THE RELATIONSHIP BETWEEN PARTIAL-PRESSURE OF OXYGEN AND PERFUSION IN 2 MURINE TUMORS AFTER X-RAY-IRRADIATION - A COMBINED GADOPENTETATE DIMEGLUMINE DYNAMIC MAGNETIC-RESONANCE-IMAGING AND IN-VIVO ELECTRON-PARAMAGNETIC-RESONANCE OXIMETRY STUDY

Changes of partial pressure of oxygen (pO(2)) and blood perfusion mere studied in MTG-B and RIF-1 tumors (n = 5 each) before and after a sin gle 20-Gy dose of X-ray irradiation. Using electron paramagnetic reson ance oximetry, we have observed an initial fast decrease of pO(2) afte r irradiation, followed by a slow increase, The time course of these c hanges was faster in the MTG-B tumors than in the RIF-1 tumors, Gadope ntetate dimeglumine (Gd-DTPA) dynamic magnetic resonance imaging studi es showed a reduction in uptake of Gd-DTPA at the time of minimum pO(2 ) and a recovery at the time of maximum pO(2) in each tumor. Previous work indicates that there is microscopic heterogeneity in tumors, with well-vascularized ''capillary regions'' being closer to capillaries t han poorly vascularized ''noncapillary regions.'' We propose a tmo-com ponent (slow and fast) model of Gd-DTPA uptake that is designed to qua ntify the kinetics of these two compartments by analyzing the total tu mor uptake kinetics without having to identify specific regions of int erest. Total perfusion in the tumors was greatly reduced at the time o f minimum oxygenation, and the volume of the slow component increased after irradiation. We conclude that a decrease in blood perfusion is o ne of the main causes of the decline in pO(2) observed after irradiati on.