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Little is known of the histological effects of radiation on low grade astrocytomas, whereas the clinical efficacy of radiotherapy on these tumors is still a matter of discussion. Since it is difficult to have large series of autoptic brains harboring astrocytomas, the wide astrocytomatous areas of conventionally irradiated secondary glioblastomas have been studied in whole mount preparations. It was assumed that these areas were already present at the time of irradiation. In these areas no histologic changes referable to ‘short-term’ effect of radiation have been found. When the astrocytomatous areas were located in the white matter, they were affected by chronic edema, which usually occurs in perighoblastomatous tissue. In one case necrotic foci in the astrocytomatous areas were interpreted as anaplastic foci damaged by radiation; radiotherapy of astrocytomas might therefore result in the eradication of undetected anaplastic foci.