Diagnostic Reproducibility, Spatial Heterogeneity, and Survival Outcomes of HER2-Null, HER2-Ultralow, and HER2-Low Categories in Invasive Breast Carcinoma.

Categorizing HER2-negative invasive breast carcinomas into low, ultralow, and null tiers continues to challenge diagnostic reproducibility, largely due to spatial heterogeneity. This study evaluated inter-pathologist agreement and spatial intratumoral concordance across multi-site specimens using a highest-tier approach in treatment-naive invasive breast carcinomas (IBC). This retrospective study analyzed 125 primary IBC excisions (evaluated via two separate topographical blocks), 78 matched core needle biopsies, and 58 lymph node metastases (LNM). Two blinded pathologists independently reviewed slides stained with Ventana 4B5. Multi-block discrepancies were managed via a highest-tier approach (null < ultralow < low). Inter-pathologist reproducibility was substantial across all specimen types (weighted Kappa: 0.697–0.744, p < 0.001). Intra-tumor block agreement was moderate (Kappa = 0.580). The final highest-tier excision status yielded substantial concordance with core biopsies (Kappa = 0.773) but moderate concordance with LNM (Kappa = 0.486), primarily due to expression downgrading in nodal metastases (15/22 ultralow primary cases reverted to null). Pooling ultralow/low cases revealed a significant association with Grade 3 tumors (p = 0.049). HER2 status showed no independent correlation with overall survival (p > 0.05). Diagnostic discrepancies in minimal HER2 expression result from a combination of intrinsic spatial heterogeneity and the subjective challenges of inter-observer interpretation. Evaluating a second tumor block, combined with confirmation by a second pathologist for HER2-ultralow cases, effectively compensates for this variation, improving diagnostic consistency and the selection of candidates for novel targeted therapies.