Preclinical radiobiology with pions: RBE--values for mouse skin damage as a function of single doses of pions.

The macroscopic reaction of the mouse skin was used to derive RBE values for negative pi-Mesons. Hind limbs of mice were irradiated with pions or X-rays. The pions were produced by the 590 MeV accelerator of the Schweizerisches Institut für Nuklearforschung (SIN). Early reaction was assessed over a period of 6--30 days after irradiation with single doses (20--45 Gy). The radiation damage was scored using an arbitrary scale of effect. The time pattern of development of the skin reaction and the subsequent healing after exposure both to pions and X-rays were similar, indicating that depletion and repopulation of the basal cells of the skin were comparable, both after pions and X-rays. RBE values as a function of pion doses at the peak (dose maximum), plateau and at the postpeak (12 mm downstream of the dose maximum) were computed with nonparametric statistical methods. The RBE at the peak and at the plateau relative to X-rays of the same dose rate was 1.15--1.25 and 0.85, respectively. The RBE of peak pions manifested a marked dependence on dose, when plateau pions were chosen as reference radiation. In this experiment there was no significant difference in RBE between peak and postpeak. The importance of some experimental condition (dose rate, irradiation volume) is discussed.