Microcystic adnexal carcinoma
MAC; sclerosing sweat duct carcinoma; syringoid eccrine carcinoma (overlapping)
Microcystic adnexal carcinoma is a rare, locally aggressive sweat-duct / pilar adnexal carcinoma that usually arises on the central face — particularly the upper lip, cheek and periorbital region — of middle-aged and elderly adults. Histologically it shows differentiation toward both eccrine and follicular structures with deep infiltrative growth and frequent perineural invasion that extends far beyond clinical margins. Diagnosis on superficial biopsy is notoriously difficult because the surface may show only bland-looking ducts or syringoma-like features. Mohs micrographic surgery is the treatment of choice. Distant metastasis is rare; local recurrence is the main risk.
Clinical features
- Slowly enlarging, indurated, skin-coloured to yellow plaque or nodule.
- Most often on the upper lip, cheek, periorbital skin or scalp; also chin, nose, forehead.
- Median age 60; F>M (slight predominance).
- Often present for many years before diagnosis (typical 5–10 year history).
- Risk factors: previous radiotherapy to the area; immunosuppression in some series.
- Symptoms of perineural extension — paraesthesia, numbness, dysaesthesia, cranial-nerve weakness — should raise concern.
Histology & pitfalls
- Superficial: bland-appearing keratin-filled microcysts and small ductal structures resembling syringoma or trichoadenoma.
- Deep: infiltrative cords and nests of basaloid / keratinising cells extending into deep dermis, subcutis, muscle and bone.
- Prominent stromal sclerosis; perineural invasion in 60–80%.
- Pitfall: a superficial biopsy may yield a benign-appearing diagnosis (syringoma, desmoplastic trichoepithelioma). For any persistent firm facial lesion, request a deep punch or incisional biopsy and remind the histopathologist of the clinical picture.
- Immunohistochemistry: CK7+, EMA+, BerEP4 variable, CK15/CD34 may help distinguish from morphoeic BCC.
Management
- Mohs micrographic surgery — first-line. Recurrence rate ~5% with Mohs vs 30–60% with conventional excision because of clinically silent infiltration.
- If Mohs is unavailable: WLE with at least 1 cm clinical margin and complete histological assessment of the entire deep margin (peripheral en-face frozen section / "slow Mohs").
- Adjuvant radiotherapy considered for: incomplete margins, named-nerve perineural invasion, recurrent disease.
- MRI of the head and neck if perineural invasion is histologically demonstrated, to assess proximal extent and skull-base.
- Systemic therapy options for advanced disease are limited; cetuximab use rests on a small number of case reports without robust evidence of EGFR-targeted benefit and should be considered anecdotal.
Prognosis
Distant metastasis is rare (<5%). Local recurrence is the main long-term risk; reported recurrence rates with conventional excision are 30–60% and substantially lower with Mohs. Long-term surveillance (annual for 10+ years) is necessary because of late local recurrence and second-primary skin cancers.
References
- Yu JB et al. Microcystic adnexal carcinoma: a population-based analysis. J Am Acad Dermatol; 2010.
- Friedman PM et al. Mohs micrographic surgery for the treatment of microcystic adnexal carcinoma. Dermatol Surg; 1999.
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