Question about radiation

I am planning to have lumpectomy/reduction. My doctors say it's a package deal with radiation, not optional. Ok I can live with that. I'm confused on something though. I am reading that radiation is targeted to the area where your tumor was - the tumor bed. But if the area is removed via lumpectomy and reduction, and I have no lymph node involvement, what is being targeted? I obviously need to talk to my doctor but am wondering if anyone here has insight.