Late changes present from several months to several years after radiation and may also develop following radiation to neighboring extracranial malignancies. They may progress and produce a mass effect that clinically and radiologically mimics tumor recurrence. In adults, the encephalopathy presents with cognitive decline and focal neurologic deficits. In children, late sequelae are learning difficulties, subnormal IQ, endocrine dysfunction, and vascular insults.
Gross pathology is typically confined to the cerebral hemispheric white matter and the periventricular zones. The affected regions are soft and granular and contain small cavities, fluid-filled cysts, and dusky areas of old and fresh hemorrhages. The cortex and arcuate fibers are usually spared.
The histology is characterized by extensive coagula-tive necrosis, demyelination, and axonal losses. Prominent vascular changes include fibrinoid necrosis, hyalinization, fibrosis, and thrombotic occlusions. Peri-vascular mononuclear infiltrates are usually not conspicuous. Hypertrophied and bizarrely-shaped astrocytes are present in the demyelinated areas. Oligodendroglial cells are variably lost. The cerebral cortex may show neuronal losses and vascular changes, and the cerebellar cortex may show Purkinje and granule cell losses (Fig. 11.47).
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