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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., Competing Interests: The authors declare no conflicts of interest., (Copyright © 2020 Wolters Kluwer Health, Inc. All rights reserved.)

Clinical information is often critical to the histopathologic interpretation of cutaneous biopsies for inflammatory skin diseases. This information is often conveyed to the dermatopathologist as list of possible diagnoses. We reviewed 348 cases of biopsied inflammatory skin disease and measured the correlation between the original clinical differential diagnoses on the pathology requisition and the patient's final diagnosis. The final diagnosis was included among the suggested diagnoses in 270 of 348 (78%) cases reviewed. In 191 of 270 (71%) correctly diagnosed cases, the final diagnosis was listed first among those included in the differential diagnoses. The total number of suggested diagnoses did not correlate with overall diagnostic accuracy. The most commonly neglected diagnoses were eczematous dermatitis, psoriasis, lichen planus, and granuloma annulare. We conclude that the differential diagnosis submitted with pathology specimens for inflammatory skin disease includes the final diagnosis in a majority of cases. The first listed diagnosis has the highest positive predictive value. Submitting longer differential diagnosis lists did not improve diagnostic accuracy.