Is a close (<1mm) posterior margin an indication for lumpectomy cavity boost in a patient with early stage breast cancer when dissection was carried down to the pectoral fascia?

In the setting of invasive breast cancer, a meta-analysis and guidelines have determined a negative margin is no tumor on ink. So as long as margin is > 0, I would not consider a close margin (often considered for 0-2 mm) to be a sole reason to boost.

In the setting of pure DCIS, current guidelines are 2 mm for negative margins with discussion for 0-2 mm. If dissection was taken to pectoral fascia, I would not consider this a sole boost factor so, for example, in a 55-year-old with an ER+ DCIS, I would not boost with this margin whereas if this was ER- or < 50 years old, I would consider boosting.