A method of assessing the susceptibility of a tumor to reduction in hypoxia, comprising delivering oxygen and CO2 to attain a starting end tidal concentration of oxygen (PetO2) between 350 and 450 mm Hg, and a starting end tidal concentration of CO2 (PetCO2) between 42 and 55 mm Hg； changing PetO2 and/or PetCO2, wherein at least one increment of change is maintained for a time sufficient to obtain a surrogate measure of tumor oxygenation reflecting change in tumor oxygenation relative to a previously measured surrogate value of tumor oxygenation, wherein the starting PetO2 and/or an incremental change in PetO2 is approximately between 375 and 425 mm Hg, and wherein the starting PetCO2 and/or an incremental change in PetCO2 is approximately between 42 and 50 mm Hg； and obtaining a surrogate measure of tumor oxygenation after changing PetO2 and/or PetCO2 for comparison to a previously measured surrogate value of tumor oxygenation.
