Radiation used in radiotherapy (aka radiation therapy or radiation oncology) is an invisible type of ionizing radiation, produced electronically or by nuclear decay of certain radioactive isotopes. Since the discovery of ionizing radiation, it has been utilized in the treatment of cancers and benign tumors since it was found that ionizing radiotherapy could penetrate the normal tissues of the body. The ionization in tissue produced by ionizing radiation causes damage to cells, primarily to the growth mechanism in the cells. Since unrestrained growth is what makes a cancer life-threatening, the ionizing radiation doses delivered to cancers by radiotherapy can cause many, most or all the cancer cells to die.
By delivering low doses of radiation once or twice daily (usually once) for weeks or months, the cancer cells usually end up with much more injury than the normal cells treated in the radiotherapy field. The carefully controlled amount of ionizing radiotherapy delivered is adjusted according to the size of the cancer, type of cancer, location of the cancer, medical condition of the patient, and goal of treatment. When targeting an area to control and kill the cancer while treating the least amount of normal tissue as possible, radiation oncologists may use advanced treatment planning and delivery techniques such as 3-D Conformal Radiotherapy, and most recently, solid modulator based Intensity Modulated Radiation Therapy (IMRT).