The process of justification requires that the potential benefit of the procedure outweighs the risk of induction of cancer in the exposed individual. The size of that risk depends on patient factors (in particular the age - children and young adults are especially susceptible), the extent and part of the body exposed (since some organs are more sensitive to IR than others) and to the nature of the examination and the imaging protocol used to perform it. For example CT scans may be obtained before intravenous iodinated contrast injection and in one or more phases post-contrast.
The risk of cancer induction by IR is a deferred risk that may occur from 5 to 15 years after exposure. The underlying clinical context in the individual patient is important, since, for example, in a patient who is undergoing imaging for an incurable cancer and also in, say, an 80 year old patient, the risk may be irrelevant.
A CT scan of the abdomen and pelvis, depending on the protocol used may expose the patient to about 20mSv of IR which, on average, increases the risk of fatal cancer by about 1 in 1000. However, this risk may be doubled in young patients, and halved in elderly patients. Remember, though, that the risk is cumulative if the patient undergoes repeated scans. This risk must be put into the clinical context and compared against other common risks. For example the risk of being killed on WA roads in a ten year period is approximately 1 in 1000.
In summary, if the potential benefit of the scan outweighs the risk, then the scan is justified. If the patient needs a scan for treatment or management then they should not be put off having one. Appropriate CT scans are good; inappropriate scans are bad.