Oxygen distributions within R3230Ac tumors growing in dorsal flap window chambers in rats

R3230Ac mammary tumors were grown in transparent window chambers implanted into the dorsal skin flap of 250 g Fischer 344 rats (see Dewhirst et al, 1992). The oxygen pressure distributions in the tumor and host tissue were measured by the oxygen dependent quenching of phosphorescence (see Vinogradov et al, 1996) after injection of Oxyphor R2 (7 mg, 0.3 ml) into the tail vein. The oxygen pressure maps show the R3230Ac tumors to be hypoxic relative to the surrounding tissue. The excitation spectrum for the phosphor has peaks at 419 nm (blue light) and at 524 nm (green light), and the emitted phosphorescence spectrum and lifetime are independent of the wavelength at which the phosphor is excited. The absorption by tissue is much greater for blue light than green light, due to intrinsic chromophores such as cytochromes, hemoglobin, myoglobin etc. Thus, blue excitation measures the oxygen pressures in a much thinner, superficial, surface layer (< 50 microns) than does green excitation, allowing "optical sectioning" of tissue oxygenation. The tissue can be further optically sectioned by making measurements from both sides of the window. Viewed from the tumor side, the superficial layers (blue excitation) of these tumors were hypoxic whereas the host tissue was well oxygenated. The oxygen pressures in the growing edge of the tumors are lower than those in the central core of the tumor, and much lower than those of the host tissue. This result is in agreement with the micro-oxygen electrode measurements of perivascular oxygen pressures reported by Dewhirst and coworkers (1992).