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Case Reports

Abstract

Merkel cell carcinoma (MCC) is a rare, cutaneous, and usually intradermal neuroendocrine malignancy which tends to occur on sun-damaged skin of older patients. Merkel cell carcinoma in situ (MCCis) is an exceedingly rare lesion which morphologically and immunohistochemically resembles MCC but is confined entirely to the epidermis. Here we describe a case of MCCis developing on the extremity of a relatively young male patient. A 40-year-old otherwise healthy male presented for evaluation of a new red, non-bleeding lesion on the left dorsal hand that had been present for several months. A shave biopsy revealed abundant single and nested cells within the epidermis with a high nuclear to cytoplasmic ratio. Numerous apoptotic bodies and pagetoid scatter were also observed. By immunohistochemistry, these cells demonstrated positivity for AE1/AE3, CK20, CK7, p63, synaptophysin, and chromogranin, and were negative for SOX10. No intradermal component was identified on multiple step sections, and a diagnosis of MCCis was rendered. MCCis can resemble other intraepidermal malignancies such as squamous cell carcinoma in situ and thus demands constant vigilance by the dermatopathologist. Classic MCC portends a relatively high mortality rate, and p63 also suggests more aggressive behavior. Conversely, MCCis is reported to carry an excellent prognosis with complete excision, but the significance of p63 positivity, as seen in the current case, has yet to be elucidated as few other cases evaluated with a p63 stain have so far been described in the literature.