Abstract

A 36-year-old Caucasian female, without a significant past medical history, presented to her primary care physician for evaluation of an asymptomatic pigmented papule on her left thigh of eight months duration. A shave biopsy was performed due to concern for malignancy. Initially, the case was signed out as a mildly dysplastic nevus by a general pathologist. However, several months later, the patient re-presented as the lesion had regrown and started to bleed. A re-biopsy was performed, with histologic features showing clear features of malignancy, prompting dermatopathology consultation for both biopsies. Examination of the initial biopsy, histologic stained sections revealed mostly bland and monomorphic small epithelioid melanocytes. However, multiple deep dermal atypical mitoses were present along with multiple focal expansile aggregates of atypical intradermal melanocytes.  Immunohistochemistry showed strong and diffuse staining with HMB45, increased proliferative index with Ki67 staining and complete loss of P16. Given the histologic findings a diagnosis was most consistent with a nevoid melanoma on the initial biopsy. Nevoid melanoma is a rare variant of nodular melanoma with characteristic features which mimic a benign nevus, especially at lower magnification. As with this case, these lesions can present as a diagnostic challenge. Important histologic clues not to overlook include the presence of multiple deep dermal atypical mitoses, impaired maturation with depth, and prominent nucleoli, which are often seen in the cells at the tumor base. We present a rare case of a misdiagnosis of a nevoid melanoma and discuss lessons learned from the histology of this case.