Molecular Profile of Advanced Endometrial Cancer (FIGO III–IV) in a Polish Multicentre Cohort: Clinicopathological Characterisation and Treatment Implications

Background/Objectives: Advanced endometrial cancer (FIGO III–IV) is characterised by poor prognosis and a heterogeneous biological profile, and molecular classification enables treatment personalisation by identifying subtypes with distinct therapeutic targets. We aimed to characterise the molecular and histopathological features of FIGO III–IV cases in a Polish multicentre cohort and to discuss the resulting treatment implications. Methods: This retrospective multicentre study included 915 consecutive patients with endometrial cancer operated on between April 2022 and May 2025 at three oncology centres in south-eastern Poland. Molecular subtyping (POLEmut, p53abn, dMMR/MSI-H, NSMP) was performed using immunohistochemistry (IHC) and next-generation sequencing (NGS). FIGO stage was assigned according to the FIGO 2009 classification. Results: Among 888 patients with a known molecular subtype, FIGO III–IV cases accounted for 15.9% (n = 141). The p53abn subtype predominated (35.5%), followed by dMMR/MSI-H (26.2%), NSMP (24.1%), and POLEmut (5.7%). The proportion of p53abn increased with stage (I–II vs. III–IV, p < 0.001), whereas dMMR/MSI-H remained stable regardless of stage (p = 0.83). POLEmut was absent in FIGO IV (0/16; 95% CI 0.0–19.4%), which should be regarded as an exploratory observation requiring prospective validation. Conclusions: The molecular profile of advanced endometrial cancer may inform treatment strategy; the therapeutic implications presented here are descriptive and hypothesis-generating, as the study did not include survival data. The dMMR/MSI-H subtype identifies patients who may benefit from immunotherapy in accordance with current clinical indications, supporting routine MMR testing regardless of disease stage. Conversely, p53abn tumours point to the need for a more intensive treatment strategy, in line with current guidelines. The absence of POLEmut in FIGO IV is an exploratory observation requiring prospective validation. Treatment de-escalation in POLEmut FIGO IIIC remains subject to further clinical validation and requires individualised assessment after complete staging.