Radiation After Mastectomy: Understanding the Necessity

Question
Had right mastectomy in Dec.Two tumors 2.1 total size. Pathologist found minimal sized cancer in sentinel node.No other node involvement(out of 15).Had close margins of 1cm. Finished chemo 2 months ago (in March).Tissue expander inserted during mastectomy now fully expanded.Am advised to get radiation since margins were close.Why is radiation necessary if surgery took the breast tissue?Didn't the chemo kill all the cancer cells that might have remained?And since chemo finished 2 months ago, isn't it late for radiation? How do I know that radiation is going to work if supposedly the chemo didn't kill everything? I'm confused.

Answer
Neither chemo therapy alone nor radiation therapy alone can guarantee a complete kill off of all cancer cells - nor a combination of them. BUT a combination does have better chances of doing so! So were I in your shoes I would accept the radiation therapy straight away! I would have administered the radiation therapy first and the chemo therapy later, but that is just details. No one can guarantee success in a cancer case like this but this is your best chance! Though I agree the therapy should have - if possible - been given earlier. But better late than never. Good luck!