Effects of multifraction irradiation on the rat kidney

Wag/Rij female rats were irradiated to the left kidney with single doses or 2, 4, 10, 20, or 40 equal dose fractions. The right kidney was removed 4 weeks after the last fraction. The kidney function was determined using three different parameters. The serum urea content indicated glomerular function. Urine osmolality and the total volume of urine excreted in 24 hr indicated tubular function. The onset as well as the rate of expression of radiation-induced kidney damage was dose-dependent. The kidney function decreased continuously. Differences in expression of damage between glomerular and tubular parameters were not observed. All parameters indicated marked sparing of the kidney by fractionation. In general, the data could be fitted to the linear-quadratic model, if the single dose data were not included in the analyses. However, the fit greatly improved when data obtained with high and low doses per fraction were analyzed separately. The Direct Analysis method was used to determine the alpha/beta ratios. No significant differences were observed between the alpha/beta ratios calculated for the different parameters. The ratios also did not change with increasing time after treatment. The alpha/beta for high doses per fraction was between 0.6 and 2.7 Gy, and that for low doses per fraction, with fractional doses in the clinical range, was between 0.5 and 3.8 Gy. The alpha/beta values for low doses per fraction were generally lower than those for high doses per fraction. These observations indicate a strong dependence of radiation-induced damage in the rat kidney on the size of the dose per fraction.