An atypical melanocytic lesion without genomic abnormalities shows locoregional metastasis

Abstract

A subset of difficult melanocytic lesions exists with histopathologic features that evade diagnostic consensus from even expert dermatopathologists. Comparative genomic hybridization (CGH) has emerged as a useful diagnostic tool to categorize these lesions, by identifying known chromosomal aberrations in malignant melanoma or the lack thereof in melanocytic nevi. However, determining a lesion's biological behavior primarily on CGH is limited by a relatively small series of corroborative cases without long term follow up. We present a case of a pigmented lesion on the right cheek of a 4 year old boy. The lesion had features of a deep penetrating nevus, but the presence of frequent mitoses, tumor infiltrating lymphocytes, and microscopic foci of tumor necrosis were concerning for an unusual melanoma. We termed this lesion a melanocytic tumor of uncertain potential (MELTUMP) for these reasons. High-resolution array-CGH performed elsewhere on the lesion demonstrated no melanoma-associated genomic abnormalities. A sentinel lymph node biopsy of this patient later revealed multiple small tumor deposits. Although the presence of nodal involvement in similar lesions often do not lead to progressive and fatal disease, this case illustrates that atypical melanocytic lesions with nodal involvement may not demonstrate genomic abnormalities by CGH, and that histopathologic assessment remains paramount in defining these difficult melanocytic lesions. Further comprehensive study of these lesions is needed.

Fig. 1

A) Scanning magnification view of the wedge-shaped lesion showing a deep dermal melanocytic proliferation (2.5×). (B) Tumor infiltrating lymphocytes are seen between large melanocytes with prominent nucleoli (20×). (C) A focus suggestive of tumor necrosis is present (40×).

Fig. 2

Mitoses present at the base of a lesion (Left, black square, higher magnification in inset) and two in the same high power field (Right, black squares and insets). There is an inflammatory cell infiltrate in this lesion.

Fig. 3

A) High power view of lymph node deposit shows cytologically atypical pigmented tumor cells that are similar to the original lesion, with (B) corresponding HMB45 immunostain positivity. (C and D). Other intraparenchymal deposits are also HMB45-positive.