Does the presence of multifocal disease impact your decision on whether or not to recommend post-mastectomy radiation?

Yes, it does however multifocality is just one of several risk factors that predict for the likelihood of local recurrence and it is far less important than % nodal positivity, tumor size, age, +margin, multicentricity or adequacy of lymph node dissection.

By itself, the presence of multifocality i...

The data doesnt suggest multifocality by itself increases risk of relapse after mastectomy. One retrospective data from MDA suggest 1-3 positive node with multicentricity ( in different quadrant) increases LRR to more than 20% and sometimes we do cosider that as a factor in 1-3 positive node