What type of rays are used to externally treat a tumour?

Hi! For external beam radiation treatments, x-rays and gamma rays are used as they can be targetted in a narrow beam (and formed into unique shapes using a metal plate called a collimator) and will be able to penetrate the skin. Beyond rays, tumours can also be treated using beams made up of a stream of electrons, and more recently, advances in machines that create beams using protons and other heavy particles (like whole carbon atoms) has shown great promise. But for now, the main ones are x-rays :)