HR receptor status changes

I have DCIS and my biopsy 1 year ago tested ER positive. I have been in a clinical trial fro a year taking an aromatose inihibitor (Lexotrole).  I just had a second biopsy that found many of the cells are loosing their Estrogen Receptivity. Has any body heard of HR status changing and how could this happen?

[Fernando Roque, MD]

Hi.  The estrogen receptor status of tumors can change over time. A mass of breast cancer is actually composed of a mixed population of cancer cells.  Some cells are strongly ER positive, some only mildly so, and some are even ER negative (do not contain estrogen receptors).  When exposed to an aromatase inhibitor like letrozole, those cells which are strongly ER positive are very susceptible to the effects of this drug and either die off or stop multiplying.  However, those cells which are only mildly ER positive or are ER negative are only slightly affected by letrozole or not at all.  These types of cancer cells now have the opportunity to grow, multiply and become the majority population.  On repeat testing, ER positivity is now observed to have decreased, since more cells are now only mildly ER positive or are ER negative.

[Deborah725]

Thank you for taking the time to respond to my question Fernando.  It helps me understand why both the MRI and mammogram suggest that my DCIS had become more dense or active in the past 6 months since my last battery of tests. It also helps me to accept the difficult decision that it is now time for the prescribed mastectomy given that my DCIS is high grade and 5 cm.  My doctor got me 90% of the way and you helped to close the deal...I kept hoping the HR test was wrong and that I might gain some benefits in surgical options from continued time on Lextrozole:-).  Again, thanks!