What dose of reirradiation would you consider for locally recurrent breast cancer after mastectomy, excised with positive margins?  

There is not one consistent approach. Based on the prior RT volume and the effect of prior RT on skin/Sc tissue can vary from comprehensive RT to limited volume RT to 45-50 Gy followed by a small volume boost.

These are complex cases and lots of factors to consider when planning adjuvant therapy (age and comorbidities, tumor biology, other pathologic features like LVI, reason for the positive margin, plans for systemic therapy, presence of reconstruction, the volume of prior radiation, extent of clinicall...