Tumor oxygenation in a transplanted rat rhabdomyosarcoma during fractionated irradiation.

PURPOSE

To quantify the changes in tumor oxygenation in the course of a fractionated radiation treatment extending over 4 weeks.

METHODS AND MATERIALS

Rhabdomyosarcomas R1H of the rat were irradiated with 60Co-gamma-rays with a total dose of 60 Gy, given in 20 fractions over 4 weeks. Oxygen partial pressure (pO2) in tumors was measured at weekly intervals using polarographic needle probes in combination with a microprocessor-controlled device (pO2-Histograph/KIMOC). The pO2 measurements were carried out in anesthetized animals under mechanical ventilation and in respiratory and hemodynamic steady state. Tumor pO2 values were correlated to the arterial oxygen pressure paO2, arterial pCO2, and pH determined with a blood gas analyzer.

RESULTS

Tumor oxygenation did not change significantly during the 3 weeks of irradiation (up to 45 Gy), from a median pO2 of 23 +/- 2 mmHg in untreated controls to 19 +/- 4 mmHg after the third week. The decrease of the number of pO2 values between 0 and 5 mmHg indicated that an improved oxygenation in the tumors occurred. However, with increasing radiation dose (fourth week, 60 Gy) a significant decrease in tumor oxygenation to a median pO2 of 8 +/- 2 mmHg and a rapid increase in the frequency of pO2 values (35 +/- 4%) between 0 and 5 mmHg was found.

CONCLUSION

Improved oxygenation in rhabdomyosarcomas R1H was only present in the early phase of the fractionated irradiation. Radiation does above 45 Gy led to a considerable decrease of tumor oxygenation in the later phase of irradiation.