Early changes in mouse urinary bladder function following fractionated X irradiation.

Functional changes in the mouse urinary bladder following single-dose or fractionated irradiation were assessed by cystometry, i.e., by measuring the intravesical volume-pressure relationship during transurethral filling. The early response presented as a dose-dependent and transitory decrease in the reservoir function of the organ as defined by the intravesical volume at a filling pressure of 10 or 20 mm Hg, V10 or V20. The quantal dose response used in the present study was a reduction in the individual bladder volume (V10 and V20) by at least 50%. After single doses greater than or equal to 10 Gy, the reaction occurred between Days 7 and 25 with maximum prevalence between Days 7 and 14. The individual duration of the response was 3-9 days. Treatment with single doses and 2, 3, 5, and 10 fractions demonstrated a significant sparing effect with ED50 values of 18.3, 24.9, 26.8, 29.8, and 38.0 Gy, respectively. The linear-quadratic model fitted the data reasonably well when tested according to Tucker (Int. J. Radiat. Oncol. Biol. Phys. 10, 1933-1939, 1984). The alpha/beta ratios estimated with different two-step techniques ranged from 11.1 to 12.4 Gy. Analysis of the data as proposed by Thames et al. (Int. J. Radiat. Biol. 49, 999-1009, 1986) yielded an alpha/beta value of 13.9 Gy (95% confidence limits 8.4 and 24.6 Gy), illustrating a fractionation effect typical for acutely responding tissues, although no clear cell depletion occurred in the urothelium.