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May 2015


Radiation Risk Assessment Made Simple 

 Samuel L. Brady 


Even for clinically indicated imaging, referring clinicians and radiologists should always adhere to the as low as reasonably achievable (ALARA) principle when ordering examinations that use ionizing radiation, especially for pediatric patients.

St. Jude medical physicist Samuel L. Brady, corresponding author of A Comprehensive Risk Assessment Method for Pediatric Patients Undergoing Research Examinations Using Ionizing Radiation: How We Answered the Institutional Review Board, talks about the tool.

What was the purpose of the study? 

To provide investigators with relative examination doses so that they may better assess the potential radiation effects and risks for research subjects and to provide simplified language that investigators can use in consent documents.

Why were you interested in this subject? 

We were first drawn to this subject because our clinical investigators, who were writing scientific protocols that included imaging examinations as part of their clinical trials, needed a simple relative risk assessment tool. Giving them an overview of relative risk based on the most common imaging examinations used in research would provide these investigators with an initial opportunity to wisely select the most effective imaging option at the lowest radiation dose. Once an imaging option was selected, the investigators would be able to weigh the risks associated with that option and determine its appropriateness for inclusion in a clinical investigation. Finally, we wanted to provide the investigators at our institution with a uniform format for crafting informed consent language that would accurately portray potential patient risk. This is an ongoing project that we are continuing to update and improve.

What makes children at greater risk of damage from ionizing radiation? 

Children are at the greatest risk of damage from ionizing radiation because of the nature of their developing bodies. In growth and development, a child’s body is more susceptible to radiation damage because of the higher proliferation and turnover of tissue cells. The greatest risk from imaging with ionizing radiation is long-term cancer induction. Accepted risk models show that cancer induction may occur at radiation dose levels measured in the diagnostic imaging range. When conducting clinical research with children, we should do our best to minimize both the number of imaging-based tests and the level of radiation used.

Why use effective dose? 

Effective dose—the sum of the equivalent doses in all tissues and organs of the body weighted for tissue effects of radiation—was developed to protect patients. Effective dose allows a risk estimate based on a nonuniformly exposed patient (e.g., imaging of the abdomen only). It is a stochastic effect calculation rather than a deterministic effect or severity calculation of tissue or organ damage.

Effective dose is calculated by multiplying a tissue- or organ-weighting coefficient that is based on evolving epidemiologic data for cancer incidence from populations exposed to ionizing radiation (e.g., atomic bomb survivors, Chernobyl cleanup workers). The weighting factor takes into account the relative risk of cancer induction; for example, breast tissue is weighted higher than brain tissue because of cancer incidence levels in the aforementioned exposed populations.

Critical to estimating effective dose is understanding organ or tissue radiation dose, which we derived using DICOM metadata. It is important to understand the limitations of effective dose. It is a population-based estimator of risk—the tissue- or organ-weighting coefficients are averaged for both sexes of all ages; thus, we do not calculate effective dose for a single patient. Because of the uncertainty in the underlying epidemiologic data, it is not appropriate to use effective dose to estimate cancer induction and mortality rates. Effective dose does, however, provide a means to compare relative radiation risk across all imaging modalities that use ionizing radiation.

You chose to develop your methods using DICOM metadata. What are the advantages of doing so? 

Basing our dosimetric analysis on metadata derived from DICOM allows us to develop customized and automated tools. DICOM metadata can be quite information rich, which allows us to parse the data in many ways and gives us the ability to analyze the data from multiple approaches. Additionally, DICOM metadata are archived with every patient image and thus are patient and examination specific, which gives us the ability to look at the variations of imaging dose due to variations within a patient population. Over all, use of DICOM metadata allows a great degree of flexibility in estimating radiation dose for analysis.

How will your research results benefit patients? 

Developing a risk assessment method for all of the ionizing radiation–based imaging modalities at our institution has allowed us to look critically at how we image our patients for both clinical trials and standard patient care. We are able to assess the radiation dose levels on a treatment protocol level, an examination level, and a patient level. By looking first at the patient level, we are able to ensure that we are providing optimal imaging care at the lowest radiation dose, customized for the patient’s body size (which is very important to account for when imaging pediatric patients). At the examination level, we can verify that we are consistent across the patient populations for a particular examination type, such as a chest CT or MIBG study. Finally, at the treatment protocol level, we can assess the total radiation risk for a particular patient over the course of his or her treatment, and we can begin to answer the question of diagnostic efficacy versus radiation dose and how can we better design treatment protocols that are safer and more diagnostically effective.