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Case Reports

Abstract

We present a case of a 66-year-old male with history of renal transplant who presented for a routine skin cancer screening examination. Examination demonstrated an approximately 6 mm ulcerated papulonodule of the back. Shave biopsy was performed, which demonstrated an ulcerated trichilemmal carcinoma, characterized by lobules of clear cells exhibiting trichilemmal keratinization and cytologic atypia. Immunohistochemical staining showed MOC 31 and keratin 5 positivity, and focal partial staining with androgen receptor stain. CD34 stain was negative. The lesion was subsequently treated with complete surgical excision.

Trichilemmal carcinoma is a rare malignant adnexal tumor with outer root sheath differentiation and features of an atypical clear cell neoplasm. While it shares the same origin as trichilemmoma, trichilemmal carcinoma varies by its atypical nuclei and high mitotic index. It typically demonstrates lobular architecture, peripheral palisading, necrosis, and large, glycogen-rich clear cells with eccentric nuclei, lacking evidence of hair follicle differentiation.

Basal cell carcinoma is an important histological mimic due to its overlapping features. However, basal cell carcinoma classically displays prominent retraction artifact, strong CK 17 positivity, and an absence of trichilemmal differentiation. Squamous cell carcinoma with clear cell change is another key mimic. It can be distinguished by recognizing trichilemmal features and folliculocentric growth. Trichilemmal carcinoma may be locally destructive and has the possibility of metastasis or local recurrence, highlighting the importance of accurate diagnosis and appropriate treatment.

This case highlights key histopathologic and immunohistochemical features that aid in distinguishing trichilemmal carcinoma, basal cell carcinoma, and squamous cell carcinoma.