How would you treat cT3N1 ER/PR (-), Her2 (+) breast cancer who had mastectomy + ALND with a pCR in the nodes but ypT1 disease in the breast?

Given increased pCR rates, this is something we are seeing more commonly. If possible, these cN1 patients are often undergoing SLN rather than ALND if SLN is negative.

I would offer PMRT for this patient given initial nodal involvement with T3 and ER- disease. Until we get data back from B51, my standard is to offer RT.

I would treat chest wall with RNI. I would not cover dissected axilla but would treat SCV, IM with CW.

With expander, I would use standard fractionation. Outside of a study, I favor standard fractionation over hypofractionation in reconstructed patients.

I generally treat any patient with clinical stage III who received neoadjuvant chemotherapy with PMRT to the chest wall and regional nodes, even patients with ypT0N0, regardless of biology, due to retrospective data showing better survival with PMRT (McGuire et al., PMID 17418973). I agree with Dr. @Dr. First Last that one way to de-escalate these patients with great clinical response, particularly HER2+ is to offer SLNB instead of ALND. There is a Chinese study showing extremely low rates of false negative with SLNB in HER2+ patients getting neoadjuvant therapy (Sun et al., PMID 32034581), lower than observed on Z1071.

If you have multiple + (at least one path proven) prexrt LN, I think it's typical to offer comprehensive (CW, SCV, AX) radiotherapy regardless of the post-chemo path end result. Some modification of the AX may be warranted if AXLND versus SLNB perhaps. It seems that AXLND are rare in the setting of post chemo cR, and large multiple nodes initially would make one cautious about limiting AX volume.