Ionizing radiation is an established risk factor for brain tumors, yet 
quantitative information on the long-term risk of different types of brain 
tumors is sparse. Our aims were to assess the risk of radiation-induced 
malignant brain tumors and benign meningiomas after childhood exposure and to 
investigate the role of potential modifiers of that risk. The study population 
included 10,834 individuals who were treated for tinea capitis with X rays in 
the 1950s and two matched nonirradiated groups, comprising population and 
sibling comparison groups. The mean estimated radiation dose to the brain was 
1.5 Gy. Survival analysis using Poisson regression was performed to estimate the 
excess relative and absolute risks (ERR, EAR) for brain tumors. After a median 
follow-up of 40 years, an ERR/Gy of 4.63 and 1.98 (95% CI = 2.43-9.12 and 
0.73-4.69) and an EAR/Gy per 10(4) PY of 0.48 and 0.31 (95% CI = 0.28-0.73 and 
0.12-0.53) were observed for benign meningiomas and malignant brain tumors, 
respectively. The risk of both types of tumors was positively associated with 
dose. The estimated ERR/Gy for malignant brain tumors decreased with increasing 
age at irradiation from 3.56 to 0.47 (P = 0.037), while no trend with age was 
seen for benign meningiomas. The ERR for both types of tumor remains elevated at 
30-plus years after exposure.