Phosphorus-31 magnetic resonance spectroscopy and blood perfusion of the RIF-1 tumor following X-irradiation.

Phosphorus-31 magnetic resonance spectra were obtained from the RIF-1 tumor in C3H mice before and up to 2 days after various doses of X rays. Parallel studies were performed to measure relative changes in tumor blood perfusion using [14C]iodo-antipyrine and changes in % tumor necrosis using Chalkley's method. Tumor ratios of phosphocreatine to inorganic phosphate (PCr/Pi) and nucleotide triphosphates to inorganic phosphate (NTP/Pi) as well as pH as measured by 31P-MRS increased significantly at most time points after irradiation with doses of 5, 10, and 20 Gy. Tumor blood perfusion was found to significantly improve after a dose of 20 Gy but not after a dose of 2 Gy. Percent tumor necrosis increased to about 3 times its control level at 1 day after a dose of 20 Gy and then declined to about twice its control value at 2 days. The magnitude of the changes in the 31P-MRS parameters makes it unlikely that any of them are entirely due to radiation-induced changes in the radiobiologically hypoxic fraction of these tumors. Changes in the necrotic fraction did not appear to influence the tumor spectra. However, the observed improvement in tumor blood perfusion may have resulted in an increase in oxidative phosphorylation of the whole tumor population as well as a clearance of inorganic phosphate and acid metabolites, so that 31P-MRS changes may indirectly reflect changes in tumor blood perfusion.