Microcystic Adnexal Carcinoma and a Summary of Other Rare Malignant Adnexal Tumours.

OPINION STATEMENT: Microcystic adnexal carcinoma (MAC) is a rare, slow-growing, infiltrative malignant tumour most commonly found on the head and neck. It often presents as a solitary skin-coloured or yellow papule, plaque or nodule. Ultraviolet radiation, immunosuppression and ionising radiation are possible risk factors. Clinical and histological differential diagnoses include morpheaform basal cell carcinoma and desmoplastic trichoepithelioma. The diagnosis is usually made by skin biopsy, and the characteristic features are small keratin-filled cysts with nests and cords which resemble ductal structures. Immunohistochemistry can assist in differentiating MAC from other tumours. The local aggressive nature of the tumour and its potential to infiltrate beyond the assessed clinical margins warrant complete excision with marginal control, and we recommend Mohs micrographic surgery. Wide local excision is widely performed but is associated with recurrence given its infiltrative nature and extensive subclinical extension. The role of radiotherapy in the management of MAC is unclear.