How does external beam radiation destroy thyroid cancer?

When high-energy rays from radioactive sources outside of the body are focused on rapidly dividing cancer cells, they act to disrupt the growth and division of these cells, killing then off in the process. Normal cells may also be affected by external beam radiation, but their slower growth usually allows them to recover without damage. In order to limit the exposure of normal tissue to high-energy rays, external beam radiation treatments are usually administered in divided doses over the course of several days. Exposure is also limited by the use of special shields that block out normal tissue to focus rays on the areas of the body requiring treatment.

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