Immunohistochemistry for PTEN testing in HR +/HER2- metastatic breast cancer

: The PTEN tumor suppressor regulates the PIK3CA/AKT1 pathway, and its inactivation significantly contributes to tumorigenesis and progression in hormone receptor-positive/HER2-negative (HR + /HER2 -) metastatic breast cancer (MBC). In ~ 5% of these patients, PTEN loss, primarily due to gene deletions, leads to aberrant PI3K signaling and enhanced oncogenic potential. Findings from the CAPItello-291 study further establish PTEN together with PIK3CA and AKT1 as a predictive biomarker for Capivasertib, a pan-AKT inhibitor, in these patients. Despite next-generation sequencing (NGS) being the most precise method for detecting gene losses, immunohistochemistry (IHC) offers some advantages, including accessibility, cost-effectiveness, and applicability when archival tissue is inadequate for NGS or when pre-analytical failure occurs. Notably, recent evidence supports a pragmatic IHC positivity criterion, defining PTEN deficiency as staining in less than 10% of tumor cells, regardless of intensity. In this manuscript, we provide a comprehensive overview of the clinical scenarios associated with PTEN IHC testing in HR + /HER2 - MBC, outline best practices to minimize the impact of pre-analytical and analytical variability, and propose a structured pathology report to standardize PTEN IHC evaluation in this context.