The effect of chronic reductions in the arterial partial pressure of oxygen on the radiation response of an experimental tumour.

A previous study by the same authors has reported the effect of acute reductions in the arterial partial pressure of oxygen (PaO2) on tumour response to radiation. The results have been extended in the present paper to investigate tumour response to radiation in animals in which the PaO2 is chronically reduced. The purpose of these experiments was to simulate the condition of cancer patients undergoing radiotherapy in the presence of chronically low PaO2 values as might be expected in patients with chronic respiratory disease. Mice bearing transplantable KHT sarcomas were kept in a 12% O2 environment prior to (10--16 days), during and following the radiation treatment of their tumours. During the period of low PaO2 (about 50 mm Hg) exposure, the mice were found to increase their haemoglobin (Hg) levels by approximately 50%. Because of this increase, the response, determined using a growth delay assay of the tumours irradiated at reduced PaO2 was found to be the same as that observed for tumours in mice breathing air throughout the experiment. In mice with reduced PaO2 levels maintained at normal Hb concentrations by periodic bleeding, tumour response was found to be similar to that of mice with acute PaO2 reductions. These results indicate that chronic PaO2 reductions in the absence of Hb compensation may have a detrimental effect on the success of a radiation treatment.