Effect of low doses of ionising radiation in infancy on cognitive function in adulthood: Swedish population based cohort study

Ionising radiation may impair the developing human brain and adversely affect cognitive processes, as documented among children exposed in utero after the bombing of Hiroshima and Nagasaki or treated with radiation for ringworm of the scalp. Evidence also comes from children treated for leukaemia or brain tumours; but in these studies it is difficult to distinguish the effect of radiation from that of underlying diseases or treatment. Existing data are based mostly on high doses of ionising radiation ( 1 Gy), therefore little is known about the effects of low doses of radiation or a possible threshold value and the effect of age at exposure. Computed tomography, which delivers high doses of ionising radiation, is increasingly being used in even young children after minor head trauma. The dose delivered to the brain of an infant during computed tomography of the skull is around 120 mGy.

Researchers analysed cognitive function in a large population based cohort of men at the time of military enlistment who had received low dose ionising radiation for cutaneous haemangioma before age 18 months. Based on previous experience, they hypothesised that damage to the frontal part of the brain would have a more severe effect on mental capacity than damage to the posterior part. During 1930-59, 4577 boys aged under 18 months received radiotherapy for cutaneous haemangioma. After exclusions 2211 men were included in the analysis of cognitive function and 2551 in the analysis of high school attendance.A decrease in high school attendance was seen in all socioeconomic groups at radiation doses greater than 100 mGy when compared with the lowest dose of 1-20 mGy. No consistent difference was seen between the two lowest dose categories (1-20 mGy and > 20-100 mGy); however, the increment of exposure was limited, with median values of only 0 and 30-40 mGy. Reasearchers found an overall statistically significant decreasing probability of attending high school in relation to radiation dose. Adjustment for potential confounders changed risk estimates only marginally. The relative decrease in high school attendance was greater for sons of low level white collar workers. The dose-response relations in the three socioeconomic classes, however, were not significantly different from each other when dose was used as a continuous variable. Irradiation of the brain with dose levels overlapping those imparted by computed tomography can, in at least some instances, adversely affect intellectual development. Although formal diagnostic protocols do not advocate computed tomography in cases of minor head injuries, clinical practice dictated by legal and financial considerations does not always adhere to these protocols. The risk and benefits of computed tomography scans in minor head trauma need re-evaluating. (Source: BMJ 2004;328:19 (3 January), doi:10.1136/bmj.328.7430.19)