Abstract
Giant congenital melanocytic nevi are benign proliferations of melanocytes that clinically present as pigmented lesions larger than 20 cm at birth. The risk of malignant transformation within these lesions reported in the literature varies. Notably, the differential diagnosis for these lesions includes proliferative nodules, which presents a challenge in diagnosis. We report a case of an 8-month-old male infant with a giant congenital melanocytic nevus with an unusual, pigmented lesion within this large nevus on the left buttock. A biopsy was performed. Histopathology showed atypical monomorphous cells arranged in a sheet-like fashion within the dermis, with no involvement of the overlying epidermis. These cells appeared epithelioid with abundant pale pink cytoplasm, nuclei of variable sizes, and scattered mitotic figures. There were scattered pigmented melanophages adjacent to this population. PRAME expression was strong and diffuse within the melanocytic population and Ki-67 highlighted scattered nuclei within the proliferation with overall less than 5% staining, although focally increased concentration of Ki-67 positive cells was observed. Given the monomorphous appearance of the tumor nodule with scattered mitotic figures and positive staining for PRAME on immunohistochemistry, fluorescent in situ hybridization (FISH) analysis of the tissue was performed. A melanoma FISH panel demonstrated several markers (3/4) that were scored at abnormal levels above threshold indicating the presence of chromosomal abnormalities that are commonly associated with melanoma. A diagnosis of a proliferative nodule within a giant congenital melanocytic nevus was also considered. However, given the additional immunohistochemical and FISH information, the diagnosis of melanoma was favored.
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