Answer to Question #10908 Submitted to "Ask the Experts"

Category: Medical and Dental Patient Issues — Diagnostic X Ray and CT

The following question was answered by an expert in the appropriate field:

Q

With the large push for patient "dose" tracking, I was wondering what guidance documents exist for evaluating risk based on a patient's cumulative "effective" dose. I use effective dose because patient-dose-tracking software is reporting effective dose per organ. Keeping in mind the Health Physics Society's position statements "Radiation Risk In Perspective" and "Uncertainty In Risk Assessment," it does not seem appropriate to sum each small exposure over the lifetime of a patient and then use that value for risk assessment. I also have concerns about software systems calculating effective dose per organ per patient. I thought International Commission on Radiological Protection (ICRP) 103, The 2007 Recommendations of the International Commission on Radiological Protection, was clear that effective dose is not for individuals. How valid are these values and should we be concerned about physicians pushing to record effective dose in patient electronic medical records or even using these values for epidemiological studies?

A

To answer your first question, the document that is currently the accepted standard for evaluating risk from radiation exposure is the Health Risks From Exposure to Low Levels of Ionizing Radiation – BEIR VII Phase 2 report. As you inferred, the effective dose is determined by multiplying the appropriate organ-weighting factor to each estimated organ dose and summing over all organs. You are also correct in your statement in that cancer incidence risks should not be applied to individuals on the basis of effective dose.  
 
To answer your second question, there are a wide range of methodologies used by the various commercially available patient-dose-tracking software solutions to estimate effective dose values. The methods range from use of the k conversion factor multiplied by the dose length product (American Association of Physicists in Medicine Report No. 96) to the use of table values generated by sophisticated Monte-Carlo calculation techniques and anthropomorphic phantoms. In any event, the systematic errors involved in determining organ dose, converting organ dose to effective dose, and taking into account all the modifying factors introduced by the patient—such as genetic susceptibility, living habits, environmental factors, etc.—results in a cancer risk estimate that is of no practical clinical use at the individual level. The recording of dose values such as the volumetric computed tomography (CT) dose index or dose length product (values displayed at the CT console) in the patient's medical record is currently required by state regulation in California and Texas, at the time of this response. Several other states and the Joint Commission have also proposed recording dose in the patient's medical record. The original intent of the regulations were to make the interpreting physicians aware of the patient's "dose" so they would identify values that were out of the norm and investigate whether such a dose was warranted or to take action to reduce future doses. Suggestions to use recorded dose values, such as minimizing future scans for patients with high cumulative doses, have not gained widespread acceptance in the medical community.  
 
Overall, further research is needed to improve our understanding of radiogenic cancers and the factors that influence them, and the medical community is continually urged to not use the "dose" information for clinical decision making. There are some excellent references on this subject that I suggest you read. How Effective is Effective Dose as a Predictor of Risk?, by C.H. McCollough, J.A. Christner, and J.M. Kofler, is an excellent article that addresses your question exactly. Cumulative Radiation Exposure and Cancer Risk Estimates in Emergency Department Patients Undergoing Repeat or Multiple CT, by R.T. Griffey and A. Sodickson, provides a description of how cancer risk estimates were considered on a clinical level. Another is A Rational Approach to the Clinical Use of Cumulative Effective Dose Estimates, by D.J. Durand. All of these articles are available for free download on the Internet.
 
Christopher Martel, MSc, CHP

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