Ionising radiation in the form of x-rays and gamma rays is the only exposure which the International Agency for Research on Cancer (IARC) classifies as a cause of brain and CNS tumours. Less than 1% of brain and other CNS tumour cases in the UK are caused by ionising radiation. Evidence on the effects of ionising radiation came initially from studies of atomic bomb survivors, though more recent studies explore the effects of radiation used to diagnose and treat illness, including X-rays, CT scans and radiotherapy.
Ionising radiation overall generally appears to be more strongly associated with meningioma risk than with glioma risk. The risk of meningioma is increased by 64-510% with each Gray (Gy) of ionising radiation exposure received, data from four cohort studies showed. In one of these studies the effect was not statistically significant. Age at exposure, sex, and time since exposure does not appear to modify the effect of radiation on meningioma risk. The risk of glioma appears to be increased by 8%-56% per Gy, though the upper estimate comes from an atomic bomb survivors study and was not statistically significant. Younger age at exposure confers a stronger effect on glioma risk.
Brain tumour risk is almost tripled in people who received 1-2 CT scans (total average X-ray dose around 60 milligrays (mGy)) during childhood or adolescence, large cohort studies have shown.[4,5]
The average x-ray dose from one head CT scan received up to and including age 20 is 28-44 mGy, depending on age and sex.
People who have ever had dental X-rays taken from the sides of the head (bitewing technique) have double the risk of adult meningioma, a US case-control study showed, though over 90% of both cases and controls had received this type of X-ray.
Radiotherapy for a primary brain tumour (compared with no radiotherapy) was associated with around 55% higher risk of secondary brain tumour, in a study of US patients treated between 1973 and 2002.
People who received radiotherapy for cancer during childhood have a 14-fold higher risk of developing glioma later in life, compared with those who did not receive radiotherapy for their childhood cancer. However this finding, from a large study of British childhood cancer survivors, was not quite statistically significant. Analysing only those children whose primary cancer was in the central nervous system revealed that those treated with radiotherapy (versus those treated without) had a significantly higher risk of subsequent brain tumour.The risk of second primary brain tumour increased linearly with increasing radiotherapy dose, with this effect much stronger for meningioma than glioma.