DS04 Dilemmas during Mohs surgery: clinical and histological challenges of adjacent trichoepithelioma and basal cell carcinoma

Abstract Trichoepithelioma and basal cell carcinoma (BCC) are basaloid cutaneous neoplasms that have significant clinical and histological similarities. Trichoepitheliomas are benign adnexal tumours, whereas BCC is a malignant epithelial neoplasm. We wish to highlight the overlapping features that complicate decision making during Mohs micrographic surgery (MMS) to prevent under- and overtreatment. We plan to show many examples of trichoepithelioma, which we hope will help Mohs surgeons distinguish between the two entities. A 71-year-old woman with a biopsy-proven BCC in the right alar grove was referred for MMS. She was noted to have several 2–3-mm smooth, dome-shaped, flesh-toned papules above and below the biopsy scar. During MMS, histology revealed multiple noncontiguous aggregates of basaloid palisading epithelial cells with jagged outlines associated with clefts between the aggregations and adjacent stroma. Absence of retraction artefact was noted, raising the diagnosis of trichoepithelioma. Interestingly, interspersed within these trichoepitheliomas an unexpected small focus of BCC was noted. Two stages of MMS cleared the BCC. The surgical defect was repaired with a cheek advancement flap and a burrows graft. Accurate differentiation between trichoepithelioma and BCC is critical. Trichoepitheliomas are characterized by round or cribriform epithelial aggregations with smooth borders surrounded by finely fibrillar stroma. Peripheral palisading is present, and follicular differentiation is a hallmark feature, often with rudimentary dermal papillae. In contrast, BCC displays irregular, jagged epithelial aggregates with clefts between the tumour islands and a mucinous stroma, which is absent in trichoepitheliomas. Mitoses and apoptotic bodies are seen in BCC. This case highlighted additional features of BCC, including increased apoptosis, asymmetrical tumour growth, and stromal retraction artefacts. These distinctions were integral in guiding surgical management. Although co-occurrence of trichoepithelioma and BCC is uncommon during MMS, an awareness of the different features can prevent extra stages of MMS. Trichoepitheliomas can transform to BCC and it could be that either the patient developed a de novo BCC or that one developed from her multiple trichoepitheliomas.