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Primary cutaneous neuroendocrine carcinoma within a cystic trichoblastoma: a nonfortuitous association?

Merkel cell (primary cutaneous neuroendocrine) carcinoma is a rare neoplasm of the skin. Its occurrence has been reported in association with other cutaneous neoplasms (Bowen disease, squamous cell carcinoma) in cases regarded as collision tumors. It has recently been described in association with cysts of the follicle apparatus. We present a unique case of rapidly growing nodular tumor on the left forearm of an 84-year-old woman, which proved to be a Merkel cell carcinoma located within a cystic trichoblastoma. The malignant component located in the center of the lesion had typical histopathological and immunohistochemical features of Merkel cell carcinoma. It was surrounded by an epithelial proliferation, made of K17-positive basaloid cells, whose aspects where those of trichoblastoma in a retiform pattern. Both lesions were intertwined, suggesting that the Merkel cell carcinoma had developed within a previously existing trichoblastoma and that it derived from the follicular Merkel cells present in the trichoblastoma. The unique features of this case, together with the reported cases of Merkel cell carcinoma arising within follicular lesions, and the fact that numerous Merkel cells are normally localized in the adult hair follicle, further support the hypothesis of a histogenetic link between normal follicular Merkel cells and Merkel cell carcinoma.