External radiation (or external beam radiation) uses a machine that directs high-energy rays from outside the body into the tumor. Most people get external radiation therapy over many weeks.
It is a common cancer treatment that used high doses of radiation to destroy cancer cells and shrink tumors.
At low doses, radiation is used as an x-ray to take picture inside our body. In cancer treatment higher doses of radiation are used to destroy cancer cells. The radiation that destroys cancer cells also injures nearby healthy cells. Therefore you have many side effects.