How would you approach a patient with invasive breast cancer and DCIS s/p mastectomy and immediate reconstruction with a negative invasive margin, but a close DCIS margins?

I would observe such a patient. With a close DCIS margin post mastectomy, there is very little evidence of a benefit to PMRT. Subsequent local recurrence is well below 10% so I see no clear role for PMRT in such a case. Even if there was a close invasive margin unless there were other aggressive factors, the role for PMRT for a close margin as the only factor is not evident in my view.

Completely agree with Dr. @Dr. First Last. Even in the setting of a positive margin, PMRT for DCIS is of questionable utility.

Klein et al., PMID 26185737, reported outcomes of all women in Ontario (N = 1,546) diagnosed with pure DCIS from 1994 to 2003 treated with mastectomy without radiotherapy. There was no difference in cumulative 10 year rates of chest wall recurrence by age at diagnosis (<40 years = 5.2%, 40-44 years = 1.3%, 45-50 years = 2.9%, >50 years = 2.1%; p = 0.19), nuclear grade (h...

Close margin is ambiguous as for some less than 1 mm is close, for others less than 2 mm is close. For T1 and T2 and close margin (< 1mm), would not treat routinely unless has T2, high grade, LVSI, and significant tumor close to margin (more than focal).