The relative biological effectiveness of fractionated doses of fast neutrons (42 MeV d→Be) for normal tissues in the pig: Pt. 1

The effects of fractionated doses of fast neutrons (42 MeV d→Be) on the early epithelial and later dermal response of pig skin have been assessed and compared with those after X irradiation. For early epithelial reaction, i.e. moist desquamation, the RBE of the neutron beam increased with decreasing size of the X-ray dose/fraction. There was an experimentally observed upper RBE value of ∼ 2.75 for X-ray doses/fraction of between 2 and 5 Gy. For the late reaction of ischaemic dermal necrosis the RBE was > 3.0 for X-ray doses/fraction of < 3 Gy and, based on assumptions made in the linear-quadratic model of cell survival, an upper limiting RBE of 4.32 ± 0.39 was calculated for infinitely small doses/fraction. These findings were compared with other radiobiological data and conclusions drawn from clinical trials. It was concluded that for the sparing of late effects in skin and subcutaneous tissues, relative to acute reactions, a relatively small number of fractions in a short overall treatment time may be optimal for fast neutron therapy. (author)