BACKGROUND AND AIMS: Lobular carcinoma (LC) of the breast exhibits diverse morphology and clinical behaviour. The pleomorphic variant (pLC) displays distinct cytonuclear features and aggressiveness compared to the classic variant (cLC). However, diagnosing pLC remains subjective. This study aims to refine LC's cytonuclear features, focusing on pLC. METHODS: Whole slide images of 59 LCs, including both in situ (LCIS) and invasive (ILC) lesions, were analysed. Nuclear measurements, including nuclear size and variability, were scored using QuPath image analysis software. For comparison, selected features were scored in normal cells (n = 10) and pleomorphism score-matched invasive breast carcinoma (IBC) of NST type (n = 33). Additional visual assessment of the pleomorphic ILC (pILC) cohort (n = 90) was conducted for cytomorphological features characterisation. RESULTS: pILC demonstrated larger nuclear area and higher nuclear variability with abundance of cytoplasm than cILC. Compared to lymphocytes, pILC demonstrated a median area ranging from 2.7 to 4.7 times larger. Cut-off values for differentiating pILC from other ILC subtypes included median nuclear area >
48.2 μm CONCLUSIONS: Objective nuclear measurements, combined with cytoplasmic and architectural features, provide a robust framework for diagnosing LC subtypes, improving diagnostic accuracy and reproducibility.