Pathological features, immunoprofile and mismatch repair protein expression status in uterine endometrioid carcinoma: focus on MELF pattern of myoinvasion

Aims: Microcystic, elongated, and fragmented (MELF) pattern of myoinvasion has been related with increased risk of lympho-vascular space invasion (LVSI) and lymph node metastasis. We analysed a cohort of endometrioid endometrial carcinomas (EECs) to examine the relationships between the MELF pattern of invasion and the clinico-pathological and immunohistochemical features of EEC. Methods and results: 129 EECs were evaluated for the presence of MELF pattern and immunohistochemically tested for Mismatch repair (MMR) proteins, p16, p53 and beta-catenin. We observed 28 MELF + EECs and 101 MELF- EECs. LVSI was observed in 20 MELF + cases and in MELF- tumors. Lymph-node metastases were observed in 7 MELF + cases (2 macrometastases, 3 micrometastases and 2 ITCs). None of the MELF- cases showed micrometastases or ITCs, 18 cases had macrometastatic lymph-nodes. Statistical analysis showed that MELF + tumors carry an increased risk of developing nodal metastasis independent of tumor dimension and LVSI. Loss of MMR proteins expression was observed in 11 MELF + cases and 45 MELF- cases, respectively. Our data showed a higher frequency of immunohistochemical MLH1-PMS2 loss in MELF- pattern of invasion (32.67% of MELF- cases vs 21.43% of MELF + cases) but a higher prevalence of MSH2-MSH6 loss in MELF + pattern (7.14% in MELF + population vs 3.96% of MELF- population) Conclusions: The morphological recognition of MELF pattern is more reliable than immunohistochemical and molecular signatures of EEC in predicting the risk of nodal involvement. The observed higher prevalence of MSH2-MSH6 loss in MELF + group and MLH1-PMS2 loss in MELF- group may suggest a different molecular signature.