Analysis of Clinicopathological and Molecular Features of Microcystic, Elongated, and Fragmented Pattern Invasion in Endometrioid Endometrial Cancer.

: Microcystic, elongated, and fragmented (MELF) invasion is a special invasion pattern in endometrioid endometrial cancer (EEC). This study aimed to investigate the clinical, pathological, and molecular features of the MELF pattern and its prognostic value in patients with EEC. : The clinical and pathological data of 342 patients with EEC were retrospectively collected at Peking University People's Hospital from January 2019 to December 2022. Some key clinicopathological features were evaluated, including the tumor grade, Federation of Gynecology and Obstetrics (FIGO) staging, cervical stromal involvement, lymph node status, and lymphatic vascular space infiltration (LVSI). Immunohistochemical staining and molecular tests were performed, and the relevant literature was reviewed. : The MELF pattern was more prevalent in low-grade EEC. A significant correlation was found between the MELF pattern and advanced FIGO staging, LVSI, the depth of myometrial invasion, cervical stromal involvement, and lymph node metastasis (LNM). The incidence of mismatch-repair-deficient (MMRd) proteins was much higher in the MELF group than in the no-MELF group. Molecular testing revealed that, after copy number-low (CNL), microsatellite instability-high (MSI-H) was the second-most frequent subtype in the MELF group. The recurrence risk did not significantly differ between the MELF and no-MELF groups, but the differences among the four molecular subtypes were statistically significant. However, the MELF group experienced a shorter recurrence time. Among the four molecular subtypes, the recurrence risk was the highest in the CNH subgroup, followed by the MSI-H subgroup. : MELF is a special invasion pattern in EEC and is associated with distinct clinicopathological and molecular characteristics, including the latest 2023 FIGO staging. Further research is warranted to explore its implications for treatment strategies and patient outcomes.