Next‐generation sequencing in breast pathology: real impact on routine practice over a decade since its introduction

The diagnosis, histomolecular classes of breast cancers (luminal A, luminal B, HER2‐enriched, and basal‐like), and accurate prediction of prognosis are commonly determined using morphological and phenotypical analyses in clinical practice worldwide. Therapeutic strategies are mostly based on the disease stage and molecular subclasses of breast cancer. Targeted therapies, such as anti‐HER2s, poly‐ADP ribose polymerase inhibitors or, to a lesser extent, phosphatidylinositol 3 kinase inhibitors, have substantially improved breast cancer patient prognosis over the past decades. Human epidermal growth factor receptor 2 (HER2) overexpression is widely determined based on immunohistochemistry, while next‐generation sequencing (NGS) is currently employed to assess the presence of molecular alterations, including breast cancer gene 1 (BRCA1) and 2 or phosphatidylinositol‐4,5‐bisphosphate 3‐kinase catalytic subunit alpha (PIK3CA) mutations, which are targets of these new approved therapies. In addition, next‐generation sequencing (NGS) can aid the pathologist in challenging situations, such as a diagnostic workup for a metastatic carcinoma in lymph nodes of unknown origin, differential diagnosis of spindle cell tumourtumor in the breast between metaplastic carcinoma, malignant PT and sarcoma, o, as well as determining relatedness between primary breast cancers and recurrences. NGS offers a powerful tool that enables the pathologist to combine morphological analyses together with molecular alterations in challenging diagnostic situations.