ranges from light and heat to radio waves, microwaves, and x-rays. In reference to cancer, the 2 main types are ionizing and non-ionizing radiation. The high-energy rays used for x-rays and some other imaging tests, as well as in higher doses for cancer treatment, are called ionizing radiation. Ionizing radiation has enough energy to remove electrons from atoms. This type of radiation can be produced by medical devices, but also comes from natural sources such as radon gas (in the ground) and outer space. See also imaging tests, radiation dose, radiation therapy, x-ray.