The purpose of radiation therapy is to use ionizing radiation, x, or gamma rays, to eradicate tumor cells, or more precisely to stop their growth without seriously damaging normal tissue. This is a difficult task since it is impossible to deliver dose solely to the target volume without affecting healthy tissues, especially when deep seated tumors are treated with external beams. In patients treated with conventional radiotherapy, the primary obstacles to achieving the maximum possible therapeutic advantage are the uncertainties in the true extent of the disease, inadequate knowledge of the exact shapes and locations of normal structures. So, the tumor dose often has to be maintained at sub-optimal levels to prevent unacceptable normal tissue complications which lead to a higher probability of local failures. Today, conformal radiotherapy is widely used in the management of cancers, especially brain tumors. Conformal radiotherapy is a new irradiation technique made possible by technological improvements, especially progress in imaging and 3D dosimetry by conforming the volume irradiated as closely as possible to the clinical anatomical target volume.