Xạ trị có thể làm tổn thương não, dây cột sống và các dây thần kinh ngoại vi. Mức độ tổn thương phụ thuộc vào liều bức xạ cho mỗi phiên và tổng liều, lĩnh vực điều trị, và thời gian của các lần điều trị. Một biện pháp thực nghiệm về tiềm năng gây tổn hại của xạ trị | 490 6 Injury to the Nervous System by Specific Physical Agents rInjury Due to Ionizing Radiation General Aspects Radiation therapy can injure the brain the spinal cord and the peripheral nerves. The extent of injury depends on the radiation dose per session and the total dose the treatment field and the timing of the treatment sessions. An empirical measure of the injurious potential of radiation therapy is given by the NSD normalized standard dose which is calculated according to the following formula NSDret TD X N-0 24 xT0 11 Here the NSD is expressed in RET rad equivalent therapy TD stands for the total dose in rads N the number of individual doses and T the duration of treatment. The latency of radiation injury may be months or years depending on the NSD. Radiation Injury to the Brain Mechanism Radiation necrosis of the brain may occur with a dose of 2800 rad 28 Gy or more. As already mentioned the extent and latency of radiation injury are strongly dose-dependent. Pathological Anatomy Fibrinoid necrosis of blood vessels with extravasation of plasma and erythrocytes is accompanied by lymphocytic infiltration and massive necrosis of nervous tissue particularly in the white matter. Differential Diagnosis Because radiation therapy is usually given to treat a tumor radiation necrosis of the brain generally requires differentiation from recurrent tumor. Cerebral ischemia is sometimes the indirect result of radiation therapy when it is due to radiation-induced occlusion of large vessels such as the middle cerebral artery or the internal carotid artery in the neck. Radiation Injury to the Spinal Cord The spinal cord too like the brain can be injured by either conventional photon-beam or high-energy electron-beam radiotherapy. Signs of spinal cord dysfunction generally appear only if the dose used is 3500 rads or higher given over a period of 28 days or less. Radiation myelopathy has been described after radiotherapy for tumors of the pharynx and neck lymphoma .
