Abstract
Spitz nevus (SN) is an uncommon melanocytic lesion that presents in childhood or adolescence and remains a diagnostically challenging entity within dermatopathology due to overlapping features with melanoma. Here we report a case of SN with anaplastic lymphoma kinase (ALK) positivity by immunohistochemistry. A 9-year-old girl presented with a 4 mm pink, dome-shaped papule on the left wrist. Biopsy of the lesion demonstrated vertically oriented nests of bland epithelioid to spindled cells in the dermal papillae and superficial dermis with prominent lymphocytic inflammation. HMB45 and tyrosinase stains were positive and Ki-67 proliferative index was <5% consistent with SN. ALK immunohistochemistry showed strong positive staining within melanocytes, indicating a genomic rearrangement involving the ALK gene. ALK-positive SN more commonly arise on the extremities in younger patients as compared to those without involvement of ALK. Histologically, these lesions have a characteristic plexiform architecture of fusiform melanocytes with background inflammation, as demonstrated by this case. ALK fusions have been reported in 10% to 50% of SN and most often involve the TPM3 or DCTN1 genes. The prognosis is excellent with rare recurrence after conservative excision and no reports of progression to melanoma. Thus, identification of ALK fusions in these challenging tumors may provide important diagnostic and prognostic information as ALK has only rarely been implicated in spitzoid melanoma and its presence suggests a SN with an overall benign clinical course. Furthermore, the use of specific kinase inhibitors may be investigated in treating residual disease in Spitz nevi and atypical Spitz tumors, thereby potentially decreasing the need for surgical re-excision.
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