Head CT does not increase patients’ risk of developing meningiomas, the usually benign but often slowly symptomatic brain tumors that have been suspected of forming more often in individuals exposed to concentrated doses of ionizing radiation.

That’s according to researchers at the Karolinska Institute in Sweden who reviewed the records of 18,388 subjects exposed to CT of the head alongside the records of 63,664 unexposed control subjects.

After excluding cases in which a meningioma was found on a first CT, which could not have contributed to causation, the team found no statistically significant difference in risk of meningioma between the two groups, lead author Arvid C. Nordenskjöld, MD, and colleagues report in a study published online Aug. 14 in Radiology.

Importantly, the researchers found that the meningioma risk in the head CT patients would have been overstated had they not taken into account—and excluded ineligible subjects by—referral notes and radiology reports.

Without access to this information, “we would have found a significant increase in the risk of meningioma after CT as well as a significant dose-response relationship,” they write. “This finding may be an important factor to consider when evaluating other studies of CT examinations and risk of slow-growing tumors.”

The authors note as a strength their long data collection period (1973 to 1992) and follow-up (through 2010).

They acknowledge as a limitation their lack of information on head CTs performed at sites other than the Department of Neuroradiology at Karolinska University Hospital.

“During the period in which our study subjects were exposed, however, CT machines were scarce, at least until the later part of the study,” they write. “We therefore think it is reasonable to assume that the lack of a statistically significant difference in risk of meningioma among exposed and unexposed subjects is a true finding, rather than being the effect of a high number of CT-exposed subjects in the control cohort.”