RADIATION RESPONSE OF THE CENTRAL-NERVOUS-SYSTEM

This report reviews the anatomical, pathophysiological, and clinical a spects of radiation injury to the central nervous system (CNS). Despit e the lack of pathognomonic characteristics for CNS radiation lesions, demyelination and malacia are consistently the dominant morphological features of radiation myelopathy. In addition, cerebral atrophy is co mmonly observed in patients with neurological deficits related to chem otherapy and radiation, and neurocognitive deficits are associated wit h diffuse white matter changes. Clinical and experimental dose-respons e information have been evaluated and summarized into specific recomme ndations far the spinal cord and brain. The common spinal cord dose li mit of 45 Gy in 22 to 25 fractions is conservative and can be relaxed if respecting this limit materially reduces the probability of tumor c ontrol. It is suggested that the 5% incidence of radiation myelopathy probably lies between 57 and 61 Gy to the spinal cord in the absence o f dose modifying chemotherapy. A clinically detectable length effect f or the spinal cord has not been observed. The effects of chemotherapy and altered fractionation are also discussed. Brain necrosis in adults is rarely noted below 60 Gy in conventional fractionation, with imagi ng and clinical changes being observed generally only above 50 Gy. How ever, neurocognitive effects are observed at lower doses, especially i n children. A more pronounced volume effect is believed to exist in th e brain than in the spinal cord. Tumor progression may be hard to dist inguish from radiation and chemotherapy effects. Diffuse white matter injury can be attributed to radiation and associated with neurological deficits, but leukoencephalopathy is rarely observed in the absence o f chemotherapy. Subjective, objective, management, and analytic (SOMA) parameters related to radiation spinal cord and brain injury have bee n developed and presented on ordinal scales.