1029 Molecular mechanism of short-patch repair of radiation-damaged DNA by in vitro reconstituted systems

Purpese/Objeetive: Linac based stereotactic radiosurgery (SRS) may be performed with a wide range of flexibility, and overall treatment times vary substantially between institutions with respect to the number of ares, isecenters, and couch rotations employed.At the present time protocols (RTOG 93-05) for SRS do not provide for a treatment delivery time requirement.However, variations in SRS treatment plans may not be biologically iso-cffective when a great deal of time is required for intermittent treatment delivery with a large number of arcs, isoeenters, and couch angles versus a continuous single isoeenter treatment with a gamma knife.The intent of this rescaxch is to quantify the decreased toxicity of intermittent radiation exposures and permit the delivery of a biologically iso-effective dose.Materials & Methods: Human tumor cells (U-87MG) were irradiated with various treatment schemes to simulate continuous and intermittent exposures during SRS.Cultured cells were irradiated in vitro with 6 MV x-rays to 12 Gy at a constant dose rate of 2.4 Gy/min."Arcs" were simulated by dividing the total radiation dose into equal increments of g or 16 rain, resulting in irradiation schemes with single, 2,4, 6, and 8 exposures.The relative percent survival of the different radiation dose delivery schemes was quantitated using standard clonogenie survival analyses, and the significance levels were determined using the Student's t-test.