Microcystic adnexal carcinoma versus desmoplastic trichoepithelioma: a comparative study

The histologic distinction between microcystic adnexal carcinoma (MAC) and desmoplastic trichoepithelioma (dTE) can be challenging in the setting of a superficial biopsy. However, accurate diagnosis has treatment implication because the standard of care for MAC is wide local excision but more conservative care for dTE. We reviewed the histologic features of 30 MAC and 39 dTE cases and performed cytokeratin (CK) 17, CK19, and epidermal growth factor receptor (EGFR) immunostains on 20 MACs and 18 dTEs. MAC cases occurred in older patients in comparison with dTE (median, 67 years vs. 34 years). The head and neck was the most commonly involved site, 88% and 89% for MAC and dTE, respectively. In addition to features previously reported as specific for MAC, such as skeletal muscle and subcutaneous tissue invasion, perineural invasion, and ductal differentiation, we found the presence of mitotic figures to be significantly more frequent in MAC cases (P < 0.0001). In contrast, the presence of keratocyst, keratin granuloma, and calcification was significantly more frequent in dTE cases (P < 0.0001). CK19 seems to be a helpful adjunct because its expression was seen in 70% (14/20) of MAC versus 22% (4/18) of dTE cases (P = 0.0044); however, the clinical usefulness in individual cases may be limited because of the overlapping immunoprofile. CK17 and EGFR expression was seen in all the MAC and dTE cases. Low polysomy of EGFR gene was observed in only one MAC case, suggesting that molecular mechanisms other than gene amplification play a role in EGFR overexpression.