I generally would not treat microinvasive disease with systemic therapy unless it were multifocal in a surrounding of high grade DCIS, as these cases could recur distantly, although even then, the risk is low. It may be higher over a life time in younger patients - with 20 year breast cancer mortal...

I do not treat microinvasive HER2+ disease with adjuvant therapy. 

This can be a difficult decision. High grade HER2+ DCIS with microinvasion always gives me pause and I take it seriously each time. I have see all too often recurrence with systemic disease or regional lymph nodes within as early as 2-3 years after surgery.  Especially when extensive, it can b...