CHANGES OF OXYGEN-TENSION IN EXPERIMENTAL-TUMORS AFTER A SINGLE-DOSE OF X-RAY-IRRADIATION

Electron paramagnetic resonance oximetry was used to measure the parti al pressure of oxygen (pO(2)) in two types of tumor in vivo in C3H/HeJ mice. The pO(2) in MTG-B (high hypoxic fraction) and RIF-1 (low hypox ic fraction) tumors was monitored prior to and at several time points after a single dose of X-ray irradiation (up to 7 days after treatment ). Initial values of pO(2) in RIF-1 (8.7 +/- 1.1 mm Hg; n = 14) were h igher than that of pO(2) in MTG-B (3.3 +/- 0.5 mm Hg; n = 19), The pO( 2) in both types of unirradiated tumors decreased slowly with tumor gr owth. Irradiation of tumors had a two-phase effect on pO(2): an initia l sharp decrease in pO(2), followed by slow reoxygenation, After a 20- Gy radiation dose, the pO(2) was 2.2 +/- 0.5 mm Hg at 6 h [significant ly lower (P < 0.0001) than in control] and 3.2 +/- 0.5 mm Hg at 48 h [ significantly higher (P < 0.02) than in control] in MTG-B, and 5.4 +/- 1.2 mm Hg at 24 h and 8.2 +/- 1.0 mm Hg at 72 h in RIF-1. The time co urse for these changes in pO(2) was found to be independent of the dos es in use in this study (10, 20, and 40 Gy). The occurrence of radiati on-induced changes in pO(2) and the different time courses of these ch anges suggest that repeated monitoring of pO(2) in tumors during treat ment could be used to enhance the efficacy of clinical treatments.