Complexities in Using Effective Doses to Guide CT Scans

Efforts to record and cumulate individual patient radiation dose are being advocated by a variety of societies, including the American College of Radiology, and even the International Atomic Energy Agency.  There is also a significant effort to include a patient’s radiation dose in the electronic medical record (EMR).  However, a recent article from Health Imaging outlines several flaws in using cumulative dose estimates alone to guide CT decision making.

The problem with this concept is that at present, we do not have a method of individualizing patient dose.  Typically, an effective dose is assigned to a procedure based on the type of study and, in the case of CT scans, an index of the x-ray output used for the procedure.  There is no adjustment for patient size or shape, factors that affect an individual’s actual radiation dose.  In the case of projection x-ray imaging exams, such as chest x- rays, an effective dose is assigned based on ‘typical’ effective doses for that procedure with no adjustment for any x-ray factors or patient size.

More fundamentally, the effective dose is not a physical measure but an estimate of the effect of exposing a part of the body to radiation compared to exposing the entire body.  The effective dose accounts for the different radiosensitivity of different organs.  Yet the estimate of effective dose for a procedure is not adjusted for sex and, in the case of adults, not adjusted for age-- both factors that affect the relative radiosensitivity of organs.  Thus, the total effective dose for an individual becomes less and less accurate as they undergo more x-ray procedures because the errors in the effective dose accumulate.

It is certainly desirable that a patient’s EMR include a clear history of any procedure that involves exposure to radiation, including the type of procedure, the date of the procedure, and the findings or outcome of the procedure.  It is also desirable to record factors that relate to the x-ray output levels, usually available for CT and angiographic procedures.

However, the fundamental concern in ordering an imaging procedure is not the cumulative dose to the patient, but rather the reason or clinical benefit for performing the exam.  If the principles of ALARA (as low as reasonably achievable) are followed, every imaging procedure should have a meaningful benefit to the patient being exposed and the radiation levels used in the procedure should be adjusted to account for patient size and to provide a diagnostic imaging exam.

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