INTRODUCTION Also known as combined adnexal tumor of the skin, sclerosing sweat duct carcinoma, microcystic carcinoma, sweat gland carcinoma with syringomatous features, and malignant syringoma, microcystic adnexal carcinoma is a rare cutaneous malignant tumor of sweat gland origin. Microcystic adnexal carcinoma typically appears in the fourth decade of life, both sexes being equally affected. In 88% of cases the lesions occur on the face. Growth is typically indolent, but the tumor is usually locally aggressive, extending into dermal, subcutaneous, and perineural planes. There is a 50% rate of local recurrence. The tumor occasionally spreads to the regional lymph nodes, but metastases are rare, amounting to only about 2%. There seems to be an association with prior radiation, and there may be an association with immunosuppression therapy.
CLINICAL PRESENTATION The lesion is solitary and usually appears as a slowly growing, flesh-colored indurated plaque or nodule, sometimes with an overlying whitish to pink discoloration. This tumor may be more diffusely infiltrative with wide perineural spread. Lesions are typically present for several years prior to diagnosis. In some cases it may be present without symptoms for decades before a diagnosis is made. The size ranges from 1 to 3 cm. The lesion may be fixed to deep tissues because of its tendency to infiltrate tissue planes. Very rarely this tumor may be multiple and bilateral. These tumors are very rare in blacks, with only a few cases having been described.
HISTOPATHOLOGY These tumors usually involve the dermis and subcutaneous tissue and may extend into muscle. The more superficial tumor has numerous keratinous cysts along with islands and strands of squamous and basaloid epithelium with variable differentiation into ductules. The deeper portion of the tumor has nests and strands of tumor and small ductules in a dense fibrous stroma. Perineural invasion is frequent and helps to distinguish microcystic adnexal carcinoma from a syringoma.
DIFFERENTIAL DIAGNOSIS Microcystic adnexal carcinoma is difficult to distinguish clinically from other malignant conditions, even on histopathology. The differential diagnosis includes syringoma, chalazion, basal cell carcinoma, squamous cell carcinoma, and sebaceous cell carcinoma.
TREATMENT While the standard of care for this tumor has not yet been established, complete surgical excision with wide tumor-free margins appears to be the treatment of choice to prevent the typical recurrences seen with this lesion. Mohs micrographic surgery is preferred but resection under frozen section control is a good alternative. Uncontrolled excision should include at least 4 to 5 mm of margin around the visible tumor. Once clear margins are obtained, additional wide excision is recommended. Radiotherapy is a useful adjunct in recurrent cases.
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