A new retrospective study has reported that estrogen receptor (ER) status tends to remain constant between primary and metastatic tumors in breast cancer. The study was intended to settle a persistent controversy as to whether ER status remains stable during progression to metastasis. The study was also designed to determine whether the location of a recurrence or the type of chemotherapy or other systemic treatment of the original tumor was related to any change in ER status (from ER+ to ER- or ER- to ER+). The study included tumors from 227 women with known ER status in both their primary tumors and metastases.

ER status was stable (concordant) in 210 (92.5%) of the women, including 147 ER+ and 63 ER- tumors. Both ER- to ER+ conversion (found in 7 women) and ER+ to ER- conversion (10 women) were observed among the 7.5% of women with discordant ER status. ER discordance was not found to be related to whether the recurrence was local (in the breast, chest wall or lymph nodes) or distant, the time elapsed between measures of ER status (less than 5 years compared to at least 5 years), or the type of chemotherapy or hormonal therapy originally used to treat the women. Variations in testing methods and marginal ER scores were common in discordant cases. The authors conclude that ER status in breast cancer is generally stable during progression to metastasis. Problems with assessing ER status may contribute to discordance in some cases. However, given the importance of ER status for treatment decisions, ER testing in metastatic breast cancer should be repeated, especially for patients whose patterns or progression are not congruent with the original ER status.