Track

Case Reports

Abstract

We present a case of a 58-year-old male who presented with a year history of a cystic enlarged mass on the ulnar region of left forearm. A local excision revealed a yellow-tan brown, firm, nodular adipose tissue measuring 4.1 x 3.3 x 2.1 cm, with a central cystic filled cavity with pale, yellow, firm, and grumous material on gross examination. Microscopic examination revealed a biphasic cyst within deep dermis and subcutis, partially surrounded by a fibrous pseudocapsule. A portion of the cyst demonstrates pilomatrical differentiation with ghost cells, basaloid cells with small, uniform round vesicular nuclei, and dystrophic calcifications. The other portion demonstrates a cyst lining composed of stratified squamous epithelium with a granular layer containing lamellated keratin, consistent with an epidermoid cyst. Significant nuclear pleomorphism, atypical mitotic figures, or an infiltrative growth pattern are not readily identified. However, a foci with an atypical epidermal keratinocytes proliferation with acanthosis, hyperkeratosis and horn cyst like inclusions reminiscent of seborrheic keratosis was noted. A final diagnosis of Hybrid cyst with features of pilomatricoma and epidermoid cyst was rendered.

The hybrid tumor with composite epidermoid cyst and pilomatricoma features are exceedingly rare with a handful of cases reported in literature, hence, description of potential histomorphologic variation of this entity is very limited. Our case extends to includes this unique atypical seborrheic keratosis like features that could be observed in this rare entity and to avoid this as a potential diagnostic pitfall.