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-γ-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 p_aO₂, arterial pCO₂, and pH determined with a blood gas analyzer. Results: Tumor oxygenation did not change signigicantly 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 doses above 45 Gy led to a considerable decrease of tumor oxygenation in the later phase of irradiation.