Response of a poorly reoxygenating mouse osteosarcoma to X-rays and fast neutrons

Abstract

A transplantable osteosarcoma, which had previously been shown to reoxygenate its anoxic cells very slowly after a single large dose of X-ray, was studied after X- and neutron-irradiation. It was shown to be relatively resistant to fractionated X-ray therapy, probably due to anoxia. Nevertheless it could be shown that a small degree of reoxygenation must also occur in this tumour. Its response to treatment with 15 MeV neutrons was unusual; RBE values for cell survival of 10 −3 were for fractionated irradiation 2·5, for single dose in vivo irradiation 1·8, and for in vitro irradiation 1·6. The small difference between the RBE values for the well oxygenated osteosarcoma cells in vitro and for the partially hypoxic cells in vivo is associated with an unexpectedly high OER of 2·0 for neutron irradiation of these cells in vitro as compared with the OER for X-rays of 2·4. The gain factor for neutrons, that is the ratio of OER values for X-rays and neutrons, is only 1·2, indicating that although hypoxia may be a limiting factor for the cure of this tumour by fractionated X-ray theory, the use of neutrons does not offer a major advantage. The search for other models of radiotherapy-resistant tumours should be continued.