Would you include the regional nodes when treating with PMRT in a patient with a high grade, large primary tumor, but low burden axillary disease with a complete axillary dissection (e.g. 1/20 nodes involved)?

This is a question we are seeing more and more. I break these cases down into two situations:

1. Postmastectomy, no neoadjuvant: In these cases, I extrapolate from MA20 which looked at patients undergoing breast conservation with ALND and a large percentage had low nodal burdens. RNI was associated with a benefit in outcomes, though no survival advantage. I extrapolate this to the mastectomy setting and treat with RNI (SCV, Axilla, IM) in most these cases.

2. Postmastectomy, positive nodes after...

I'm assuming this patient went directly to surgery and has received adjuvant chemotherapy. Most patients on the NCIC MA.20 trial had small tumors (~ 52%< 2 cm, ~99% < 5 cm) that were ER+ with relatively low nodal burden. A large majority would have also had grade 1-2 disease (i.e. "favorable biology"). However, an interesting subset to be included that has helped inform my practice were the 177 node negative patients with high-risk features defined as primary tumor measuring 5 cm or more or 2 cm...

If the patient was ER+ and had a low Oncotype score, I would consider enrollment in MA.39, which would randomize this patient to PMRT with comprehensive nodal RT versus no PMRT.

Otherwise I agree with the response by @Dr. First Last, and would treat the undissected axilla and the supraclaviular fossa, +/- the IMNs depending on the clinical scenario.