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

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 clinically apparent normal tissue radiation damage, etc.). Ideall,y the margin would be re-excised. In the setting of a local recurrence after mastectomy excised to negative margins, I typically recommend 40 Gy in 15 fractions for reirradiation. The RTOG 1014 partial breast reirradiation regimen (1.5 Gy twice daily for 30 treatments) would be another reasonable evidence-based extrapolation. My rationale for preferring the hypofractionated START-B approach...

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.