The radiosensitivity of some tumors may depend upon the sensitivity of their micro-vasculature to radiation. Heretofore, the dose-response of human endothelial cells replicating in tissue culture has not been published. In studies reported here, we exposed flasks containing 4 to 7× 104 genetically identical human endothelial cells to doses of X irradiation from 125 to 1000 rad. During the phase of logarithmic growth, cell counts were compared to those of an unirradiated control to construct a dose-response curve. Similar studies were performed with normal fibroblasts. We found that 160 rad suppressed endothelial cell replication by 37%. Although recovery was evident with doses of 500 rad, no net increase in cell number occurred in 3 weeks in flasks of endothelial cells that received 750 or 1000 rad. Fibroblasts were slightly less sensitive under these conditions. To our knowledge, this is the first report of a radiation dose-response curve for human endothelial cells replicating in culture.

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