Squamous Islands in Clear Cell Hidradenoma : A Potential Diagnostic Pitfall

Abstract

Clear cell hidradenoma is a benign eccrine tumor with nodular‐cystic architecture, ductal differentiation and poroid cytology. Squamous islands occurring in eccrine neoplasms are classified into three types, according to their location, histologic characteristics and degrees of cellular atypia. Type I squamous islands resemble normal epidermis and are present in the superficial to middle portion of eccrine poromas. Type II squamous islands are localised in the deeper portion of eccrine poromas and show lumen formation suggestive of acrosyringeal differentiation. Type III squamous islands show atypical features and are limited to malignant eccrine tumors. A 55‐year‐old man was biopsied for a dermal nodule on his left arm. The initial shave biopsy revealed the surface of an intradermal cystic nodule with numerous whorls and horn pearls of squamatization, reminiscentÂof a well‐differentiated squamous cell carcinoma; however, no cytologic atypia, necrosis or mitotic activity were identified; focal clear cell and ductal differentiation were also present. Complete excision revealed clear cell hidradenoma with extensive mixed type I and type II squamous islands. The presence of whorls and pearls of squamoid cells is a rare finding in clear cell hidradenoma that, moreover, in the setting of a superficial biopsy, may be a diagnostic pitfall.