BACKGROUND: The response of an experimental tumor to hyperfractionated irradiation with different time intervals between the two daily fractions has been investigated. MATERIAL AND METHODS: Tumors were exposed to irradiation five days per week over six weeks. A standard treatment of 30 fractions, i.e. one fraction per day (200 kVp X-rays) was compared with a hyperfractionated schedule of 60 fractions, i.e. two fractions per day, with time intervals of either one, two, three, five or six hours between the two daily fractions. RESULTS: Compared with standard treatment a significant reduction (p <0.005) of net growth delay was observed for the tumors treated with two daily fractions separated by two hours. However, at a time interval of five and six hours between the two daily fractions net growth delay increased considerably (p <0.0001 and p <0.01) as compared with the standard treatment. CONCLUSION: Our results indicate the importance of the time interval between the daily fractions in hyperfractionated and accelerated radiotherapy.
