Article publication date
14 April 2025
ARPANSA review date
19 May 2025
Summary
This computational modelling study evaluated the relationship between current computed tomography (CT) scanning practices and future incidence of cancer in the USA. Data on CT use was extracted from a market outlook survey of hospitals and imaging facilities in the USA. CT scans were subdivided into categories and an average dose per category was determined using data from a US CT dose registry. Subsequently, absorbed doses were estimated for different organs through radiation transport simulations. Using information from the US National Research Council Biological Effects of Ionizing Radiation (BEIR) VII report, lifetime cancer risks corresponding to the calculated absorbed organ doses were computed. A sample of 121212 examinations was used to determine the proportion of scans for each CT scan category. By combining the lifetime cancer risk per CT scan type with the data on total CT scans, the total risk to the USA’s population was estimated.
The article computed that approximately 93 million CT scans were undertaken in the USA in 2023, with persons in the 60-69 age bracket undergoing the most examinations. Examinations performed in the last year of life were excluded for cancer estimates leaving 84 million CT scans which were modelled to result in 102700 cancers over the projected lifetime of the exposed patients. This is currently equivalent to approximately 5% of new cancers in the USA each year.
Published in
Journal of the American Medical Association Internal Medicine
Link to study
Commentary by ARPANSA
The use of ionising radiation, such as CT scans, in medicine involves a balance between benefit and risk. The article makes good use of some large databases, and models derived from epidemiological data, to assess long-term cancer risks from CT utilisation in the USA but does not attempt to address the benefit side of the equation. CT scans provide significant benefits to patients by enabling accurate and timely diagnosis, often avoiding the need for more invasive procedures, or identifying disease at an early stage when it can be effectively treated. Medical staff requesting CT scans must justify them by considering the balance of benefit and risk for each patient. Imaging staff must optimise scan settings to balance radiation exposure with the image quality needed for the diagnostic task. The risks identified in the paper underline the importance of continued and effective implementation of these principles of justification and optimisation.
The study uses cancer estimates from the BEIR VII risk model and as such inherits its limitations. The primary limitation is that the model is based on cancers observed in cohorts of survivors from the atomic weapons detonations in Hiroshima and Nagasaki and uncertainties remain in applying this information to other situations. The BEIR model estimates for the relationship between cancer risk and ionising radiation exposure are not new and have not changed for decades. In this sense the current paper does not present new information on the relationship between exposure and cancer risk but rather is using cancer risk as a tool to highlight potential issues with scan over-use. Additionally, the study's risk calculations factored in average life expectancies, which may overestimate future cancer risk for patients with shorter life expectancies due to underlying illness.
While our understanding of the relationship between exposure and cancer risks remains the same, advancements in CT scanning technology have significantly lowered patient doses and improved scan quality. ARPANSA’s National Diagnostic Reference Level (DRL) Service has tracked these optimisations, with data demonstrating that doses for common procedure types have been steadily decreasing over the last decade.
The study estimated 273 CT scans per 1000 population in the USA. In comparison, the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) 2021 global survey reported 238 scans per 1000 population for the USA and 157 scans per 1000 population in Australia, both of which are comparable to the average for countries at the highest income level (159 scans per 1000 population). The lower prevalence of scans, in conjunction with the dose reductions mentioned above, indicate that similar modelling for Australia would give a substantially lower estimate on the projected number of cancers from CT scans.
Most importantly, results from the study should be considered in the clinical context of CT scans. CT scans are performed for diagnostic purposes on unwell patients and the diagnostic benefits of the scan must be weighed against the potential harms. This principle of justification, along with the optimisation mentioned above, must be applied to all medical ionising radiation exposures in Australia as outlined in the Code for Radiation Protection in Medical Exposure RPS C-5. Simply, the conclusion presented by the paper does not account for the diseases treated and prevented by undergoing a scan and should not be cause for hesitancy in patients who have been prescribed a scan by their medical professional. However, it does underline the importance of continued monitoring and vigilance in the usage of ionising radiation in medical procedures, including the use of referral guidelines and comparison of doses to diagnostic reference levels. These principles, advocated by the International Commission on Radiological Protection and highlighted in the Bonn Call for Action, a joint position statement by the International Atomic Energy Agency and the World Health Organization, continue to guide international best practice for the use of ionising radiation in medicine.
