Patterns of HER2 expression and genomic correlates in lung cancer, with a focus on preanalytical variables impacting immunohistochemical staining results.

Aims: Fam-trastuzumab deruxtecan-nxki (T-DXd) was recently approved for advanced stage or metastatic solid tumours with human epidermal growth factor receptor 2 (HER2) immunohistochemical (IHC) 3+ staining. Data on HER2 IHC testing and knowledge of genomic correlates in lung cancer are scarce. This study analyses genomic characteristics of HER2-expressing tumours and addresses issues with preanalytical variables for lung cancer specimens.

Methods: HER2 IHC staining was performed on selected archival cytology and surgical pathology lung cancer specimens for patients eligible for T-DXd therapy. Patient and tumour characteristics and next-generation sequencing (NGS) data were correlated with HER2 IHC results.

Results: 166 patients with thoracic tumour samples had HER2 expression assessed: 46% were IHC 0, 28% IHC 1+, 13% IHC 2+ and 13% IHC 3+. HER2 IHC scores were overall lower for cytology cell blocks as compared with surgical pathology specimens; 79% of cases with paired specimens had a decrease in their HER2 IHC score from their surgical specimen to their paired cytology specimen. Of specimens with HER2 IHC 3+ and NGS available, only 14% (3/21) had concomitant ERBB2 alterations. Among all specimens, ERBB2 point mutations were noted in 4% (4/110) and ERBB2 amplification in 3% (3/110). The majority of HER2 3+ cases with paired NGS (17/21, 81%) had non- genomic alterations, including: , and mutations.

Conclusions: HER2 IHC 3+ is seen in a small but clinically significant proportion of samples and is associated with a variety of co-occurring non-ERBB2 genomic alterations. Preanalytical variables including specimen fixation can significantly impact the assessment of HER2 expression via immunohistochemistry.