Histologic Discordance Between Primary Tumor and Nodal Metastasis in Breast Cancer: Solving a Clinical Conundrum in the Era of Genomics.

A degree of molecular concordance is expected for tumors originating from a common stem or progenitor cell. The histological discordance and absence of any genomic overlap should raise suspicion for two separate primary tumors. Paired DNA sequencing of the primary tumor and nodal metastasis can inform clinical decisions when primary breast tumor and axillary metastasis are histologically discordant. Molecular/Precision Oncology Tumor Board is the best setting to facilitate such decisions in these challenging cases. Paired DNA sequencing under these rare circumstances may suggest an occult breast tumor.