It May Not Be the Cause, but Study Links Childhood CT Scans to Cancers
The likelihood of malignant brain tumors increases up to fourfold, but the reason remains unclear.
Radiation exposure from CT scans in childhood is associated with an increased risk of brain tumors, according to a recent large-scale study conducted in the Netherlands. The findings, published July 18 in the Journal of the National Cancer Institute, suggest the possibility exists that relatively high radiation doses from head CT scans may increase brain tumor risk. But experts urge caution against drawing the conclusion that there is a direct cause-and-effect relationship.
CT greatly improves diagnostic capabilities and is being used more frequently among children in Western countries. But the technique delivers higher radiation doses than many other imaging procedures. In particular, children may receive higher doses than adults if the CT settings are not adjusted for their smaller body size. Children are also more susceptible to the negative health consequences. Moreover, epidemiological studies have demonstrated an increased cancer risk associated with pediatric CT scans. Research has shown there is a clear dose-response relationship for both leukemia and brain tumors: Risk increases with increasing cumulative radiation dose. For a cumulative dose of between 50 and 60 milligray or mGy -- a unit of estimated absorbed dose of ionizing radiation -- to the head, there is a threefold increase in the risk of brain tumors; the same dose to bone marrow results in a threefold increase in the risk of leukemia.
If typical current scanner settings are used for head CT in children, then two to three head CT scans would result in a dose of 50-60 mGy to the brain. The same dose to red bone marrow would be produced by five to 10 head CT scans under current scanner settings for children under age 15. However, the absolute cancer risks associated with CT scans are small -- the lifetime risks of cancer due to CT scans are about one for every 1,000 people who are scanned.
In the new study, researchers further examined this link by collecting data on 262,227 pediatric CT scans performed on 168,394 children in 42 Dutch hospitals between 1979 and 2014. Starting five years after the first CT scan, the overall cancer incidence was 1.5 times higher (454 observed cases) than expected based on data from the general Dutch population, with significant increases for malignant central nervous system tumors, nonmelanoma skin cancer, and cancers of the colon, bone, large bowel, soft tissues, and thyroid. Compared with the general population, the incidence of brain tumors was higher in children who had CT scans. Based on the findings, one excess case of brain tumors per 10,000 head CT scans is estimated to occur during the decade after the first head CT scan.
The researchers also calculated absorbed radiation doses to the bone marrow and the brain. For all brain tumors combined, and for malignant and nonmalignant brain tumors separately, relative risks increased by a factor of two to four for the highest dose category. However, the risk for leukemia was not associated with bone marrow dose.
“It is very important that physicians only use CT imaging when medically necessary for disease diagnosis and management and when no alternative options that do not use ionizing radiation, such as ultrasound or MRI, are possible,” said Diana Miglioretti, at the University of California Davis School of Medicine.
“When CT imaging is medically necessary, it is important that they child-size the dose and use a dose that is as low as possible while maintaining sufficient diagnostic quality,” added Miglioretti, who is an expert in radiation exposure from medical imaging and was not involved in the study.
But the authors note important limitations of the study. “Confounding factors such as the indication of the test may influence the a priori chance of finding an abnormality,” said study author Bernard Zonnenberg of the University Medical Center Utrecht in the Netherlands. “Such is the case in TSC [tuberous sclerosis complex] patients, and this may also apply for other diseases that were not studied.”
TSC is a rare genetic disorder characterized by the growth of numerous benign tumors in the brain and many other vital organs. The authors ruled out TSC as a confounding factor because excluding these patients did not significantly affect brain tumor risks. But they were not able to obtain individual information for other syndromes.
According to Richard Wakeford, an expert in radiation-related health effects at the University of Manchester in the U.K., the main weakness of the study was the lack of knowledge as to why the CT scans were conducted. “As with almost all studies of radiation exposure in medicine, exposure occurs because of a known disease, as in radiotherapy, or a suspected disease, as in diagnostic imaging, and this could affect the results of epidemiological studies in a number of ways, so caution is merited,” said Wakeford, who was not involved in the new study. “Doubts over interpretation will remain until the reasons for CT scans can be incorporated in the studies.”
These concerns are more relevant for brain tumors than for leukemia because CT scans are not routinely used for diagnosing leukemia. Moreover, the absence of clearly increased risks for leukemia is puzzling because this malignancy originates in the bone marrow, a tissue more radiosensitive than the brain. “One would have expected leukemia to show an increased risk with dose if a brain cancer response was found, and one has to wonder whether factors other than radiation were playing a role in the brain cancer finding,” Wakeford said. “Why were so many brain CT scans being given to patients with the highest brain doses?”
No matter what underlies the association between radiation exposure and brain tumors, experts agree on the clinical implications of the study. “Diagnostic examinations using radiation should be conducted with care, in the knowledge that they could carry with them some very small risk,” Wakeford said. “It should not dissuade people from having a CT scan, since in the vast majority of instances, not having the CT scan is much more risky than having the scan.”