Injury to the CNS from radiation is appropriately discussed here, since it occurs mainly in relation to therapy for brain tumors. Three syndromes of radiation damage have been delineated: acute, early delayed, and late delayed, although these syndromes often blend into one another. The acute reaction may begin during the latter part of a series of fractionated treatments or soon thereafter. There may be a seizure, a transitory worsening of the tumor symptoms, or signs of increased intracranial pressure. Although the condition has been attributed to brain edema, this is not always visible on MRI and its basis is unknown. The symptoms subside in days to weeks. Corticosteroids are usually administered, but with the exception of cases with demonstrable edema, their effect is uncertain. A novel syndrome of migraine-like headache and focal neurologic deficits, usually developing many years after cranial radiation, is described in more detail below.
The early radiation syndrome has been more troublesome in our experience than has the very acute form. As with the acute syndrome, focal tumor symptoms may increase, and as seen on MRI (Fig. 30-26), the tumor mass enlarges, raising the possibility of further tumor growth, but again the symptoms usually reflect extensive demyelination, loss of oligodendrocytes beyond the confines of the tumor, and varying degrees of tissue necrosis. The administration of dexamethasone or a similar corticosteroid may hasten resolution of this condition.
The late-delayed process is the most serious of radiation complications. Here one finds—in structures adjacent to a cerebral neoplasm, the pituitary gland, or other structures of the head and neck—necrosis of the white matter of the brain and, occasionally, of the brainstem. In some areas, the tissue undergoes softening and liquefaction, with cavitation. With lesser degrees of injury, the process is predominantly a demyelinating one, with partial preservation of axons. Later reactions are thought to be caused by diffuse vascular changes as a result of radiation exposure. Endothelial cells multiply frequently and, because ionizing radiation injures dividing cells, the vessels are most vulnerable. The result is hyaline thickening of vessels with fibrinoid necrosis and widespread microthrombosis. There is a lesser degree of damage to glial cells. Neurons are relatively resistant though they can be secondarily affected by loss of glial support as well as reduced tissue perfusion.
The symptoms of delayed injury, coming on 3 months to many years after radiation therapy, are either those of a subacutely evolving mass, difficult to separate from those of tumor growth, or of a subacute dementia. The clinical pattern varies with the site of the lesion: focal or generalized seizures, impairment of mental function, and, sometimes, increased ICP. Whole-brain radiation for metastatic tumor or acute lymphoblastic leukemia can lead to multifocal zones of necrosis and holohemispheric spongiform changes in the white matter, with diffuse cerebral atrophy and enlarged ventricles. Progressive dementia, ataxia, and urinary incontinence are the main clinical features of this state (DeAngelis et al). In its mildest form there are no radiographic changes aside from the tumor, but the patient becomes mentally dull, slightly disinhibited, and often sleepy for large parts of the day. Panhypopituitarism is another complication of whole-brain radiotherapy, particularly in children, who may also suffer growth retardation. Radiation necrosis of the spinal cord is described in Chap. 42.