MELF pattern invasion in endometrial carcinoma: association with low grade, myoinvasive endometrioid tumours, focal mucinous differentiation and vascular invasion

SummaryAimsTo investigate the frequency of microcystic, elongated and fragmented (MELF) pattern invasion in endometrial carcinoma and its association with other pathological findings.MethodsThe histology slides from 170 consecutively accessioned endometrial malignancies were reviewed to identify cases showing MELF type invasion. Histological subtype, tumour grade, presence of squamous and mucinous differentiation, and frequency of lymphovascular space invasion (LVSI) were compared in MELF positive and negative cases. Additional stains for cytokeratin and stromal mucin were performed in the MELF positive tumours and immunohistochemistry to demonstrate vascular endothelium was performed in 12 selected cases.ResultsThere were 133 endometrioid adenocarcinomas (EEC) and 37 non-endometrioid carcinomas. MELF type invasion was identified in 27 cases (15.9%) all of which were FIGO grade 1 or 2 myoinvasive adenocarcinomas of endometrioid type. The histochemical and immunohistochemical stains were useful in demonstrating the relatively subtle invasive foci in some tumours. MELF positive cases more frequently showed focal mucinous differentiation and LVSI.ConclusionsMELF type invasion appears restricted to low grade, myoinvasive carcinomas of endometrioid type and is more common in tumours exhibiting focal mucinous differentiation. These associations suggest that MELF changes represent a specific tumour-stromal interaction rather than a degenerative process. Tumours exhibiting MELF pattern invasion should be carefully examined for LVSI. Ancillary techniques, in particular cytokeratin immunohistochemistry, are useful in diagnostic assessment.