Diagnostic pitfalls related to morular metaplasia in endometrioid carcinoma: an underestimated issue

Here, we present a case that highlights the crucial pitfalls related to the presence of morular metaplasia (MM) in endometrioid carcinoma, which are insufficiently recognized in the routine pathology practice. A 45-year-old woman underwent hysterectomy with rectosigmoidectomy due to a 11-cm mass involving uterus, right ovary, and rectosigmoid colon. Histologically, the lesion appeared as a predominantly solid carcinoma with a minor glandular component. Results of the first immunohistochemical analysis suggested a colorectal origin (PAX8-, CK7-, WT1-, hormone receptors-, and CDX2+ in the absence of mucinous features). Subsequent immunohistochemistry (nuclear β-catenin+, CD10+, and low ki67 in the solid areas) supported a diagnosis of endometrioid carcinoma with diffuse MM. This case remarks that morphological and immunohistochemical features of MM may conceal the glandular architecture and the typical immunophenotype of endometrioid carcinomas. Acknowledging the diagnostic issues related to MM appears crucial to avoid misdiagnosis and inappropriate patient management.