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The relative biological effectiveness of fractionated doses of fast neutrons (42 MeV_dBe) for normal tissues in the pig

CRC Normal Tissue Radiobiology Research Group, Research Institute (University of Oxford), Churchill Hospital, Oxford 0X3 7LJ, UK ^* MRC Radiobiology Unit, Chilton, Didcot, Oxfordshire 0X11 ORD, UK

The effects of fractionated doses of fast neutrons (42 MeV_dBe) on the radiation response of the pig kidney have been assessed and compared with those observed after X irradiation. Following X irradiation there was a marked increase in the total dose at which renal function was preserved with decreasing fraction size. The rate of this increase was dependent on the overall treatment time; for fractionated irradiation given over 18 or 39 days the exponents related to fraction number, N, were 0.36 ± 0.03 and 0.48 ± 0.003, respectively. In contrast, there was no significant change in the iso-effect dose for renal injury following fractionated irradiation with fast neutrons where there was also little effect of varying the overall treatment time. Analysing these data by means of the linear-quadratic (LQ) model, using both an Fe-plot and the Tucker test, gave /β ratios of 2.42 ± 0.16 Gy and 2.99 ± 0.16 Gy, respectively, for X-ray doses given in 18 days. For fractionated doses of X rays given in 39 days the β ratios were 0.40 ± 0.01 Gy and 0.47 ± 0.02 Gy, respectively. The β ratios for renal tissue following fast neutron irradiation obtained bythe two methods were also similar, i.e. 15.00 ± 0.60 Gy and 15.72 ± 3.76 Gy, respectively. The pronounced fractionation effect seen with X irradiation, particularly for doses administered over 39 days as opposed to 18 days, coupled with the absence of any such effect with fast neutrons, resulted in a marked increase in relative biological effectiveness (RBE) with decreasing X-ray dose/fraction. The slopes of the resulting regression lines were –0.73 ± 0.05 and –0.33 ± 0.02, respectively. The lack of dose sparing associated with fractionation, or variation of the overall treatment time for fast neutron irradiation, suggests that doses administered to tumours adjacent to the kidney can be given as a few relatively large dose/fractions in a short overall treatment time without an increased risk of complications related to renal tissue. This may be of therapeutic advantage in the treatment of rapidly proliferating tumours where dose may be wasted using more conventional protracted fractionated irradiation schedules.

Key Words: Fast neutrons • Relative biological effectiveness • Pig kidney • Renal function

Received for publication November 1, 1990. Revision received February 1, 1991.