Histological grading of invasive breast carcinoma in the digital era: a practical framework for whole slide imaging

Whole slide imaging (WSI) is increasingly embedded in routine histopathology; however, histological grading of invasive breast carcinoma continues to rely on methodologies developed for conventional light microscopy. This narrative review aims to establish a practical and conceptually refined framework for applying the Nottingham grading system in the digital setting, with emphasis on improving reproducibility, standardisation and biological interpretability. An evidence-informed, methodological analysis was undertaken, integrating published literature with expert practice to examine the impact of WSI on the assessment of tubule formation, nuclear pleomorphism and mitotic activity. Key technical, perceptual and interpretative challenges specific to digital pathology were identified, and practical, standardised approaches were developed. The three grading components are differentially affected by WSI. Tubule formation remains highly reproducible and is well suited to low-power digital assessment. Nuclear pleomorphism demonstrates moderate variability, reflecting both intrinsic subjectivity and the influence of display and perceptual factors. Mitotic assessment represents the principal challenge, with variability arising from hotspot identification, area calibration and recognition of mitotic figures in the absence of z-axis information. Defining counting areas in mm² (optimally 2-3 mm²), prioritizing regions of increased tumour cellularity, and using calibrated high-power screen fields improve reproducibility. Systematic underestimation of mitotic counts in WSI compared to light microscopy is recognised and is most relevant in cases near grading thresholds. Overall, histological grading can be performed reliably using WSI but requires methodological adaptation rather than direct transfer of glass slide-based approaches. Standardization of hotspot selection, area calibration, and morphological interpretation is essential. This work provides a practical, evidence-based framework to support consistent and biologically meaningful grading in contemporary digital pathology practice.