Detection of ctDNA in patients with stage III cutaneous melanoma with satellite/in-transit or nodal metastases

e21559 Background: Circulating tumor DNA (ctDNA) has emerged as a biomarker to monitor disease status in cancer patients. Limited data exists on the sensitivity of ctDNA for detection of satellite/in-transit or nodal metastases in melanoma. Methods: We conducted a retrospective study of stage III cutaneous melanomas with clinically detected satellite/in-transit and/or lymph node metastases who underwent ctDNA analysis using an mPCR-NGS-based ctDNA assay ("Signatera") prior to surgery or systemic therapy at Inova Schar Cancer Institute. Results: ctDNA was detected in 4 of 14 patients including 1 of 3 patients with both satellite/in-transit metastases and nodal metastases; 3 of 6 patients with nodal metastases only; none of 5 patients with satellite/in-transit metastases only. Nodal tumor burden (sum of short axis of involved lymph nodes) for ctDNA detected patients was greater than 1.5cm (5.0cm, 3.0cm, 3.0cm, and 1.6cm, respectively), and 1.5cm or less in patients whose ctDNA were not detected. Conclusions: ctDNA may not detect satellite/in-transit metastases or small nodal metastases in melanoma. A larger study is needed to validate this finding. [Table: see text]