Effects of hyperbaric oxygen and normobaric carbogen on the radiation response of the rat rhabdomyosarcoma R1H

Purpose: Hypoxic tumor cells are an important factor of radioresistance. Hy perbaric oxygen (HBO) and normobaric carbogen (95% oxygen, 5% carbon dioxid e) increase the oxygen delivery to tumors. This study was performed to expl ore changes of tumor oxygenation during a course of fractionated irradiatio n and to determine the effectiveness of normobaric carbogen and HBO during the final phase of the radiation treatment. Methods and Materials: Experiments were performed on the rhabdomyosarcoma R 1H growing on WAG/Rij rats. After 20 X-ray fractions of 2 Gy within 4 weeks , oxygen partial pressure (pO(2)) was measured using the Eppendorf oxygen e lectrode under ambient conditions, with normobaric carbogen or HBO at a pre ssure of 240 kPa. Following the 4-week radiation course, a top-up dose of 1 0-50 Gy was applied in 2-10 fractions of 5 Gy with or without hyperoxygenat ion. Results: HBO but not carbogen significantly increased the median pO(2) in i rradiated tumors. The radiation doses to control 50% of tumors were 38.0 Gy , 29.5 Gy, and 25.0 Gy for air, carbogen, and HBO, respectively. Both high oxygen content gas inspirations led to significantly improved tumor respons es with oxygen enhancement ratios (OERs) of 1.3 for normobaric carbogen and 1.5 for HBO (air vs. carbogen: p = 0.044; air vs. HBO: p = 0.02; carbogen vs. HBO: p = 0.048). Conclusion: Both normobaric carbogen and HBO significantly improved the rad iation response of R1H tumors. HBO appeared to be more effective than normo baric carbogen, both with regard to tumor oxygenation and response to irrad iation. (C) 2001 Elsevier Science Inc.