What evidence supports the use of high tangents for pN1mic breast cancer?  

In my experience, high tangents are commonly used for patients with pN1mic disease, and occasionally for higher risk patients with pN0 disease. The recent ARO publication (Arbab et al.) on RT planning for residents endorsed this approach.

However, I am not aware of any studies that have explicitly studied and validated this approach for pN1mic patients. In fact, pN1mic patients have generally been eligible for studies investigating comprehensive RNI: For example, pN1mic patients are eligible for the ongoing MA.39 trial which is randomizing patients between comprehensive RNI and no nodal irradiation. So it's not entirely clear to me how we know that only the low-axilla is at risk in patients with pN1mic disease, rather than the entire axilla being at risk suggesting that a more comprehensive RNI approach should be taken.

Are there any trials that have actually defined outcomes using high tangents for pN1mic patients? Even better, are there any trials that have compared high tangents with a) no RNI (and shown superiority) or b) comprehensive RNI (and shown equivalence) in the pN1mic setting?