Follicular induction overlying a dermatofibrosarcoma protuberans

Follicular induction and CK20+ Merkel cells overlying cutaneous focal mucinosis

BACKGROUND

Cutaneous focal mucinosis (CFM) or focal dermal mucinosis is a benign reactive process categorized as a primary mucinosis. Skin biopsy is essential for diagnosis, as the clinical appearance is often non-specific. Follicular induction is a phenomenon whereby the epidermis is induced by an underlying process to form primitive or mature hair follicles, and is commonly seen overlying dermatofibromas. Follicular induction has been rarely described in CFM.

METHODS

We performed a retrospective histological review of lesions of CFM confirmed by skin biopsy from 2010 to 2015 in our department.

RESULTS

We found that 11% (11/98) of CFM lesions showed follicular induction. Cytokeratin 20 (CK20) immunostaining was performed on all 11 of these biopsies that showed follicular induction and highlighted an increased density of CK20+ Merkel cells within the basaloid epidermal proliferations.

CONCLUSION

As superficial basal cell carcinomas (BCC) often show a mucinous stroma around the basaloid islands, CFM with follicular induction may closely mimic a BCC histologically, particularly in superficial shave biopsies. Therefore, it is important that dermatopathologists be aware of this phenomenon. Furthermore, CK20+ staining within the basaloid epithelial proliferations may be helpful in differentiating CFM with follicular induction from a BCC.

Induction of Desmoplastic Trichoepithelioma in a Dermatofibroma

Induction of follicular germinative structures above a dermatofibroma (DF) is a common finding. Rarely, induction of a trichoblastoma in a DF has been observed. Here, we report the case of a desmoplastic trichoepithelioma induced by a DF. The lesion with clinical and histological appearance of a DF situated on the left dorsal foot showed an associated adnexal proliferation that fulfilled histopathological criteria of desmoplastic trichoepithelioma. Immunohistochemistry (Ber-EP4, Bcl-2, CK17, CK20, CK7, EMA, and Ki67) helped to confirm the diagnosis and to exclude possible differential diagnoses.