Effects of irradiation by single or multiple fractions per day on transplantable murine mammary carcinoma.

Effects of different fractionation schedules of irradiation were studied in an experimental mammary adenocarcinoma transplanted in the back of mice. The number of fractions per day varied from one to three, keeping the daily dose constant. It was found that an increase of the number of fractions per day did not necessarily lead to a decrease in the tumour response, as might be expected for the same total dose. At a twelve-day overall time, three fractions per day did not give such a good result as one fraction per day, but at a nineteen-day overall time three fractions per day were about as good as one fraction per day; and below 45 Gy total dose, the result with three fractions per day even seemed to be better. The effects of actinomycin D administered during the irradiation treatment show that schedules with three fractions per day possibly take better advantage of repair of sublethal and potentially lethal damage than schedules with one fraction per day. Administration of misonidazole during the irradiation treatment led to a dose-modifying effect of 1.2 in one fraction per day schedules but had no significant effect in three fractions per day schedules. The present results may provide guidelines for clinical application of irradiation schedules with more than one fraction per day.