The relationship between o.e.r., r.b.e. and number of fractions for X-ray- or neutron-induced skin damage

The skin reactions in aerated and hypoxic mouse tails after single or fractionated doses of 250 kV X-rays of fast neutrons (6 MeV deuterons on beryllium) were measured. The oxygen enhancement ratio (o.e.r.) for one to sixteen fractions of X-rays remained constant, while that for one to ten fractions of neutrons decreased with increasing neutron fractionation and decreasing neutron dose/fraction. The o.e.r. for X-rays was 1.7, for single neutron doses 1.4, and for ten fractions of neutrons 1.25. It was anticipated that the o.e.r. for neutron-induced damage would decrease further as neutron fractionation was increased because the contribution to damage from the highest LET components of dose, the α and heavy recoil particles, would increase relative to the lower LET components. The relative biological effectiveness (r.b.e.) values obtained for skin damage were higher at all neutron doses/fractions examined in this study on tails than all those previously obtained in studied on skin at other sites on four species. This may be due to the influence of hypoxia on the r.b.e. measurements in the mouse tail. (author)