Discordance of human epidermal growth factor receptor 2-low status between breast primary and distant metastases with clinical-pathological correlation.

Aims: Breast cancer with human epidermal growth factor receptor 2 (HER2) immunohistochemistry (IHC) 1+ or 2+ with negative in-situ hybridisation (ISH) (HER2-low) can now be targeted by HER2 antibody drug conjugates. We set out to compare HER2 status between matched primary invasive breast carcinoma (IBC) and distant metastases (DM) with clinical-pathological correlation, with specific interest in HER2-low.

Methods: Biomarker studies and clinical-pathological features of primary IBC with matched DM diagnosed between 2021 and 2022 were retrospectively analysed. HER2 status was assessed per 2023 American Society of Clinical Oncology/College of American Pathologists (ASCO/CAP) guidelines for IHC (4B5) and ISH (IQFISH pharmDX). Bilateral breast primaries were excluded. HER2 IHC 0 to 1+ were reassessed.

Results: One hundred and forty-seven cases of primary IBC with matched DM were identified. Biomarkers were performed on core biopsy (n = 74) and resection (n = 73). One hundred and twenty-six (86%) were initially classified as 'HER2-negative'; of these, 67 (46%) were reclassified as HER2-low. Patients with HER2-positive primaries were younger (P = 0.01) and had an increased incidence of micropapillary carcinoma (P = 0.02). HER2-low primaries also had an increased incidence of micropapillary carcinoma (P = 0.02) and oestrogen receptor (ER) positivity (P = 0.02) compared to HER2 0. One hundred and sixty-nine matched DM cases excluding bone metastasis were identified (range = one to seven metastases per IBC). The most common sites of metastases were liver (50 of 169, 30%), lung (36 of 169; 21%), distant lymph node (26 of 169, 15%); 138 DM cases (82%) were previously classified as 'HER2-negative', and 62 (37%) were reclassified as HER2-low. Like HER2-low primaries, HER2-low metastases were frequently ER-positive (52 of 62; 84%) (P = 0.02). Brain metastases were more frequently HER2-positive (five of 32; 16%) (P = 0.04). Comparing HER2 status in matched primaries and DM, HER2 status was discordant in 62 cases (37%). Most changes occurred from HER2-low to HER2 0 (33 of 169, 20%), HER2 0 to HER2-low (17 of 169, 10%) and HER2-low to positive (10 of 169, 6%). All HER2-low to HER2 0 changes were HER2 1+ to 0. In 30 patients with multiple DM sites (47 cases), HER2 status among different DM samples was discordant in 16 patients (53%), mainly from HER2-low to HER2 0 (16 of 47, 34%).

Conclusion: A significant proportion of previous 'HER2-negative' primaries and DM cases were reclassified as HER2-low. Discordant HER2 status between IBC primary and metastasis and between different DM sites demonstrated tumour heterogeneity and highlights the need for HER2 retesting in distant metastasis.