Repair in mouse lung for up to 20 fractions of X rays or neutrons.

Local irradiation of the mouse thorax followed by the measurement of lung damage up to 17 months after irradiation has been carried out with up to 20 fractions of 3 MeV neutrons or of 240 kV X rays. Doses per fraction down to 0.28 and 1.5 Gy respectively were used. Repair capacity and RBE values were assessed by measuring breathing rate and lethality at monthly intervals up to 17 months. Only a small sparing of neutron damage was found. Sparing with X rays continued to increase as the size of each fraction was decreased, and was the main influence on the RBE values. The single-dose RBE was approximately 1.8, increasing to approximately 5 at the lowest dose per fraction measured. Dose-response curves derived for each fraction were well fitted by the formula alpha d + beta d2 where the repair parameter alpha/beta has values of 2-4 Gy after X irradiation. A slight fall of alpha/beta with time after X irradiation was observed, from about 4 Gy for pneumonitis to about 2 Gy for late fibrosis. This was significant for lethality but not for the increase of breathing rate. With neutrons the value of alpha was much higher than with X rays and a trend of increasing value of alpha at later times after irradiation was seen. Use of the linear quadratic dose-response formula predicts a continuing increase in the sparing of X-ray damage in lung as doses per fraction are decreased below those used here, and a limiting low-dose RBE of about 7.