The response of C3H mammary tumours to Irradiation in single and fractionated doses

Abstract

Both spontaneous and 1st generation isotransplanted mammary tumors in C3H mice were irradiated with 14 MeV electrons. Irradiation was in 1 or 2 equal fractions, split by 24 hours, with the mice breathing either oxygen or nitrogen at atmospheric pressure during irradiation. The response of the tumors was assessed in terms of the number of volume doublings required for the irradiated tumor to re- grow to its size at time of irradiation. Using spontaneous tumors, with irradiation while the mice were breathing oxygen, a dose in 2 equal fractions was more effective at total doses above 3,000 rads than a single dose. Below this dose no difference in response was detected. For transplanted tumors in these conditions a small amount of recovery was observed at low doses but above 2,500 rads it was again found that 2 fractions were more effective. Irradiations while the mice were breathing nitrogen (i.e. severely hypoxic) were performed with spontaneous tumors in 1 or 2 fractions. The responses to the 2 treatments were almost identical and showed a protection factor of about 2. The greater effectiveness of the fractionated doses is due to an increased oxygenation of the tumor due to the previous doses of irradiation. The similarity of the 2 responses for nitrogen breathing supports this interpretation.