Study on Different Staining Platforms for HER2 Cytoplasmic Granular Staining Pattern in Pure Apocrine Carcinoma of the Breast.

OBJECTIVE

Accurate interpretation of HER2 low/ultralow expression has attracted increasing attention. This study aimed to explore the characteristics and interpretation strategies for the HER2 cytoplasmic granular staining pattern observed in pure apocrine carcinoma (AC), while investigating its impact on the interpretation of HER2 low/ultra-low expression cases.

METHODS

The clinicopathologic information of 74 patients with pure AC and their previous HER2 (PATHWAY 4B5, Ventana platform) IHC results were retrospectively collected. All enrolled cases underwent Dako HER2 (HercepTest [poly]) IHC staining, among which 43 cases were further subjected to FISH testing. Clinical targeted drug information was collected concurrently.

RESULTS

Previous PATHWAY 4B5 staining revealed punctate and/or diffuse cytoplasmic granular staining in 35.14% (26/74) of pure AC. In contrast, only 9 cases (12.16%, 9/74) of HercepTest staining exhibited cytoplasmic granular staining, all of which belonged to the PATHWAY 4B5 cytoplasmic granular staining subset (34.62%, 9/26). HercepTest IHC interpretation demonstrated 89.19% (66/74) concordance with PATHWAY 4B5 interpretations. Furthermore, 88.46% (23/26) of PATHWAY 4B5 cytoplasmic granular staining cases exhibited concordant interpretations between both antibody platforms. 44.59% (33/74) were HER2-positive, and 55.41% (41/74) were triple-negative apocrine carcinoma (TNAC). About 18.19% (6/33) of HER2-positive cases and 48.78% (20/41) of TNAC cases showed cytoplasmic granular staining on PATHWAY 4B5, and 95.00% (19/20) of the latter cases were HER2 low expression.

CONCLUSIONS

Pure ACs on the PATHWAY 4B5 platform primarily present HER2 cytoplasmic granular staining in TNAC and HER2 low expression cases. The rate of HER2 cytoplasmic granular staining on HercepTest platform was significantly lower than that of PATHWAY 4B5, and is more suitable for the detection and interpretation of HER2 0 and low expression cases.