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

Abstract

Both spontaneous and first generation isotransplanted mammary tumours in C3H mice have been irradiated with 14 MeV electrons. Irradiation has been in one or two equal fractions, split by 24 hours, with the mice breathing either oxygen or nitrogen at atmospheric pressure during irradiation. The response of the tumours has been assessed in terms of the number of volume doublings required for the irradiated tumour to regrow to its size at time of irradiation. Using spontaneous tumours, with irradiation while the mice were breathing oxygen, it was found that a dose in two 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 tumours in these conditions a small amount of recovery was observed at low doses but above 2,500 rads it was again found that two fractions were more effective. Irradiations while the mice were breathing nitrogen (i.e. severely hypoxic) were performed with spontaneous tumours in one or two fractions. The responses to the two treatments were almost identical and showed a protection factor of about two. It is suggested that the greater effectiveness of the fractionated doses is due to an increased oxygenation of the tumour due to the previous doses of irradiation. The similarity of the two responses for nitrogen breathing supports this interpretation.