The effect of age and the proportion of renal tissue irradiated on the apparent radiosensitivity of the pig kidney.

In 14-week-old (immature) and 45-week-old (mature) pigs either the right kidney (unilateral irradiation, UI) or both kidneys (bilateral irradiation, BI) were irradiated. The kidneys of immature pigs received single doses of 7.0-12.6 Gy of 250 kV X-rays; mature pigs received single doses of 7.8-14.0 Gy of 60Co gamma-rays. These were assumed to be approximately equivalent to the X-ray doses using a RBE of 0.9 gamma-rays/X-rays. The glomerular filtration rates (GFR) and effective renal plasma flows (ERPF), were determined for up to 24 weeks after irradiation. From these data dose-effect curves were obtained by determining, at each dose level, the percentage of irradiated kidneys which exhibited a greater than or equal to 50 per cent reduction in GFR and ERPF. 60Co gamma-ray doses were normalised to approximately equivalent X-ray doses. The dose-effect curves were fitted by probit analysis, and ED50 values (+/- SE) calculated for both GFR and ERPF. Similar conclusions could be drawn from the results obtained following the determination of both GFR and ERPF. However, the ED50 values obtained for ERPF were significantly lower than those for GFR (p less than 0.05). In 14-week-old animals the kidney of UI animals was significantly more radiosensitive than that of BI animals, i.e. the ED50 values, in terms of GFR, were 8.74 +/- 0.31 Gy and 10.97 +/- 0.23 Gy, respectively (p less than 0.001). In 45-week-old pigs the reverse was true; the ED50 values were 12.67 +/- 0.34 Gy and 8.78 +/- 0.15 Gy (p less than 0.001) for unilateral- or bilateral-irradiated animals, respectively. The kidney of BI mature animals appeared to be as radiosensitive as the UI immature pigs. Thus the renal response to radiation was markedly influenced by the age of the animals and by the proportion of the renal tissue irradiated.