Post-mastectomy radiation with tissue expander - immediate dents?

My sister had a nipple-soaring SMX with a nerve graft in August. She's thrilled with the results and only needed 3 fills. She had multifocal IDC and one positive node so she is doing radiation. She had her first treatment yesterday and she woke up with a dent in her breast. Then had her second treatment a few hours ago and the expander is even more dented. She's calling her plastic surgeon in the morning, but I wondered if this happened to anyone else? Is she likely to need to need the expander removed? She had her surgery at Cleveland Clinic, but we are in Michigan and she is having to do radiation locally since it was not feasible to be out of state for 5 weeks. We have low trust in the facilities in our area, so she is nervous to take guidance from the radiation oncologist.