What is the risk that an adult in his 20's will get brain cancer from getting one ct scan?

Not very significant: With one cat scan of head the risk in men is 1 in 11080 and in women is 1 in 8100, there is also lot of variations in radiation exposure from one place to other.This was published in archives of internal medicine in dec 2009.

0: One head ct scan radiation dose is about the same as one chest x-ray. One will not get enough radiation exposure from one ct to cause any issues.

Zero chance: Not enough irradiation to cause any problem, and certainly does not cause brain cancer.

None: No measurable risk.

More common than: Winning the powerball lottery, in the range of being struck by lightening (about 1/10, 000).

Very low: The risk of brain cancer or leukemia is 1/10, 000 for a head ct in a child 10 years or younger, if the ct scan is done at normal radiation dose. Most places now have a low dose protocols for head ct scans for children. In the adults, the risk is lower. The benefits in clinically indicated ct scans outweight the risks.