ESR News October 2015
Estimation and communication of radiation-induced cancer risks from x-ray examinations
John Damilakis, Professor of Medical Physics
It has been well documented that high doses of radiation may cause cancer. However, a firm link between low radiation doses, such as those received by patients during diagnostic and interventional procedures, and cancer has not been established yet. Even so, radiation protection regulations conservatively assume that any amount of radiation may pose some risk of carcinogenesis.
The radiation-induced lifetime risk of cancer incidence can be determined by multiplying the effective dose with risk factors published by the International Commission on Radiological Protection (ICRP) (1) or the Biological Effects of Ionizing Radiation Committee (BEIR) (2). The ICRP states that the overall fatal cancer risk from low dose ionising radiation is about 5% per Sv (1). As expected, radiation risk coefficients are highest in newborns, children and adolescents because of their greater radiosensitivity and longer remaining life expectancy. Alternatively, to derive lifetime risks of site-specific cancer incidence associated with radiation exposure from a medical examination, organ doses can be multiplied by appropriate sex-specific and age-specific risk factors provided by BEIR (2). The total lifetime attributable risk can be estimated by summing up site-specific lifetime attributable risks. Using this method, radiation-induced cancer risks are calculated for radiosensitive organs primarily exposed to x-ray photons. Contribution to total radiogenic risk from organs exposed to scattered photons can be assumed to be negligible.
Cancer risk to the embryo or foetus associated with x-ray examinations is not certain. ICRP recommendations on pregnancy and radiation indicate an excess cancer risk for fatal childhood cancer due to irradiation in utero to be 6% per Gy (2). If the dose to an embryo from a CT examination is 20 mGy, the risk of excess childhood fatal cancer is 0.12%. The background risk of fatal childhood cancer is 0.2%. Thus, the probability of fatal childhood cancer for this embryo is increased from 0.2% (natural risk) to 0.32%. The risk factor for genetic effects is assumed equal to 1% per Gy (2).
It should be noted, however, that there are many assumptions and uncertainties related to the radiation-induced cancer risk estimations. Risk coefficients for radiogenic cancer are based on the linear-no-threshold model. This model assumes that cancer risk is directly proportional to the dose. Current knowledge of the biological effects of ionising radiation comes from the survivors of atomic bombs and individuals who have received radiation from radiotherapy. However, the validity of the linear-no-threshold model for low radiation doses has not been confirmed yet.
Ionising radiation is frightening because, for patients and relatives, radiation is often linked to atomic bombs and nuclear reactor accidents. Proper communication about the risks from medical x-ray examinations between radiologists and patients is needed to increase understanding and eliminate unfounded fears. Dauer et al (4) discuss four approaches typically used by medical professionals to communicating risks of radiation to patients: the ‘paternalistic’ approach (‘the physician advises the patient what procedures and treatments are recommended and the patient is expected to unquestionably follow such advise’), the ‘risk comparisons only’ approach (‘risk comparisons using the concept of effective dose’), the ‘risk numerology’ approach (‘relative risks, excess cancer rates, increased rates over background levels, and log-based hazard comparisons) and the ‘quality assurances’ approach (‘our protocols are designed to deliver doses as low as reasonably achievable so you don’t really need to worry about it’). In the same publication (4), authors provide suggestions for improved benefit-and-risk communication.
- The 2007 Recommendations of the International Commission on Radiological Protection. ICRP publication 103. In Ann ICRP 2007; 37(2-4).
- Pregnancy and medical radiation. Publication 84, Annals ICRP 2000; 30(1).
- Committee to Assess Health Risks from Exposure to Low levels of Ionizing Radiation, Nuclear and Radiation Studies Beard, Division on Earth and Life Studies, National Research Council of the National Academies. Health risks from exposure to low levels of ionizing radiation: BEIR VII phase 2. Washington, DC: The National Academy Press; 2006.
- Dauer L, Thornton R, Hay J, Balter R, Williamson M, Germain J, AJR 2011;196:756-761.