Multifraction irradiation of mouse bladders.

The response of mouse bladders to multifraction irradiation was assessed from increases in urination frequency or the reduction in bladder capacity after irradiation. A range of electron doses were given as 1, 2, 5, 10 or 20 equal fractions in overall treatment times of 1-2 weeks. Dose-related increases in urination frequency were measured from 10 to 14 months after irradiation and a dose-related reduction in bladder capacity (at inflation pressures of 20 mm Hg) was apparent at the time of sacrifice. The extent of repair of sublethal and potentially lethal damage was estimated from a comparison of the isoeffective doses in fractionated regimes and single dose treatments. After small doses per fraction (2.5-6 Gy), the extent of repair in bladder was very similar to that in mouse skin. After larger doses per fraction (greater than 8 Gy) slightly more repair was seen in bladder than skin. Linear-quadratic analysis of the data suggests quite a high value for the ratio alpha/beta, in the region of 5 to 10 Gy. This is higher than the alpha/beta ratios which have been reported for most other slowly dividing normal tissues.