<P> One fractionation schedule that is increasingly being used and continues to be studied is hypofractionation . This is a radiation treatment in which the total dose of radiation is divided into large doses . Typical doses vary significantly by cancer type, from 2.2 Gy / fraction to 20 Gy / fraction . The logic behind hypofractionation is to lessen the possibility of the cancer returning by not giving the cells enough time to reproduce and also to exploit the unique biological radiation sensitivity of some tumors . </P> <P> Historically, the three main divisions of radiation therapy are: </P> <Ul> <Li> external beam radiation therapy (EBRT or XRT) or teletherapy; </Li> <Li> brachytherapy or sealed source radiation therapy; and </Li> <Li> systemic radioisotope therapy or unsealed source radiotherapy . </Li> </Ul> <Li> external beam radiation therapy (EBRT or XRT) or teletherapy; </Li>

Exposing tumors to x-rays or gamma rays is called