THE PO(2) IN A MURINE TUMOR AFTER IRRADIATION - AN IN-VIVO ELECTRON-PARAMAGNETIC-RESONANCE OXIMETRY STUDY

Using electron paramagnetic resonance (EPR) oximetry with the oxygen-s ensitive paramagnetic material, fusinite, we have measured the partial pressure of oxygen (pO(2)) in the mouse mammary adenocarcinoma MTG-B. The average pO(2) in untreated tumors was low (about 5 mm Hg) and dec reased with tumor growth. Magnetic resonance imaging and histological examination were used to localize the position of the fusinite with re spect to tumor margins and vascularization. The pO(2) was generally hi gher in the periphery than in the center of the tumors, but there was considerable variation among tumors both during normal growth and afte r radiation treatment. After a single 20-Gy dose, a characteristic pat tern of change in tumor pO(2) was observed. In irradiated tumors, ther e was an initial reduction in pO(2) (minimum occurred 6 h postirradiat ion) which was followed by a transient increase in pO(2) to levels hig her than the preirradiation pO(2) (maximum occurred 48 h postirradiati on). This work demonstrates postirradiation changes in pO(2) of potent ial radiobiological significance. Compared to other oxygen assessment techniques, EPR oximetry is very useful because it can assess pO(2) in the same region of the tumor over the course of tumor growth and duri ng response to treatment. Thus EPR could be used to identify potential ly radioresistant tumors as well as to identify tumors with slow reoxy genation. (C) 1995 by Radiation Research Society