Tumors commonly exhibit pronounced inter-individual and intra-individual differences in the oxygenation status. Paramount factors contributing to this variability are the tumor growth stage or size, changes in the O2 transport capacity of the arterial blood (e.g., during tumor-induced anemia) as well as inherent properties of the tumor cell line investigated and different tumor growth sites as shown in the present study. In most cases, variations of nutritive tumor blood flow and changes in the amount of arterio-venous shunt perfusion within the tumor tissue have to be considered as basic pathomechanisms for these inter-tumor variations of tissue oxygenation.