Previous Studies have demonstrated that X-ray irradiation affects angiogenesis ill tumors. Here, we Studied the effects of gamma-ray irradiation on boron-10 compound accumulation in a murine tumor model. The Mouse squamous cell carcinoma was irradiated with gamma-ray before BSH ((10)B-enriched borocaptate sodium) administration. Then, the boron-10 concentrations in tumor and normal muscle tissues were measured by prompt gamma-ray spectrometry (PGA). A tumor blood flow assay was performed, and cell killing effects of neutron irradiation with various combinations of BSH and gamma-rays were also examined. BSH concentrations of tumor tissues were 16.1 +/- 0.6 mu g/g, 16.7 +/- 0.5 mu g/g and 17.8 +/- 0.5 mu g/g at 72 hours after gamma-ray irradiation at closes of 5, 10, and 20 Gy, compared with 13.1 +/- 0.5 mu g/g in unirradiated tumor tissues. The enhancing inhibition of colony formation by neutron irradiation with BSH was also found after gamma-ray irradiation. In addition, increasing Hoechst 33342 perfusion was also Observed. In this Study, we demonstrated that gamma-ray irradiation enhances BSH accumulation ill tumors. The present results suggest that the enhancement of (10)B concentration that occurs after gamma-ray irradiation may be due to the changes ill the extracellular microenvironment, including in tumor vessels. induced by gamma-ray irradiation.