Cutaneous mastocytoma (CM) is a localized variant of mastocytosis, characterized by an over-accumulation of mast cells in the skin, without extra-cutaneous organ involvement. It is defined as the presence of up to 3 isolated mast-cell skin lesions and commonly develops in newborns and children. We report the case of a 35-year-old healthy Caucasian woman presenting with a 4-year history of a pruritic brown plaque on her left breast. Hematoxylin-eosin staining showed a dense dermal infiltrate of atypical mast cells extending to the subcutis. The cells presented a marked nuclear pleomorphism with bilobed and multilobed nuclei. Immunohistochemical studies revealed strongly expressed KIT (CD117) and CD25 proteins. Serum tryptase levels and bone marrow biopsy were normal. The diagnosis was a solitary cutaneous pleomorphic mastocytoma. This case can be added to 17 other cases of adult mastocytoma documented in the literature, although, unlike other reported cases, and as far as we are aware, this is the first case of pleomorphic mastocytoma in an adult.

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