Response of avian embryonic brain to spatially segmented X-ray microbeams

Duck embryo was studied as a model for assessing the effects of microbeam r adiation therapy (MRT) on the human infant brain. Because of the high risk of radiation-induced disruption of the developmental process in the immatur e brain, conventional wide-beam radiotherapy of brain tumors is seldom carr ied out in infants under the age of three. Other types of treatment for ped iatric brain tumors are frequently ineffective. Recent findings from studie s in Grenoble on the brain of suckling rats indicate that MRT could be of b enefit for the treatment of early childhood tumors. In our studies, duck em bryos were irradiated at 3-4 days prior to hatching. Irradiation was carrie d out using a single exposure of synchrotron-generated X-raps, either in th e form of parallel microplanar beams (microbeams), or as non-segmented broa d beam. The individual microplanar beams had a width of 27 mum and height o f 11 mm, and a center-to-center spacing of 100 mum. Doses to the exposed ar eas of embryo brain were 40, 80, 160 and 450 Gy (in-slice dose) for the mic robeam, and 6, 12 and 18 Gy for the broad beam. The biological end point em ployed in the study was ataxia. This neurological symptom of radiation dama ge to the brain developed within 75 days of hatching. Histopathological ana lysis of brain tissue did not reveal any radiation induced lesions for micr obeam doses of 40-160 Gy (in-slice), although some incidences of ataxia wer e observed in that dose group. However, severe brain lesions did occur in a nimals in the 450 Gy microbeam dose groups, and mild lesions in the 18 Gy b road beam dose group. These results indicate that embryonic duck brain has an appreciably higher tolerance to the microbeam modality, as compared to t he broad beam modality. When the microbeam dose was normalized to the full volume of the irradiated tissue, i.e., the dose averaged over microbeams an d the space between the microbeams, brain tolerance was estimated to be abo ut three times higher to microbeam irradiation as compared with broad beam irradiation.