Radiation therapy, also called radiotherapy, uses high-energy rays to damage cancer cells and stop them from growing. Radiation therapy can be given externally by a machine or internally by placing radioactive material in the body near the cancer.
There are two types of radiation therapy: external beam and internal.
External beam radiation comes from a machine outside the body that aims high-energy beams at cancer cells. Internal radiation therapy involves putting radioactive material inside the body near cancer cells. The most common type is brachytherapy, which uses short bursts of radiation.
Radiation therapy damages DNA in cancer cells, making it difficult for them to grow and divide. When DNA is damaged, the cell either dies or cannot divide properly. Radiation therapy does not discriminate between healthy tissue and cancerous tissue—some normal cells are also affected by the treatment. However, most normal cells can recover from this damage more easily than cancer cells can because they have a better ability to repair themselves. This is one reason why side effects are usually more severe with higher doses of radiation therapy (to kill more cancer cells) or when treating larger areas (healthy tissue as well as tumor). Another reason for side effects is that different parts of the body react differently to treatment; for example, radiosensitive tissues such as bone marrow and skin are more likely to be damaged by radiation than other tissues.
Side effects depend on many factors including the type and dose of radiation used, how sensitive the area being treated is to radiation, whether you receive other treatments along with radiation (such as chemotherapy), your age and general health, etc. Some common side effects include fatigue, skin reactions like redness or dryness, nausea/vomiting, diarrhea, appetite changes, weight loss/gain, hair loss (if scalp is treated), mouth sores/dryness/swelling/trouble swallowing or eating (if head/neck area is treated), urinary frequency/urgency/incontinence or kidney problems (if pelvis is treated), shortness of breath (if lung area is treated). Most side effects go away after treatment ends but some may last a long time or be permanent