Clinical Molecular Testing for Clonal Relatedness of Second Melanoma Tumors.

Context.—: Patients with melanoma can develop second tumors representing either metastases or new primary melanoma. This distinction has profound implications for management. Although clinicopathologic features are often sufficient, molecular assays can support the presence or absence of clonal relatedness in challenging cases. However, the potential for false-positive and false-negative results in this context is not well described.

Objective.—: To evaluate clinical molecular assays used to determine whether melanoma tumors represent primary-metastasis pairs or unrelated tumors.

Design.—: We identified clinical cases at our institution in which paired melanocytic tumors were analyzed for clonal relatedness by molecular assays. Results were compared against data sets and/or controls to establish the likelihood that paired tumors were clonally related.

Results.—: In total, 12 pairs were evaluated by single-nucleotide polymorphism (SNP) array, targeted next-generation sequencing (NGS), or both. SNP array predicted relatedness in 5 of 9 cases and unrelatedness in 4 cases. In SNP comparisons, whole-chromosomal and arm-level changes were often nonspecific (coincidentally similar between unrelated tumors). Targeted NGS predicted relatedness in 2 of 4 cases and unrelatedness in 1 case, and was equivocal/noncontributory in 1 case. For targeted NGS, nonspecific (coincidentally similar) results were related to recurrent oncogenic drivers or pairs lacking detected oncogene mutations.

Conclusions.—: The genome-wide analysis provided by SNP array was optimal for assessment of clonality. Targeted NGS can be informative but may be equivocal in some cases. The choice of assay may rely upon considerations including the amount of DNA, likelihood of distinctive mutations, and need for therapeutic target identification.