Which of the following organs is the least radiosensitive?

• A. Brain
• B. Breasts
• C. Stomach
• D. Large Bowel

Answer: (A)

Radiosensitivity follows how rapidly and how successfully cells divide and differentiate. Tissues with many actively dividing cells are more sensitive to radiation, while those with mostly mature, non-dividing cells are more resistant. The brain is made up largely of mature neurons that rarely divide, so it is relatively radioresistant compared with organs whose cells are constantly renewing, like the stomach and large bowel epithelium or glandular breast tissue. The stomach and large bowel have high cell turnover, so they are more susceptible to radiation damage; breast tissue also contains actively renewing cells. Therefore, among these options, the brain is the least radiosensitive.