The rate of estrogen receptor-conversion associated with tumor progression in estrogen receptor-positive breast cancer patients following adjuvant Tamoxifen administration.

Background: Hormone Receptor (HR)-discordance between primary breast cancer and metastasis is a known biological phenomenon. Discordance studies usually comprise a heterogeneous group of HR-positive and negative patients and allow for the comparison of changes in HR-status from the primary to the recurrent disease. However, in a clinical setting, the rate of estrogen receptor-conversion following endocrine therapy with agents such as Tamoxifen (TAM) in estrogen receptor-positive cancers is of primary interest as opposed to total receptor discordance.