Does the presence of an ATM mutation impact your fractionation regimen for a woman with breast cancer?

ATM mutation is not a contraindication for RT. Techniques should be used to reduce scatter dose to contralateral breast. This is very good review on the topic.

https://www.ncbi.nlm.nih.gov/pubmed/31381960

Though not a lot of data, I have treated patients with hypofractionation (breast or breast and RNI) and not noticed any issues.