Late functional response of mouse urinary bladder to fractionated X-irradiation

Purpose: Late functional changes in mouse urinary bladder were studied afte r fractionated irradiation in order to define parameters of repair of suble thal radiation damage. Materials and methods: In the repair capacity study, 1 to 10 fractions were applied within 5 days. To assess the kinetics of repair, dr fractions were applied with intervals of 0, 0.5, 1, 2, 4, or 8h. The endpoint used to est ablish dose-effect relationships was a reduction in the cystometrically mea sured bladder capacity by greater than or equal to 50% of the individual pr e-treatment value. Data analysis was performed with a linear-quadratic mixt ure model, which includes latent-time analysis in an effect-dependent manne r. Results: The alpha/beta values were found to be 4.4 Gy (95% confidence inte rval [2.0-8.4]) for the repair capacity data alone and 3.7 Gy (1.7-6.7) in combination with the repair kinetics data. The halftime of repair, assuming mono-exponential repair kinetics, was 0.39h (95% CI [0.23-1.3]). The mean latent time decreased from around 40 weeks to 12 weeks, inversely dependent on the biologically effective dose. In animals experiencing an acute respo nse during the first month after treatment, the incidence of severe late re sponses was significantly increased (p=0.0025). Conclusions: The present results confirm that the sparing effect in the mou se urinary bladder by dose fractionation is in the lower range of chronical ly responding tissues. The half-time of repair is in the lower range report ed for mouse tissues.