Measurement of tumor oxygenation.

There is now clear evidence that the oxygenation status of cells in tumours can influence the response of those tumours to therapy. The classic example is for radiation, to which oxygen deficient hypoxic cells are more resistant than cells under well oxygenated conditions (1). Most solid animal tumours have been shown to contain such resistant cells in different proportions (2) and it is a major factor that influences the overall response of the tumour to radiation. There is even evidence from clinical studies indicating the presence of such cells in certain types of human tumours, and that by eliminating these cells significant improvements in radiation response can be obtained (3). Tumour oxygenation status has also been shown to be an important factor in the response of tumours to certain chemotherapeutic agents, cytokines, hyperthermia, and photodynamic therapy (4). More recent studies are now providing evidence that tumour oxygenation, especially hypoxia, may also influence malignant progression through effects on signal transduction pathways and the regulation and transcription of various genes (5, 6). It naturally follows that if one could accurately measure the oxygenation status of individual tumours one should be able to better predict treatment outcome and select appropriate therapies to improve it. In this review we will define the parameters of interest, discuss the most clinically applicable methods for measuring these parameters, and make some suggestions as to how future studies in this area should proceed.