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...