Repair in mouse lung of multifraction X rays and neutrons: extension to 40 fractions

We have extended our previous multiple irradiations of mouse lung from 20 to 40 fractions of both X-ray and neutron radiation in order to test whether the repair parameters previously derived will hold for lower doses per fraction, down to 1.1 Gy of X rays and 0.18 Gy of 3 MeV neutrons per fraction. Repair parameters were calculated from measurements of breathing rate and lethality at monthly intervals up to 17 months after irradiation with 1, 10, 20 or 40 equal fractions. Sparing of neutron damage was negligible when the neutron dose was divided into multiple fractions, but progressively greater repair of lung damage was seen after increasing numbers of X-ray fractions. A significant increase in the iso-effect dose for 40 fractions of X rays was found compared with 20 fractions, even when two fractions per day were given at intervals of about 6 hours, as was the case in the 40 fraction experiment. The data were well fitted by the linear quadratic formula for response vs. dose per fraction and the ratio alpha/beta yielded values of approximately 3 Gy after X rays and 30 to 40 Gy after neutron irradiation; these values are not different from alpha/beta ratios found for up to 20 fractions. The single dose RBE was less than 2, increasing to about 6 at the lowest dose per fraction measured, in agreement with previous results. The ratio of the alpha component for neutrons to that for X rays was about 8, which is therefore the limiting RBE predicted for infinitely small doses per fraction.