LAUSR

Microcystic stromal tumour (MST) is a rare, usually benign, ovarian neoplasm characterised morphologically in its classic form by a distinctive triad of features comprising microcysts, solid cellular regions and fibrous stroma. Variant morphology also occurs, including the presence of nests, tubules, cords, and signet ring cells. Immunohistochemically, this neoplasm is characterised by diffuse nuclear expression of β-catenin, cyclin D1, WT1 and SF1, as well as diffuse staining with FOXL2 and CD10. Inhibin and calretinin are typically negative. At the genomic level, these neoplasms harbour mutually exclusive mutations in CTNNB1 or APC genes with the former being significantly more common. This molecular characteristic raises possible links to other rare ovarian lesions including solid pseudopapillary tumour, signet-ring stromal tumour and Sertoli cell tumour. Rarely, MST is an extracolonic manifestation of familial adenomatous polyposis (FAP) and serves as a sentinel event that could trigger the identification of the syndrome. Herein, we review the published literature on ovarian MST and provide practical advice for pathologists reporting these rare neoplasms.