Radiation treatment for breast cancer has no effect on fertility.
But if you're also having chemotherapy and definitely want to get pregnant in the future, you may want to start fertility treatment BEFORE you start radiation. Fertility treatment may involve collecting eggs, fertilizing them, and saving them until after your treatment is completed.
The eggs for fertility treatment should be collected before radiation treatment starts. This is because the eggs that are beginning to mature for ovulation may be affected by tiny amounts of radiation that may scatter from the breast (the main treatment area).
Waiting until after your eggs are collected is unlikely to make radiation therapy any less effective.
It's also unlikely that the tiny amount of radiation that may scatter from the breast will affect the immature eggs in your ovaries.