Influence of hyperthermia on the oxygen enhancement ratio for X-rays, measured in vivo

The skin of mouse tail was used to study the effect of hyperthermia on the oxygen enhancement ratio (OER). Heating was by immersion of a portion of the tail in hot water. Radiation was given either immediately before or after hyperthermia. The average skin reaction between 15 and 50 days after treatment was taken as the end-point. The OER in the absence of hyperthermia was 1.77, suggesting significant hypoxia of the skin. When hyperthermia was given after irradiation the measured value for the OER was not significantly different, but with prior hyperthermia the OER was increased to an average value of 2.3. This increase in OER was probably due to a transient increase in blood circulation following hyperthermia and causing improved tissue oxygenation during irradiation. As a consequence a greater thermal enhancement ratio would be expected for heat given before irradiation than afterwards, which has frequently been observed with other normal tissues. There was no evidence that heat reduced OER, as has been reported in some studies performed on cells in vitro. (author)