Would you recommend PMRT for a patient with a right breast mastectomy with closest margin less than 0.1cm?  

I would consider this if there were other high risk factors like T2 disease, high grade or LVSI. In absence of these factors, favor systemic treatment alone.  Here is one reference.

If it's a T2N0 with close but negative margins (no tumor on ink), I do not offer PMRT in most cases. It can be considered in conjunction with factors such as triple negative/ER-, LVSI. 

Part of the decision-making process would depend on the location of the close margin. If it was posterior margin that was close (assuming no other adverse risk factors), then I would be much less likely to offer PMRT.