Measurements of renal damage in the mouse were made to determine if there was an equal effect per fraction during a course of repeated 240-kVp X-ray doses. An X-ray dose of 2 Gy was given 2, 8, 14, or 20 times with interfraction intervals of 12 h. Some animals were also irradiated with twenty 2-Gy doses using a 5-h interfraction interval. The underlying effect per fraction (<tex-math>$-{\rm log}_{e}{\rm SF}$</tex-math> of the notional target cell population) was determined from the additional top-up dose of d(4)-Be neutrons needed to produce measurable renal impairment assessed by decreased clearance from the plasma of <tex-math>$[{}^{51}{\rm Cr}]{\rm EDTA}$</tex-math> and by a reduction in the hematocrit at 25, 29, 33, and 39 weeks after treatment. There was no significant influence of the time of assay on the values of underlying effect measured. A mean value of underlying effect was therefore calculated for the two different assays of each mouse, from the measurements at the four times. This gave ∼40 estimates (one for each animal assessed) with each assay of the effectiveness of 2-Gy fractions in each of the four fractionation schedules, a total of 321 determinations in the study with 12-h intervals. Regression analysis showed that there was no significant trend in underlying effect per fraction with number of fractions, i.e., the damage per fraction was constant regardless of the number of fractions used. With underlying effect normalized to 1 unit of damage for a single 2-Gy dose, the slope of this plot was -0.0013 per <tex-math>$\text{fraction}^{2}$</tex-math> ± 0.0097 (95% CL). The assumption of equal effect per fraction was therefore not invalidated in the kidney of the mouse. With a 5- instead of a 12-h interfraction interval, the 20-fraction schedule was 7% more effective as measured by the two assays analyzed together; this was significant at P = 0.0001. This shows that 5 h is not sufficient time between fractions for full repair to occur in the kidney, and underlines the need for intervals of at least 6 h between the doses in clinical radiotherapy using more than one fraction per day. The data are consistent with an α/β ratio ∼1.6 Gy, with a repair half-time ∼1.3 h. However, these experiments were not designed to determine these parameters and their values should be regarded only as rough estimates.

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