What is the definition or description of: external beam radiation therapy?

Cancer treatment: It is the use of ionizing radiation (usually photons) to treat cancer and a few benign conditions. Radiation tends to have more of an effect on rapidly dividing cancer cells, than on normal tissue cells in the body, which makes it useful in the treatment of cancer. The treatment is delivered from a machine called a linear accelerator, through multiple fields aimed at a particular body site.

Radiation can be: Delivered internally, using unsealed liquid sources or pellets or seeds implanted temporarily or permanently. More commonly, it is delivered by a machine that aims beams from the outside at targets located internally, and passes through healthy cells on entry and exit, as the # of beams increase, and focus on the target, the dose to healthy tissue diminishes. Ebrt is common.