A method of assessing the susceptibility of a breast tumor to a reduction inhypoxia,comprising delivering controlled amounts of oxygen and carbon dioxide toattain astarting end tidal concentration of oxygen (PetO2) which lies in the range of350 to 450mm Hg and a starting end tidal concentration of carbon dioxide (PetCO2) whichlies inthe range of 42 to 55 mm of Hg: incrementally increasing or decreasing the endtidal ofat least one of oxygen and carbon dioxide wherein at least one increment ismaintainedfor at least as long as the time required to obtain a surrogate measure oftumoroxygenation reflecting at least an increase or decrease in oxygenation of thetumorrelative to a previously measured surrogate value of tumor oxygenation in atleast oneregion of the breast tumor, wherein at least one of -- the starting PetO2andanincremental increase or decrease in PetO2--, is in the range of approximately375 toapproximately 425 mm of Hg, and wherein at least one of -- the starting PetCO2and anincremental increase or decrease in PetCO2-- is in the range of approximately42 toapproximately 50 mm of Hg； and obtaining at least one surrogate measure oftumoroxygenation after incrementally increasing or decreasing the end tidal of atleast one ofoxygen and carbon dioxide for comparison to a previously measured surrogatevalue oftumor oxygenation, with respect to at least one region of the breast tumor.
