For a young female (<40) with HR-/HER2+ cT1-2N1, ypT1aN0 s/p mastectomy with SLNB, would you offer PMRT?

Thanks for the question. In the seminal PMRT randomized trials of patients treated with adjuvant systemic therapy, PMRT improved overall survival. B-51 was not designed to evaluate the non-inferiority of PMRT omission. Beyond being less conservatively designed to test for superiority of "RNI" with regard to invasive breast cancer recurrence-free interval (IBCRFI) in both breast-conserving surgery (where the benefit of "RNI" for N+ disease in prior studies is modest) and mastectomy treated patien...

I would be very uncomfortable omitting radiotherapy in this patient.

The patient is young which is associated with higher locoregional recurrence rates. The B-51 follow-up is short and the patient had residual disease in the breast. I appreciate the excellent summary of Dr. @Dr. First Last.

Had the patient received breast conservation, concerns would have been addressed.

I agree with Dr. @Dr. First Last's points about patients undergoing mastectomy and SLNB being underrepresented on B-51 and therefore using caution when applying the conclusions of B51 to these patients.

I would also use caution when interpreting ypN0 in this case. There is evidence that after neoadjuvant chemotherapy, the false negative rate of SLNB is higher (Boughey et al., PMID 24101169 and Sun et al., PMID 32034581), especially with only 1 LN removed. Z1071 reported FNR 18% and Peony reported...

I would favor no RT although I doubt there is consensus with people having different interpretations of B-51.

Our institutional policy is to offer RT based on the initial stage (preneo adj) so yes we would offer.