Evaluation of fluorescence in‐situ hybridization in monomorphic endometrial stromal neoplasms and their histological mimics: a review of 49 cases

Aims To perform a population‐based review of monomorphic endometrial stromal tumours and their histological mimics presenting over a 20‐year period, including an evaluation of fluorescence in‐situ hybridization ( FISH ) for the JAZF 1 and YWHAE breakaparts. Methods and results Forty‐nine tumours were examined, comprising 13 histological mimics and 36 endometrial stromal tumours [six stromal nodules (ESNs), 25 low‐grade stromal sarcomas (ESSs), and five monomorphic undifferentiated sarcomas (m UES s)]. Nine ESSs showed variant histological patterns, including smooth muscle, sex cord‐like/glandular, fibrous or rhabdoid differentiation. Three ESSs were initially misclassified as benign uterine lesions, and, conversely, three benign mimics were originally reported as ESSs. One m UES showed a prominent pseudopapillary pattern. Fluorescence in‐situ hybridization demonstrated JAZF1 breakaparts in five of six ESNs and 16 of 25 ESSs; however, only three of nine ESS variants were positive. YWHAE breakaparts were present in four of five m UES s. Analysis of a subsequent metastasis in the YWHAE breakapart‐negative m UES demonstrated a YWHAE deletion. None of the histological mimics was positive in FISH analysis. Diffuse cyclin D1 expression was restricted to m UES s in this series. Conclusions Endometrial stromal neoplasms continue to present diagnostic difficulty. Fluorescence in‐situ hybridization analysis is helpful in distinguishing stromal tumours from their histological mimics and in distinguishing ESS from m UES .