Computer-assisted morphometric analysis of late renal radiation injury

Quantitative morphology of late renal radiation injury

Late radiation injury manifests itself in all morphologic compartments of the kidney, but loss of cell mass is most significant in the proximal convoluted tubules. Development of an end stage or nonfunctional kidney requires 12 or more months after single fraction X ray exposures of about 12 Gy (2, 5, 13) and is associated with marked morphologic alterations of renal tubules. Radiation induced changes were studied at 6 months after irradiation, a time interval when histological alterations appear minor, but in previous studies were shown to correlate with the later end stage alterations (5, 8, 9). Renal alterations were graded objectively based on renal weight, variation in size of tubule cell nuclei, and glomerular nuclear volume fraction. Irradiation was associated with loss of renal weight, increased variability of tubule nuclear size, and previously unappreciated changes in glomerular nuclear volume fraction. A classification index derived from a weighted combination of renal weight ratio and tubule cell nuclear variability correlates with radiation dose and with previously established subjective histologic grading of renal damage, and allows objective comparisons of various fractionation schedules.

Fraction size, dose and time dependence of X ray induced late renal injury

Histologic quantitation of renal radiation injury based on an intact mouse model correlates well with loss of renal mass, as measured by the ratio of right to left renal weight, radiation dose, and time after irradiation. Use of the radiation dose at which 50% of animals show renal tubular changes of a specified grade (ED50), allows comparison of 1, 5 and 15 fraction exposures. The X-ray dose necessary to effect significant injury (Grade 3 change) by six months after irradiation (ED50) was 11.6, 25.7, and 44.0 Gy for single, 5, and 15 fractions, respectively. Isoeffect plots of total radiation dose as a function of fraction number are linear on logarithmic coordinates, indicating that this relationship followed a power function. When plotted as inverse total dose versus dose per fraction, the six month post-irradiation data were linear, but the 12 month data were not. Lack of linearity is not in agreement with the multi-fraction isoeffect linear quadratic model.