Dose fractionation and tumour repopulation in radiotherapy for bladder cancer.

Local control of the 77 transitional cell carcinoma of the bladder was analysed with respect to total dose after normalization for variations in fraction size and to overall treatment time. For the TCD50 of 63.3 Gy protraction of overall treatment time from 40 to 55 days average gave the decrease in local control rate from 50% to about 5%. The decrease in local tumour control with extension of overall treatment time likely reflects accelerated tumour repopulation during the treatment. To determine the time onset of the accelerated tumour clonogen repopulation, data taken from the literature were analysed. Results show that on average tumour clonogen repopulation in transitional cell cancer of the bladder accelerates after a lag period of about 5-6 weeks after the start of treatment and that a dose increment of 0.36 Gy per day is required to compensate for this repopulation. Such a dose increment is consistent with about 5-8 day clonogen doubling time. It suggests that overall treatment time is an important factor in the dose fractionation and protraction of time may have a significant impact on treatment outcome. Thus, radiotherapy for bladder cancer should be completed as soon as possible and total dose for high probability of local tumour control should consequently be delivered in more than 5 fractions per week.