The question of doing a whole body CT scan
to look for a disease is an interesting one that is becoming more relevant as more and more radiologic testing is used in medicine. It is thought that the increased exposure to radiation will result in a very small increased level of some types of cancer later in life.
This is likely not a significant risk for a single person, since the odds remain very low, but when thousands and millions of people are exposed every year, some people will have long term effects. In your specific case, it would be important to know what condition your doctor
is attempting to discover. The general rule in medicine is that tests should only be done if they will alter the management of a patient's care. If this CT scan is necessary to diagnose a disease, to give a prognosis, or to discover how the best treatment would be, then it might be a very necessary risk to take, especially since your personal risk remains low, as explained above.
CT scans are excellent for many disease processes, but should be used with thoughtfulness. Your doctor will appreciate your concerns, and would welcome the opportunity to discuss these risks with you in your specific situation. Please talk to him or her in more detail about this.