In the production of radiation necrosis, the total and fractional doses of radiation and the time over which treatment is administered are obviously important factors, but the exact amounts that produce such damage cannot be stated. Accepted levels of large-field radiation are tolerated in amounts approaching 6,000 cGy, provided it is given in small daily doses (200 to 300 cGy) 5 days per week over a period of 6 weeks. Other factors, still undefined, must play a part, as similar courses of radiation treatment may damage one patient and leave another unaffected. The severe necrotizing encephalopathy that has followed the combined use of methotrexate (intrathecally but also intravenously) was discussed earlier, under “Involvement of the Nervous System in Leukemia,” the condition in which it was first described and formerly was most prevalent.
CT and MRI show a contrast-enhancing lesion, and by angiography there is an avascular mass. Small calcifications may appear many years after the radiation. MRI is somewhat more sensitive in distinguishing radiation necrosis from tumor and peritumor products, but PET is the most reliable way of differentiating the two, perhaps obviating the need for biopsy (Glantz et al, 1991). Single-photon emission tomography (SPECT) can be useful for this purpose as well (Carvalho et al). CT or MRI perfusion imaging can also be used to differentiate radiation necrosis from tumor progression; cerebral blood volume is reduced in the former and most often elevated in the latter.
Treatment has consisted of the administration of corticosteroids, which may cause regression of symptoms and of edema surrounding the lesion. Very high doses may be necessary, 40 mg or more of dexamethasone or its equivalent daily. Rarely, surgical resection of an edematous necrotic mass is attempted; this would only be considered to abrogate severe mass effect or herniation.
Stroke-like migraine attacks after radiation therapy (SMART) syndrome A difficult to classify migraine-like syndrome following cranial irradiation has been described by Partap and colleagues and by Pruitt and coworkers. This syndrome has been given the acronym SMART, for “stroke-like migraine attacks after radiation therapy.” The typical case is of a young adult who, years or decades after receiving radiation as a child for an intracranial neoplasm, develops episodes of severe headache and simultaneous symptoms such as aphasia, hemiparesis, or hemianopia, sometimes lasting days. There may be an independent headache suggestive of migraine. The syndrome can occur in the absence of an intracranial malignancy supporting the notion that the syndrome is the result of radiation exposure alone.
In many cases there is diffuse gyriform enhancement and edema over a large region of cortex spanning several arterial territories (Fig. 30-27). Perfusion imaging studies have shown that a transient period of regional hyperperfusion precedes these imaging abnormalities as well as epiphenomena such as seizures (Olsen et al). This suggests that radiation-induced endothelial injury leading to impairment of cerebrovascular autoregulation is the underlying pathology. In the early phase of the disorder, headache and seizures may not be present. The clinical symptoms and imaging abnormalities typically resolve over the course of weeks to a month or two. SMART syndrome can recur, and different parts of the cortex may be affected during different episodes. Corticosteroids seem helpful in some cases.
It is also known that tumors, usually sarcomas, can be induced by radiation, as mentioned earlier (Cavin et al). While well documented, this occurs rarely and only after an interval of many years. We have seen two cases of fibrosarcoma of the brachial plexus region in the radiation field for breast tumors (Gorson et al). These lesions appeared more than 10 years after the initial treatment, and many cases of even longer latency are on record.