I recently had a similar patient- 36 yo six months postpartum, not breastfeeding. I had the same concern, so I sent her to the radiation therapist pre-op to get his OK. He felt there was no problem, even without having seen her, but I had her go for an exam to be sure. We did the usual lumpectomy; she is getting adjuvant chemotx, to be followed by her radiation.
The alternate course, modified radical mastectomy, has its own problems in such a patient, due to the lopsided cosmetic result.