Should the justification of medical exposures take account of radiation risks from previous examinations?

With the growing availability of dose histories for patients, the question of whether previous diagnostic radiation exposures should affect decisions on future examinations is coming into sharper focus. This article discusses ways in which cumulative dose information may affect our thinking in justifying exposures. Based on a common tendency to see a connection between past and future events even where we know them to be independent-the gambler's fallacy-we may find ourselves treating past risks as if they contribute to the present risk. We take the example of two patients scheduled for CT scans, one with no previous diagnostic radiation exposures, the other with a history of previous CT scans, to show that the risks, and justification process, are equivalent in both cases. For the patient with a history of diagnostic exposures, there are only two possibilities: either harm has been caused or there has been no effect. If previous CT examinations have not caused harm, then, as past risks, they are irrelevant. The patient is in precisely the same position with regard to risk as a patient with no dose history. If harm has been caused, avoiding further diagnostic exposures does not change this outcome; again in this case, a justified radiation examination should proceed. We argue that bringing dose history into the decision process for justifying examinations is contrary to our understanding of risk for low-dose radiation and, rather than improving patient safety, would unnecessarily restrict access to radiation-based diagnostic examinations.