How does radiotherapy help brain tumors?

Killing cancer cells: Radiaiton therapy uses high energy x-rays to cause injury to cancer cells. It is used in many cases were surgery is either not possible or even despite surgery, there will be cancer cells left behind. Radiation in general works by either killing or injurying cancer cells so that they either die off directly or die off as they try and grow/divide. The normal body is able to recover by healing.

Kills cancer cells: Radiation uses high-energy x-rays that are focused on the tumor. They kill cancer cells by damaging their dna. The treatment is generally delivered daily (5-days each week) for about 6 weeks.

Circuits still work: Both the above are correct. The reality is that cancer cells anywhere can be kill by radiation. The brain offers a special problem in that very important areas are often right next to tumors, even completely surrouding them. Radiation can kill cancer while normal tissue can repair itself day to day. Technology, perhaps the best being protons, can stop the radiation even millimeters from a tumor.