Using electron paramagnetic resonance (EPR) oximetry with the
oxygen-sensitive paramagnetic material,
fusinite, we have measured the partial pressure of
oxygen (pO2) in the mouse mammary
adenocarcinoma MTG-B. The average pO2 in untreated
tumors was low (about 5 mm Hg) and decreased with
tumor growth. Magnetic resonance imaging and histological examination were used to localize the position of the
fusinite with respect to
tumor margins and vascularization. The pO2 was generally higher in the periphery than in the center of the
tumors, but there was considerable variation among
tumors both during normal growth and after
radiation treatment. After a single 20-Gy dose, a characteristic pattern of change in
tumor pO2 was observed. In irradiated
tumors, there was an initial reduction in pO2 (minimum occurred 6 h postirradiation) which was followed by a transient increase in pO2 to levels higher than the preirradiation pO2 (maximum occurred 48 h postirradiation). This work demonstrates postirradiation changes in pO2 of potential radiobiological significance. Compared to other
oxygen assessment techniques, EPR oximetry is very useful because it can assess pO2 in the same region of the
tumor over the course of
tumor growth and during response to treatment. Thus EPR could be used to identify potentially radioresistant
tumors as well as to identify
tumors with slow reoxygenation.