Radiotherapy, called also Radiation Therapy, is treatment of cancer and other diseases with ionizing radiation. This approach is completely non-invasive and consists in depositing energy on the tumor area.

This dose of radiation is measured in Grays [Joule/Kg] and kills the cells in two general events: the production of free radicals into nuclei, for example hydroxyl radicals, and the damage of DNA strings, through single strand breaks (SSB) and especially through double strand breaks (DSB).

The branch of science studying interaction between radiation and biology is called radiobiology. From this branch comes the Linear Quadratic model, base model for all radiotherapies, that gives the probability of cell survival with a specific dose and with different Linear Energy Transfers (LET) of the radiation used.