What is your cut off on ER percent positivity to treat breast cancer as functional triple negative cancer?

I usually use less than 10% ER positivity as the cut off. In this case, I would treat as if the tumor is TNBC.

The answer is dependent on the setting:

• Neoadjuvant (biopsy alone used to determine ER/PR):
  • I sometimes place patients as high as ER 20-30% on KEYNOTE 522. Often, I seek a second opinion on ER/PR IHC in these settings, as I have had cases of false positives at my local institution.
• Adjuvant:
  • I use ER >1% to justify hormone-positive treatment, placing on adjuvant endocrine therapy. However, I have a low threshold to halt adjuvant endocrine therapy in patients with ER<10% who develop toxicities.
• Metastatic:
  • I definitely treat ER<10% as met TNBC, but I sometimes will treat ER 30% or less as TNBC, especially when PR is 0% AND there has been a quick relapse on adjuvant endocrine therapy or very high Ki-67 or concomitant CNS mets, employin...