As the name, External Beam Radiation Therapy, already tells the radiation is sent to the tumor from the outside of the body. Mostly the radiation comes from X-rays or gamma rays and has to be applied multiple times over a time period of six to eight weeks. The high-energy radiation beam is produced by a linear accelerator. The beam is sent deep through the body to achieve a high dose of radiation at the tumor cells [1]. In combination with  CT or MRI the treatment can be monitored. With these state-of-the art image-guided techniques (see image) the radiation can be sent more directly to the tumor what reduces side effects [1,2]. With EBRT the radiation can be delivered to a variety of area sizes. So only a part of the brain or the whole brain can be affected. The latter is recommended for large or spreading tumors [1].

As mentioned above mostly X-rays or gamma rays are used for EBRT, but it is also possible to use protons as the source of radiation. With a technique similar to nuclear reactors, the proton is separated from an atom. The emerging proton beam can be directed to and shaped as the tumor by a computer program [1]. There is a great advantage of proton beams in contrast to X-rays. While X-rays affect the tissue in front of the tumor during treatment, proton beams deliver most of their energy directly to the tumor. Also the spreading radiation is lower so the damaged tumor cells can be easily distinguished from healthy brain cells [1].

Stereotactic Radiation Therapy is a noninvasive approach which tries to treat only the tumor by using pencil-thin beams of radiation. Most of the time it is used with imaging techniques like CT and MRI to orientate the high dose of radiation towards the tumor. This is supported by a computer program planning the therapy [1]. Meanwhile the SRT has become a standard treatment, but at the beginning it was used especially for tumors in locations like the brain or the spinal cord that are very risky for operation. Besides the reachability of considered inpoperable areas the SRT has other advantages. Not only the location but also the size is an important factor when treating tumors. The SRT can be used for the treatment of tumors one and a half inches in diameter and smaller. The last point to mention is that the SRT has to be applied only once [1].