Microcystic adnexal carcinoma mimicking basal cell carcinoma.

Clinical presentation

A 42-year-old man presented with a 5-year history of an asymptomatic slow-growing papule on the left scapular region. The physical examination revealed a well-defined, 1.2-cm diameter, firm, dome-shaped papule with a white-pinkish center, peripheral small papules with a pearly appearance, and a hyperpigmented outer rim (Fig 1). The initial clinical impression suggested basal cell carcinoma.

Fig 1

Clinical image showing a firm, pearly, dome-shaped papule with a white-pinkish center, cystic structures, and a hyperpigmented outer rim.

Dermoscopic appearance

Dermoscopic assessment revealed a white structureless central area, white-yellowish clods of variable sizes distributed peripherally, and a brown pigmented outer rim (Fig 2).

Fig 2

Dermoscopy revealed a white, structureless, central area with white-yellowish clods of variable sizes and perilesional light-brown pigmentation.

Histologic diagnosis

Histopathologic analysis revealed a dermal neoplasm composed superficially of keratin-filled cysts and aggregates of basaloid cells in slender cords deeply infiltrating the dermis. The immunohistochemical study revealed a positive pattern for the expression of carcinoembryonic antigen and epithelial membrane antigen, demonstrating follicular and eccrine differentiation, and negative stain for Ber-Ep4, features consistent with microcystic adnexal carcinoma (MAC) (Fig 3).

Fig 3

A, Microcystic adnexal carcinoma: dermal neoplasm composed superficially of keratin-filled cysts and infiltrative nests of basaloid cells in strands in the underlying reticular dermis. Immunohistochemistry was positive for (B) carcinoembryonic antigen and (C) epithelial membrane antigen, markers of eccrine and apocrine ducts, favoring microcystic adnexal carcinoma. D, Immunohistochemistry negative for BerEp4, a marker of eccrine secretory and follicular germinative cells favoring microcystic adnexal carcinoma. (A, Hematoxylin–eosin stain; original magnification: left, ×10; right, ×20.)

MAC is a rare malignant adnexal tumor with pilar and eccrine sweat gland differentiation. Clinically, it is characterized by a slow-growing, asymptomatic, firm, papule, plaque, or nodule, affecting middle-aged adults, usually located on the head and neck.1, 2 MAC is commonly misdiagnosed clinically and histopathologically on account of similar features with other adnexal tumors or basal cell carcinoma. In addition, MAC has a local aggressive growth pattern with the capacity to infiltrate deeply and rarely metastasize. Consequently, inadequate biopsy techniques showing only the superficial component should be avoided.1, 2 Clinical suspicion of this infrequent tumor supported by dermoscopic findings can aid in early recognition to establish a correct diagnosis and management. Adequate excision with Mohs micrographic surgery to ensure appropriate margin clearance is essential to prevent local recurrences.