Sebaceous Adenoma of the Caruncle Mimicking Papilloma: Clinicopathological Features and the Ophthalmic Relevance of Mismatch Repair Immunohistochemistry.

Sebaceous adenoma is a rare benign sebaceous neoplasm that only exceptionally involves the ocular adnexa. Caruncular lesions are particularly uncommon and often resemble more frequently encountered lesions such as papilloma. Correct diagnosis of these tumours is important because sebaceous neoplasms may be associated with Muir-Torre syndrome (MTS), a hereditary cancer predisposition syndrome within the Lynch syndrome spectrum. Modern pathology reports incorporate mismatch repair (MMR) immunohistochemistry to help identify patients who may require genetic assessment and cancer surveillance. We present the case of an 81-year-old woman who presented with a right caruncular lesion associated with epiphora and mucoid discharge. Clinical examination demonstrated a cream coloured papillomatous lesion that was mobile and not adherent to adjacent structures. The lesion was presumed clinically to represent a benign papilloma and was excised during combined lacrimal surgery. Histopathological examination demonstrated a well-circumscribed lobulated sebaceous proliferation composed predominantly of mature sebocytes with peripheral basaloid germinative cells and no significant atypia, consistent with sebaceous adenoma. Immunohistochemistry demonstrated preserved nuclear expression of MutL Homologue 1 (MLH1), postmeiotic segregation increased 2 (PMS2), MutS homologue 2 (MSH2) and MutS homologue 6 (MSH6), indicating intact MMR function and reducing suspicion for MTS. Sebaceous adenoma should be considered in the differential diagnosis of cream-white papillomatous caruncular lesions. Ophthalmologists should understand the significance of MMR immunohistochemistry, as abnormal results may identify patients requiring genetic referral, whereas preserved expression is generally reassuring and supports a sporadic lesion. Retained nuclear staining for MLH1, PMS2, MSH2 and MSH6 indicated preserved MMR function (MMR proficient; pMMR), which predicts microsatellite stability (MSS). In conjunction with the patient's age and absence of a personal history of malignancy, these findings strongly favoured a sporadic sebaceous adenoma. Although intact MMR expression does not completely exclude MTS, abnormal staining would have prompted consideration of genetic referral and systemic evaluation. Our report expands upon previous literature by providing a practical explanation of MMR immunohistochemistry aimed at ophthalmologists, who may be the first clinicians to recognise sebaceous neoplasia and initiate appropriate MTS screening.