Impact of time between X-ray fractions on acute changes in the function of the urinary bladder in the mouse

Noninvasive transurethral cystometry was used in mice anesthetized with pentobarbitone to quantify the effect of the interval between X-ray fractions on acute radiation-induced changes in the function of the urinary bladder. Bladder capacity was defined at intravesical pressures of 10 and 20 mm Hg. A reduction in the storage capacity of the bladder by > or = 50% of the individual preirradiation control value, occurring at any time during the initial 30 days after radiotherapy, was the quantal end point used for dose-response analysis. Four equal-sized dose fractions were applied with intervals of 0, 0.5, 1, 1.5, 2, 4 and 8 h, respectively. The ED50 values increased from 18.2 Gy (95% confidence interval 16.8-20.6) for single-dose treatment to 28.1 Gy (26.2-29.9) in the 8-h protocol. Linear-quadratic analysis of the results, using an indirect repair model and the direct analysis approach, yielded a half-time of repair of 1.2 h (0.9-1.9) with an alpha/beta ratio of 10.4 Gy. If combined with previous data for split doses where 1 to 10 fractions were given within < or = 7 days (Dörr and Schultz-Hector, Radiat. Res. 131, 35-42, 1992), an even longer repair half-time of 1.7 h (1.3-2.7) was determined.