor DNA reflects tumor burden and detects early recurrence in patients with Merkel cell carcinoma

Merkel cell carcinoma (MCC) is an aggressive skin cancer that recurs in 40% of cases. Effective surveillance crucial for disease management. This prospective, multicenter study assessed the utility circulating tumor DNA (ctDNA) to assess burden and detect early recurrence MCC patients. Whole-exome sequencing was performed on tissue identify tumor-specific, single nucleotide variants each patient, which were tracked subsequent blood samples. A total 562 samples collected from 167 patients (pts) over time, with a median follow-up 10 months April 2020 August 2022. Of pts, 66 had clinically evident at baseline majority (63/66) ctDNA positive (sensitivity: 95%; 95% CI: 87-99%). Ninety-six pts showed no evidence most them (86/96, 90%) being negative. There 30 where acquired prior initial treatment. Among these primary diameter (median size 2.2cm; range 0.4-12) strongly correlated levels (median: 17.2 MTM/ml, 0.08-4490; r=0.82, p<.001). 119 (71%, samples: 369) followed longitudinal testing during surveillance, while negative disease. Compared who remained negative, ctDNA-positivity associated nearly 7-fold higher rate (HR=6.8, 2.9-16, After test (n=298 tests), risk low next 90 days (2.2%, 0.7-4.2%), contrast after (31%, 17-50%, n = 71 tests). These data demonstrate promising blood-based tool detection recurrence. Pts active may not need imaging within test.