Nodal Cellular Blue Nevus in Sentinel Lymph Node Biopsy: A Case Report With Emphasis on Avoiding Misdiagnosis of This Important Mimicker of Metastatic Melanoma

Melanoma arising in blue nevus (MBN) is a rare, aggressive malignancy that can develop from a preexisting blue nevus or resemble a cellular blue nevus without a clear precursor. We present a diagnostically challenging case of MBN on the foot of a 13-year-old female, with two sentinel lymph nodes (SLNs) showing heavily pigmented cells within the capsule and trabeculae. In contrast to the primary tumor, the cells within the SLNs contained bland spindled to epithelioid melanocytes with low proliferation and no significant cytologic atypia. Chromosomal microarray analysis was utilized to compare genetic profiles in both the primary melanoma as well as the lymph node. The melanoma revealed five copy number aberrations. Conversely, the SLN exhibited a normal profile. The findings were compatible with multiple nodal cellular blue nevi. The distinction between metastatic melanoma and nodal nevus is an important one with significant clinical implications. This case underscores the diagnostic challenge nodal cellular blue nevi can pose and the importance of correlating morphology, immunophenotype, and in difficult cases, molecular findings, to avoid misdiagnosis as metastatic melanoma. In particular, array-based comparative genomic hybridization (aCGH) was helpful in this case.