Ki-67 proliferation index for diagnostic standardization of canine hepatoid gland tumours.

Canine hepatoid gland tumours (HGTs) lack standardized histopathological guidelines, leading to diagnostic subjectivity. This study characterized clinicopathological and immunohistochemical features of canine HGTs, assessed interobserver agreement and evaluated the prognostic values of Ki-67 and vascular endothelial growth factor (VEGF) markers. Twenty-five canine HGT samples from 23 dogs (2016-2022) were analysed retrospectively and three pathologists independently classified the samples. Immunohistochemistry (IHC) for Ki-67 and VEGF was performed. Interobserver agreement was measured via kappa coefficient, and IHC data were correlated with clinical follow-up using Kaplan-Meier and Freeman-Halton tests. Diagnostic agreement among all pathologists was achieved in only 44% of cases (fair agreement; kappa = 0.3341). Discrepancies were most frequent between carcinoma and epithelioma. The Ki-67 index correlated significantly with diagnosis (P <0.05), showing a clear gradient: carcinomas (18.3%), epitheliomas (10.2%) and adenomas (3.4%). Infiltration (P = 0.0267), loss of architectural trabecular arrangement (P <0.0001) and cellular atypia (P = 0.0021) were the most discriminative morphological criteria. VEGF immunolabelling and other histomorphological features showed no significant associations (P >0.05). While one pathologist's diagnosis correlated with survival (P = 0.0199), limited clinical follow-up prevented cohort-wide consistency. High Ki-67 expression serves as a robust objective indicator of biological aggressiveness in canine HGTs. Despite interobserver variability, both malignant and borderline forms (epithelioma and carcinoma) often require a similar initial aggressive surgical approach, although definitively distinguishing between them remains critical due to the known metastatic potential of carcinomas. Integrating Ki-67 assessment with traditional morphology is recommended to standardize diagnosis and improve therapeutic decision-making in canine HGTs.