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

Abstract

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.

Trichilemmal cysts with proteinaceous material: A potential diagnostic pitfall

“…Another hypothesis regarding the cause of TCPMs is that these lesions arise from the VHF, based on their characteristic distribution. All five TCPMs were located outside of the scalp, although approximately 90% of TCs are known to develop on the scalp 20 . Immunohistochemistry on normal VHFs and THFs, which demonstrated no difference in staining patterns.…”

“…All five TCPMs were located outside of the scalp, although approximately 90% of TCs are known to develop on the scalp. 20 Immunohistochemistry on normal VHFs and THFs, which demonstrated no difference in staining patterns.…”

“…18 Two cases (Cases 2 and 3) had a focal area of ductal structures. TC with ductal differentiation has been rarely reported, 19,20 in which the ducts are considered sweat duct differentiation. However, we considered the ductal structures in two TCPMs to be the differentiation into sebaceous ducts.…”