The RBE of euoxic and hypoxic Ehrlich ascites tumour cells irradiated with negative pions in vitro and in vivo.

Abstract

Ehrlich ascites tumour cells (EAT-F5) were irradiated in vitro with the sparsely ionizing "plateau pions" and with the more densely ionizing "peak pions". From cell survival curves obtained under euoxic and hypoxic conditions, the RBE for the production of irreparable lethal and potentially lethal damage was derived. Cells of the same strain were used to produce solid tumours in the leg muscle of mice. Survival of these cells irradiated in vivo was compared with the corresponding in vitro data. It can be concluded that a large fraction of hypoxic cells is present in these tumours and that repair takes place in vivo at least as much as under the in vitro conditions. These solid tumours were irradiated with 5 dose fractions of fast protons as reference radiation, and the probability pT of tumour destruction was determined as a function of dose. Irradiation with a spot of "plateau pions" indicated the same pT as a function of dose, but side effects in the intestinal tract increased also with dose, leading to the death of the animals. The probability for avoiding these side effects in the animals, pS, was also determined. Using both values, pT and pS, the probability for local tumour control was estimated. Irradiation with "peak pions" resulted in an RBE of about 2.5 for tumour destruction corresponding to the RBE found in vitro for the production of irreparable lethal lesions in hypoxic cells. Side effects in the mouse gut showed a RBE of 2.0, corresponding to irreparable lethal lesions in euoxic in vitro cells.