What is the principle of radiotherapy?


Radiation therapy is a localized treatment to kill cancer cells by irradiating them while sparing the surrounding normal tissue. The source of radiation can be outside (external radiotherapy) or inside (brachytherapy) the body.

During irradiation, the X-rays enter the cells where they produce physicochemical reactions that damage the cell DNA, causing the death or preventing the multiplication of such cells. Therefore, the radiation must be directed very precisely to the area to be treated, to preserve as much as possible the healthy tissues and the surrounding organs, which are called “organs at risk”. Moreover, radiation outcomes will be different in cancer and normal cells. Indeed, normal cells can often manage to repair DNA damage between radiotherapy sessions and they will survive, but not cancer cells that are also more sensitive to irradiation and will be killed during treatment.

The key parameters of radiotherapy are the:

    • Total radiation dose(in Gray or Gy, the unit of prescription for radiotherapy)
    • Treatment duration (the total time between the first and last treatment session)
    • Dose fractionation (the total number of sessions)
    • Irradiated volume

Before treatment initiation, these different parameters will be optimally defined by the radiation oncologist. The total radiation dose will be chosen in function of the tumor type (some cancers are more “radiosensitive” than others) and on the tolerance of the surrounding organs.