Trichilemmal Cysts With Divergent Ductal Differentiation: A Series of 4 Cases.

Trichilemmal cysts are common clonal tumors with a predilection for the scalp. They are composed of an outer epithelial wall resembling the outer root sheath in the isthmus of the hair follicle and a central core of compact keratin. Sweat duct differentiation is exceptional with only one convincing case reported to date. Here, we sought to characterize the clinicopathological characteristics of sweat duct differentiation in trichilemmal cysts. We reviewed all cases of trichilemmal cyst diagnosed at our institution between 2008 and 2019. Ductal structures were found in 4 of 411 cases (0.97%). Subjects included 2 male and 2 female patients with a median age of 37.5 years (range 34-55). The ducts were lined by attenuated epithelial cells and immunoreactive for polyclonal carcinoembryonic antigen and cytokeratin 7. Ductal differentiation involved a median of 7.5% (range 1%-50%) of the cyst wall. All 4 cases were from the scalp and treated with local excision. No recurrence was identified with a median follow-up period of 1.5 years (range 1-12 years). In summary, sweat duct differentiation in trichilemmal cysts is rare but likely under recognized. Conceptually, we suggest it represents a type of divergent cellular differentiation within a clonal neoplasm rather than a retention cyst or hybrid cyst.