What is the definition of a critical organ in radiation biology?

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In radiation biology, an organ is considered critical when damage to it would meaningfully impair function and reduce a person’s quality of life. The emphasis is on the consequences of injury, not on where the organ is or how easy it would be to treat afterward. This is why dose limits in radiotherapy are set to spare these organs at risk, to prevent side effects that would significantly affect daily living.

For example, protecting the lenses of the eyes to prevent cataracts, the spinal cord to avoid paralysis, the salivary glands to prevent dry mouth, or the heart to preserve cardiac function—all of these are about preventing outcomes that would diminish quality of life. The other ideas—being located in the skin, being easy to treat, or never being affected—do not define a critical organ, because they don’t capture the real consequence of damage.