The risk of developing a tumor of the nervous system in humans is analysed in 
several studies of populations, exposed to ionising radiation for medical 
reasons, or exposed to military or occupational radiation. The main data come 
from series of patients who underwent radiotherapy during childhood: a high 
incidence of tumors of the nervous system is found after irradiation of one to a 
few grays as treatment of a benign disease (especially tinea capitis), as well 
as after irradiation at higher doses of a few tens of grays for the treatment of 
cancer (in particular cerebral irradiation in acute lymphoblastic leukaemia). 
The type of radiation-induced tumors is variable, but meningioma is more 
frequent after low doses and glioma and sarcoma after higher doses used in the 
treatment of neoplastic diseases. A dose-effect relationship appeared between 
the risk of tumor of the nervous system and the radiation dose. The risk was 
higher when radiation was delivered at a younger age. Much less data are 
available after radiotherapy in the adulthood, but an increased risk of cerebral 
tumor appears in the series of ankylosing spondylitis patients. As for the 
exposures to radiodiagnosis exams, the main problem is the risk of cerebral 
tumor in children whose mother has undergone abdominal or pelvic X-rays during 
pregnancy. No risk of neurologic tumor was found in the A-bomb survivors 
irradiated at Hiroshima and Nagasaki. Occupational exposure to ionising 
radiation has been incriminated in the first radiologists exposed to high doses. 
In nuclear industry workers, the results of epidemiological studies are 
contradictory and at the present time it is not possible to link their 
radiologic exposure with a risk of tumor of the nervous system. In populations 
living near nuclear plants, mortality due to tumors of the nervous system was 
not increased.