This study summary is an excerpt from the book 2 Minute Medicine’s The Classics in Medicine: Summaries of the Landmark Trials
1. In this retrospective cohort analysis of children and young adults without previous malignancy, patients exposed to multiple computed tomography (CT) scans had over 3 times the risk of developing leukemia and brain tumors.
Original Date of Publication: August 2012
Study Rundown: While the connection between radiation exposure and malignancy was previously identified, research on the subject matter drew from high-dose radiation exposure secondary to atomic bombs in Japan. At the time of its publication, this study was the first to use a cohort to examine the malignancy risk associated with low-dose radiation from CT scans. Through retrospective analysis, researchers investigated the relationship between CT-induced radiation exposure during childhood and young adulthood and development of leukemia and brain cancer, as these malignancies occur in particularly radiosensitive tissue. Children under the age of 15 who received 5-10 head CTs were exposed to around 50 milligrays (mGy) of radiation to their marrow, while 2-3 head CTs exposed these children to about 60 mGy of brain radiation. For comparison, the average chest radiograph exposes a child to 0.05 to 0.3 mGy.
Investigators reported an over 3 times increased risk of developing leukemia with exposure to ≥30 mGy and developing a brain tumor with ≥50 mGy of radiation. Limitations included using standardized CT machine settings as a proxy when assessing radiation exposure due to missing data regarding machine-specific settings. However, the use of a cohort from Great Britain’s national database ensured a large study group with low risk of loss to follow-up. This was the first cohort study to investigate the potential association between lower doses of radiation than the estimated 100 mGy of radiation exposure experienced by many Japanese atomic bomb victims. While the overall incidence of these malignancies is small within the pediatric population, this study does indicate significant risks associated with over-imaging developing children. With this in mind, the Alliance for Radiation Safety in Pediatric Imaging within the United States has started the “Image Gently” campaign, a movement designed to increase awareness of healthcare practitioners and caregivers to the benefits of reducing unnecessary radiation exposure.
In-Depth [retrospective cohort]: A total of 355 191 patients seen at a Great Britain National Health Service hospital without a history of prior malignancy, but who underwent a first CT scan between 1985-2002 at ages younger than 22, were included in the study. The study group was divided such that 178 604 patients were investigated with respect to leukemia and 176 587 patients were studied with respect to brain tumors. Malignancies investigated included acute lymphoblastic leukemia, acute myeloid leukemia, myelodysplastic syndromes (MDS), leukemia excluding MDS, gliomas, meningiomas, and schwannomas. Relative risks were calculated to assess the relationship between the radiation doses expressed in mGy units and malignancy. Data were collected from a national registry and ended on December 31, 2008, at time of death, or when patients were lost to follow-up. During analysis, radiation doses were adjusted to reflect the time it typically takes to develop a malignancy. This resulted in a 2 year lag for leukemia and a 5 year lag for brain tumors.
Among the patients included in the leukemia analysis, 283 919 CT scans were completed (64% head) and 74 patients had developed leukemia on follow-up. 279 824 scans were completed for patients included in brain tumor analysis with 135 patients ultimately receiving diagnoses of brain tumors on follow-up. Significant positive associations between leukemia and CT radiation dosing (p <0.01) and brain tumors and radiation (p <0.001) were observed. Mean radiation dose was 51.13 mGy for patients who developed leukemia and with exposure to ≥30 mGy of radiation. Among those who received ≥30 mGy, patients were more than 3 times as likely to develop leukemia when compared to those who received <5 mGy (RR 3.18; 95%CI 1.46-6.94). Mean radiation dose for patients who received ≥50 mGy and developed brain tumors was 104.16 mGy. Those who received ≥50 mGy of radiation were at greater than 3 times the risk of developing a brain tumor when compared to those who were exposed to <5 mGy (RR 3.32; 95%CI 1.84-6.42).
Pearce MS, Salotti JA, Little MP, McHugh K, Lee C, Kim KP, et al. Radiation exposure from CT scans in childhood and subsequent risk of leukaemia and brain tumors: a retrospective cohort. The Lancet. 2012 Aug 4;380(9840):499–505.
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