Microcystic adnexal carcinoma (MAC) is a rare, locally aggressive malignant sweat gland neoplasm. It is commonly found on the head and neck, with a predilection for the central face.
MAC may also be referenced under a number of synonyms:
Microcystic adnexal carcinoma is a rare disease. Based on data from the United States, the estimated yearly incidence is five cases per 10 million people, with the majority (83%) being non-Hispanic white individuals.
MAC largely affects middle-aged and older adults, though cases in children have been reported. Rates between men and women are comparable, though some sources indicate a slight female skew.
Predisposing factors include:
Although its rarity makes it difficult to establish a precise aetiology, microcystic adnexal carcinoma is believed to develop from a pluripotent adnexal keratinocyte population that is capable of both eccrine and follicular differentiation.
Microcystic adnexal carcinoma typically presents as a solitary, asymptomatic, poorly defined, skin-coloured or yellow papule or plaque. It most commonly appears on the head and neck. Additional reported sites include the scalp, trunk, extremities, breast, vulva, axilla, and perineum.
Typically, the tumour surface is smooth and non-ulcerated, although atrophic, scaly, or fissured MAC have been described. MAC may appear inconspicuous and is often considered benign at first, or it may be mistaken for superficial basal cell carcinoma.
MAC tends to be indolent, deeply infiltrative, and locally aggressive with a high propensity for perineural invasion and infiltration beyond clinically apparent margins. Although metastasis is rare, local invasion of muscle, adipose, and nerve tissue can result in significant morbidity.
Microcystic adnexal carcinoma is diagnosed by histopathology. A deep (incisional) skin biopsy is crucial for diagnosis, as superficial biopsies may reveal only benign features.
The characteristic histopathologic features of MAC include the following:
Perineural invasion is frequently present and immunohistochemistry can assist in differentiating MAC from other tumours, although MAC does not have a unique immunohistochemical profile.
It is important to recognise that MAC has subtle clinical and histologic features which can often lead to a delay in diagnosis.
The risk of microcystic adnexal carcinoma can be mitigated by photoprotective measures to reduce UV exposure and minimise other sources of unnecessary ionising radiation.
Microcystic adnexal carcinoma has a favourable prognosis in terms of survival. The major prognostic concern for MAC is a high risk of tumour recurrence due to its locally aggressive nature and propensity for perineural invasion. It rarely metastasises but often requires extensive surgery, posing functional and cosmetic challenges especially on the head and neck. Mohs micrographic surgery is the preferred treatment to minimise tissue loss and to ensure margin control.
