Is pectoralis muscle invasion an indication for post-mastectomy radiation (PMRT) for an otherwise early stage, node negative breast cancer with clear margins?

In and of itself no. Given the inclusion of specific node negative patients in MA.20 and EORTC 22922 and small but significant DFS benefit in both, by extrapolation to mastectomy it behooves us to consider carefully the potential benefit of PMRT in node negative patients without over-treating. In re...

The data in a node negative situation is lacking /limited but in 1-3 positive node, pec fascia involvement was associated with high risk of recurrence even when surgeon excised muscle with negative margin.

https://www.ncbi.nlm.nih.gov/pubmed/11395242

Plus Danish study did allow pec fascia involvement ...

I think if this is the only potential indication for PMRT and the involved muscle has been removed with negative margins, I would not treat.