Optimal Assessment of Metastatic Breast Carcinoma: The Value of Cytopathology Combined with Molecular Analysis

Abstract

Metastatic breast cancer (MBC) remains in most cases an incurable disease with genetic complexity and heterogeneity. Improvements in classification and management have been introduced, in addition to the development of endocrine and anti-HER2 targeted therapies. Currently, efforts are being made to delineate the best approach for the genomic landscape of MBC and, as result, molecular therapeutic targets. Here, we highlight the recent developments in the cytopathology of MBC, discussing cytological diagnostic approaches in the characterization of hallmarks, such as immunocytochemistry and genomic biomarkers. Cytological material can be processed for ancillary testing for diagnostic and therapeutic purposes. Reassessment of receptor status is indicated due to changes in tumor biology and metastatic presentation. PD-L1 expression is the only approved biomarker for predicting immune checkpoint inhibitor response in metastatic TNBC, evaluated by immunostaining. The feasibility of applying PD-L1 assays in MBC cytological samples can be recommended, with the adoption of a combined positive score. Non-formalin cytological samples provide higher purity, cellular yield, and better tumor fraction for single-multi gene assays. In MBC, molecular tests enable personalized therapy such as PIK3CA, NTRK fusion genes, and MSI. Cytopathology combined with molecular analysis must be performed effectively in routine clinical practice, through procedure standardization and experience dissemination.