SUBACUTE NEUROPATHOLOGICAL EFFECTS OF MICROPLANAR BEAMS OF X-RAYS FROM A SYNCHROTRON WIGGLER

Microplanar beam radiation therapy has been proposed to treat brain tu mors by using a series of rapid exposures to an array of parallel x-ra y beams, each beam having uniform microscopic thickness and macroscopi c breadth (i.e., microplanar). Thirty-six rats were exposed head-on ei ther to an upright 4 mm-high, 20- or 37-mu m-wide beam or to a horizon tal 7-mm-wide, 42-mu m-high beam of mostly 32- to 126-keV, minimally d ivergent x-rays from the X17 wiggler at the National Synchrotron Light Source at Brookhaven National Laboratory. Parallel slices of the head , separated at either 75 or 200 mu m on center, were exposed sequentia lly at 310-650 grays (Gy) per second until each skin-entrance absorbed dose reached 312, 625, 1250, 2500, 5000, or 10,000 Gy. The rats were euthanized 2 weeks or 1 month later, Two rats with 10,000-Gy-entrance slices developed brain tissue necrosis, All the other 10,000- and 5000 -Gy-entrance slices and some of the 2500- and 1250-Gy-entrance slices showed loss of neuronal and astrocytic nuclei and their perikarya. No other kind of brain damage was evident histologically in any rat with entrance absorbed doses less than or equal to 5000 Gy, Brain tissues i n and between all the 312- and 625-Gy-entrance slices appeared normal. This unusual resistance to necrosis is central to the rationale of mi croplanar beam radiation therapy for brain tumors.