Pre-operative circulating tumour DNA in high-risk primary cutaneous melanoma: prospective feasibility in molecular pathology

BACKGROUND AND PURPOSE: Circulating tumour DNA (ctDNA) has emerged as a prognostic biomarker in melanoma, but its detectability in pre-operative blood from patients presenting with primary cutaneous melanoma remains incompletely evaluated. We assessed the feasibility of pre-operative ctDNA detection in high-risk primary melanoma using routinely available methods in molecular pathology. PATIENTS/MATERIAL AND METHODS: In a prospective single-institution cohort enrolled between September 2021 and December 2022, pre-operative plasma was obtained from patients with clinically suspected primary cutaneous melanoma. Patients with pathologically confirmed invasive melanoma and high-risk features (≥ T3a, ≥ N1a, or ≥ M1a) were selected for molecular analysis. Tumour tissue was analysed using next-generation sequencing (NGS) to identify targetable BRAF or NRAS driver mutations, and pre-operative plasma was analysed for ctDNA using droplet digital PCR (ddPCR) for BRAF V600E or targeted NGS for other driver mutations. RESULTS: Of the 288 consented patients with pre-operative blood samples, 21 met high-risk criteria, and 12 had a targetable BRAF or NRAS driver mutation in tumour tissue and underwent tumour-informed plasma ctDNA analysis. Pre-operative ctDNA was not detected in any of these 12 patients (0 of 12; 95% confidence interval [CI] 0 to 26.5%). All ddPCR and NGS assay controls performed as expected, and wild-type copy counts were consistent across ddPCR samples. INTERPRETATION: The finding is concordant with two independent studies using different methods, and a mathematical prediction of ctDNA shedding from small primary tumours. Reliable pre-operative ctDNA detection in primary melanoma may require alternative cell-free DNA (cfDNA) approaches, such as bespoke multivariant or mutation-agnostic methods.