Radiotherapy and radiosurgery

A) In radiotherapy, radiation is delivered to both normal and cancer tissues. The therapeutic efficacy is derived from the increased DNA repair capacity of normal cells relative to tumor cells when repeatedly exposed to small doses of radiation. In the illustration, the dashed line represents the radiation field. The solid square represents the intracranial space. The striped circle represents the tumor volume. In this theoretical illustration, both the tumor and the normal tissue receive 2 Gy of ionization radiation. B) In radiosurgery, multiple, non-parallel radiation beams are converged on the tumor volume. The radiation received by the normal tissue in each beam path is minimal relative to the point of beam convergence. In this illustration, the dashed line represents the radiation field. The solid square represents the intracranial space. The striped circle represents the tumor volume. The tumor volume receives 8 Gy of radiation from four radiation sources while the surrounding tissues receive significantly lower doses of radiation.