Radiation is used in both X-Rays and radiation therapy for treating cancer. Yet it also causes cancer?

Medical radiation comes in the form of diagnostic x-rays and other tests, as well as from radiation therapy. Radiation therapy is currently used to treat some types of cancer and involves dosages many thousand times higher than those used in diagnostic x-rays.

Overall, radiation therapy alone does not appear to be a very strong cause of second cancers. This is probably due to the fact that doctors try to focus the radiation on the cancer cells as much as possible, which means few normal cells are exposed to radiation. Still, some studies have linked radiation therapy with an increased risk of leukemia, thyroid cancer, early-onset breast cancer, and some other cancers. The amount of increased risk depends on a number of factors, include the dose of radiation, the location in the body, and the age of the person getting it (younger people are generally at greater risk later on).

If cancer does develop after radiation therapy, it does not happen right away. For leukemias, most cases develop within 5 to 9 years after exposure. In contrast, other cancers often take much longer to develop. Most of these cancers are not seen for 10 years after radiation therapy, and some are diagnosed even more than 15 years later.

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What is so confusing?