Proceedings of the 41st Annual ASTRO Meeting 285 20 13 IDENTIFICATION OF PATIENTS WITH INCREASED SENSITIVITY TOWARDS IONIZING RADIATION - WHICH TEST IS RELIABLE? Distel LV, Neubauer S, Grabenbauer GG, Sauer R University, Erlangen, Germany Purpose: During radiotherapy (RT) of cancer, about 1% of patients suffer from severe side effects caused by ionizing radiation. Increased radiosensitivity could be due to a genetic background. Therefore it would be of great interest, to detect these patients before starting RT. Intrinsic radiosensitivity is based on several different cellular functions such as DNA repair, homologous recombination, cell cycle control etc. It seems very unlikely that an increased intrinsic radiosensitivity could be detected by a single method. Till now, we have been using four different methods detecting non-repaired DNA double-strand breaks (dsb), chromosomal aberrations, cell cycle control and clonogenity. The reason looking for the non-repaired DNA dsb’s was to check the repair capacity of the cells, for the cell cycle control was to check the proper functioning of the cell cycle checkpoints. Chromosomal aberrations were investigated to check the correctness of the repair processes and recombination mechanisms. A clonogenity assay should show the entire ability of the cells to handle the induced damages. It was already proven, that detecting genetically based radiosensitivity is possible by the analysis of chromosomal aberrations and clonogenity of the cells. Because there may be cells which have an elevated radiosensitivity, which is not shown by the two established methods, our intend was now, to find out if detecting additional defects of these cells by other methods would be possible. Twenty patients were screened, and five of them had a genetic disorder related to elevated radiosensitivity. Materials & Methods: Twenty patients were selected with the assumption to have an elevated risk of increased sensitivity to ionizing radiation. Five patients had a clinical disorder known to be associated with elevated radiosensitivity. One patient had a hereditary severe chronic immunodefiency syndrom (defect in homologous recombination), one had a Gorlin Goltz syndrome (defect in cell cycle control), one had a homozygous Nijmegen breakage syndrom (NBS) (defect in DNA dsb repair) and his two parents had a heterozygous NBS. Biopsies from the skin and peripheral blood lymphocytes, each taken from the same cancer patient, were cultured and compared with a control group. Fluorescence in situ hybridization (FISH): Peripheral blood samples were irradiated in vitro with 0.7 and 2 Gy. A three color FISH-painting technique was performed to estimate the cytogenetic damage. Clonogenic assay: A clonogenic assay with fibroblasts was done in the dose range from 0 to 10 Gy. Non-repaired DNA double-strand breaks: Cells were irradiated with doses up to 150 Gy. The repair time for leukocytes was 8 h and for fibroblasts 24 hours. Cell-cycle control: The correct function of the GUS-Phase checkpoint and the G2 checkpoint were monitored by feulgen image cytometry. Results: Lymphocytes of the five patients with genetic disorders and lymphocytes of two from the remaining fifteen patients showed clearly elevated break rates of chromosomes. The same patients were identified by the clonogenic assay. Lymphocytes of the two heterozygous NBS patients showed no elevated numbers of non-repaired DNA dsb’s. One of the two patients with higher chromosomal break rates and an additional person had an increased number of non-repaired breaks. Fibroblasts of all patients showed Gl and G2 blocks, although six patients showed a distinct weakened Gl block. Twelve patients had no detectable abnormalities. Conclusion: Chromosomal aberrations and survival of fibroblasts seem to provide corresponding results. The non-repaired DNA dsb method could not detect all patients indicated by the FISH and clonogenic assay. However, an additional person was found. To assess the value of these findings, we will have to wait for a clinical outcome of the therapy related side effects. To get more reliable results with the cell-cycle control method, the time course of the cell cycle checkpoints should be taken into consideration. 20 14 NUCLEAR SCINTIGRAPHIC ASSESSMENT OF RADIATION-INDUCED INTESTINAL DYSFUNCTION Kirichenko AV’, Rich TA’, Teates CD”, Straume M3, Mason KA4 University of Virginia Health Sciences Center, Departments of Therapeutic Radiology and Oncology’; Radiology’; Center for Biological Timing and Department of Internal Medicine, Charlottesville, VA3; University of Texas M.D. Anderson Cancer Center, Department of Experimental Radiation Oncology, Houston, TX, USA4 Purpose: Changes in the intestinal absorption occur after irradiation because of loss of the intestinal absorptive surface and the consequent decreased active transport. In our study the jejunal absorption of 99m-Technetium-pertechnetate, an actively transported y-emitter, was assessed in C3WKam mice given total body irradiation of 4, 6, 8, and 12.5 Gy, single 9-Amin- ocamptothecin (9.AC) intramuscular injection or combination of both agents, and correlated with morphological changes in the intestinal epithelium. Materials and Methods: 99m-Technetium-pertechnetate absorption from the intestinal lumen into the circulation was studied with a dynamic gamma-scintigraphy assay combined with a multichannel analyzer to record the radiometry data automatically in a time-dependent regimen. The resulting radioactivity/time curves obtained from irradiated animals were compared to the control animals. Results: A dose-dependent decrease in absorptive function was observed 3.5 days following irradiation. Absorption of crater infused into the jejunal lumen was slowed in a radiation-dose-dependent manner. The mean absorption rate was reduced to 78.8 i 9.3% of control in response to a sublethal 4-Gy total body irradiation (mean ? SEM absorption lifetime 237 + 23 s versus 187 ? 12 s in non-irradiated controls) and to 28.3 i 3.7% in response to 12.5 Gy (660 i 76 s). By 3.5 days the 99m-Technetium-pertechnetate absorption rate was significantly delayed by single sublethal 2 mg/kg 9.AC injection. Combi- nation of 4 TBI with 2 mg/kg 9-AC injection decreased the 99m-Technetium-pertechnetate jejunal absorption in an additive fashion. Morphological evaluation of mouse jejunum showed significant dose-dependent correlation between changes in the absorption function, number of cells lost per villus and percentage of apoptotic cells. In contrast classical jejunal micro colony assay failed to show any difference in jejunal crypt survival between the control group and mice treated with 4, 6 or 8 Gy irradiation alone, 9-AC alone or 9-AC + irradiation.