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 an d Materials: Rhabdomyosarcomas R1H of the rat were irradiated with Co- 60-gamma-rays with a total dose of 60 Gy, given in 20 fractions over 4 weeks, Oxygen partial pressure (pO(2)) in tumors was measured at week ly intervals using polarographic needle probes in combination with a m icroprocessor-controlled device (pO(2)-Histograph/KIMOC). The pO(2) me asurements were carried out in anesthetized animals under mechanical v entilation and in respiratory and hemodynamic steady state. Tumor pO(2 ) values were correlated to the arterial oxygen pressure p(a)O(2), art erial pCO(2), and pH determined with a blood gas analyzer. Results: Tu mor oxygenation did not change significantly during the 3 weeks of irr adiation (up to 45 Gy), from a median pO(2) of 23 +/- 2 mmHg in untrea ted controls to 19 +/- 4 mmag after the third week. The decrease of th e number of pO(2) values between O and 5 mmHg indicated that an improv ed oxygenation in the tumors occurred. However, with increasing radiat ion dose (fourth week, 60 Gy) a significant decrease in tumor oxygenat ion to a median pO(2) of 8 +/- 2 mmHg and a rapid increase in the freq uency of pO(2) values (35 +/- 4%) between O and 5 mmHg was found. Conc lusion: Improved oxygenation in rhabdomyosarcomas R1H was only present in the early phase of the fractionated irradiation. Radiation doses a bove 45 Gy lied to a considerable decrease of tumor oxygenation in the later phase of irradiation.