Clinico-Morphological Correlations with Ki-67 and p53 Immunohistochemical Expression in High-Grade Gastrointestinal Neuroendocrine Neoplasms

Background/Objectives: The 2019 WHO classification redefined high-grade gastrointestinal neuroendocrine neoplasms (GI NENs), encompassing not only poorly differentiated neuroendocrine carcinomas (NECs), but also well-differentiated grade 3 neuroendocrine tumors (NETs G3). Since both subtypes share a Ki-67 index > 20%, distinguishing them based solely on morphology is challenging. Prior studies have shown TP53 alterations in NECs but not in NETs. This study aimed to evaluate clinico-morphological parameters and the immunohistochemical (IHC) expression of p53 in high-grade GI NENs to identify relevant correlations. Methods: Tumors were stratified by Ki-67 index into two groups: >20–50% and >50%. p53 IHC expression was assessed as “wild-type” (1–20% positive tumor cells) or “non-wild-type” (absence or >20% positivity). Correlations were analyzed between Ki-67, p53 status, and various pathological features. Results: Significant correlations were found between the Ki-67 index and maximum tumor size, pT stage, lymphovascular invasion, perineural infiltration, and diagnostic classification. Similarly, p53 immunohistochemical status was significantly associated with lymphovascular invasion, lymph node metastasis, and tumor classification (NET G3 versus NEC, including NEC components of MiNENs). Conclusions: The findings support the value of Ki-67 and p53 as complementary biomarkers in the pathological evaluation of high-grade GI NENs. Their significant associations with key morphological parameters support their utility in differentiating NETs G3 from NECs, particularly in cases showing overlapping histological features. The immunohistochemical profile of p53 may serve as a useful diagnostic adjunct in routine practice.