A Rare Case of a Pilar Cyst With Ductal Differentiation

Trichilemmal cysts with proteinaceous material: A potential diagnostic pitfall

BACKGROUND

Cysts of the skin are observed frequently and their diagnoses are generally straightforward. However, atypical cystic lesions for which differentiation is indistinct have been noted.

METHODS

We examined five cases of trichilemmal cyst with proteinaceous material (TCPM), which required differentiation from sweat duct/gland tumors. We investigated the histopathological findings of TCPMs and evaluated the immunohistochemical expression of cytokeratin (CK) 10, CK13, CK17, CK19, CD8, and CD117. Immunohistochemical analysis was performed on the five TCPMs, 10 trichilemmal cysts (TCs), five clear cell hidradenomas, five poroid hidradenomas, and cutaneous normal adnexa.

RESULTS

Apoptotic cells were present in the cyst wall with a small amount of keratin or calcification in the cavity of TCPMs. The TCPMs and TCs were negative for CK19 and CD117, whereas clear cell hidradenoma and poroid hidradenoma were positive for CK19 and CD117. The restricted positivity for CK10 was detected in the suprabasal layers of the cyst walls of TCPMs and TCs. The immunostaining patterns of TCPMs and TCs were similar to those of normal follicular isthmus.

CONCLUSIONS

The histopathological findings with characteristics of TCs and a panel of immunohistochemical antibodies including CD117, CK19, and CK10 contributed to a correct diagnosis of TCPM. This article is protected by copyright. All rights reserved.

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.