Conjunctival ‘mucoepidermoid carcinoma’ revisited: a revision of terminology, based on morphologic, immunohistochemical and molecular findings of 14 cases, and the 2018 WHO Classification of Tumours of the Eye

In 2018, the consensus meeting for the WHO Classification of Tumours of the Eye decided that conjunctival mucoepidermoid carcinoma should be reclassified as adenosquamous carcinoma, as this represented a better morphological fit. To examine the applicability of this terminology, we studied the clinical, histopathological, immunohistochemical and molecular pathology of 14 cases that were originally diagnosed as conjunctival mucoepidermoid carcinoma. There were 7 (50%) females and 7 (50%) males. The median age was 64 years. The left eye was affected in 8 and the right eye in 6 patients. In-situ carcinoma was present in 11/14 (79%) cases and comprised in-situ squamous cell carcinoma (SCC) and conjunctival intraepithelial neoplasia with mucinous differentiation (CIN-Muc). Invasive carcinoma was present in 11/14 (79%) cases. Group 1 (1/11 cases, 9%) comprised invasive SCC only. Group 2 (6/11 cases, 55%) comprised SCC with mucinous differentiation, manifesting as scattered intracellular mucin, occasionally together with intercellular mucin, with no evidence of true glandular differentiation. Group 3 (3/11 cases. 27%) comprised true adenosquamous carcinoma. Group 4 (1/11 cases, 9%) comprised pure adenocarcinoma. Thirteen of 14 cases (93%) underwent FISH for MAML2 translocation and none were rearranged. Two cases harboured high-risk HPV (type 16 and 18). The combined findings confirm that all lesions in our study were not mucoepidermoid carcinoma, but represented predominantly SCC with mucinous differentiation and adenosquamous carcinoma. We, therefore, recommend future revision of the WHO classification to include SCC with mucinous differentiation alongside adenosquamous carcinoma.