Would you treat a premenopausal woman with T2N0 ER-, PR+ (15-20%), HER2- breast cancer with neoadjuvant chemotherapy like a triple negative breast cancer?

Yes, I would treat this patient with neoadjuvant chemotherapy as I would for a "triple-negative" breast cancer as certainly the biopsy of an ER 0%, weakly PR positive tumor is similar. I would not send an OncoType on this tumor - if it was low or intermediate, I still would not trust this patient would not benefit from chemotherapy, thus it would not help with my clinical decision making. Only 54 of the ~10,000 patients included in the TAILORx trial using Oncotype for chemotherapy decision makin...

Yes, I would treat her as a triple-negative patient and would not expect any benefit from adjuvant endocrine therapy. I also wouldn't waste the time or money to send a genomic assay, such as Oncotype, Mammoprint, or EndoPredict, as any of these are sure to confirm that this is a high-risk cancer that needs to be treated with chemotherapy. For her (essentially) triple-negative breast cancer, I would administer a carboplatin-containing regimen, as several studies, including CALGB 40603, BrighTNess...

Multiple pathology papers report that this is more likely than not a technical artifact and repeat staining should be done. Most of the time it will not be ER-PR+ on repeat stain (for example see Foley et al., PMID 28891017). This was specifically addressed in the recent ASCO/CAP guidelines for ER and PR testing (Allison et al., PMID 31928404).

That said, if after retesting this still was ER- PR+, then you have a very rare and probably molecularly heterogeneous entity. I treat these like I do ER...

First, I would repeat the PR. Also, calculate an Allred score. There are only 1% of cases where PR is + with a negative ER. Shouldn't happen. This is most times an anomaly and will be negative upon repeat. Then treat as triple-negative. There is no role for any other testing such as Oncotype or Endopredict which would be for ER+ women only.

I would get an OncoType Dx assay or a Mammoprint/Blueprint on the biopsy and treat it as per the results of that. She is not triple-negative breast cancer, but may well benefit from chemotherapy. There is increasing experience with using OncoType DX on neoadjuvant biopsies (Yardley et al., PMID 26507191; Bear et al., PMID 28407247).

We are starting to use EndoPredict in order to determine which patients can be "de-escalated" and avoid neoadjuvant chemotherapy, and which HR+/HER2- patients would benefit from it.

Almstedt et al., PMID 32436145