I have had breast implants for many years. I was worried about what the radiation would do to my implant, which is under the muscle. Both the radiation oncologist and the plastic surgeon said that the radiation could cause the muscle to contract and squeeze the implant, which would be uncomfortable and make it so that I would need another surgery after treatment to replace the implant with a smaller one. I assume I would have also needed or wanted a smaller one in the other breast. But nothing happened to the size or shape of my breast. It looks and feels the same.
