Radiobiology of pulsed dose rates

Any departure from CLDR will yield an increased biological effect, which is greater for tissues with smaller /spl alpha///spl beta/ ratios and shorter T/sub 1/2/ of repair. The Relative Effectiveness (RE=1+g.d/(/spl beta///spl alpha/)) for late responding tissues will increase more than the RE for early responding tissues including tumors. This could obviously lead to a reduced therapeutic ratio. Fortunately the effect is small (less than 10%) unless doses per pulse greater than 1 Gy are used. Dose-per-pulse is paramount (together with total dose) in determining the biological effect. The effect of between-pulse interval is next in importance, (longer intervals helping therapeutic ratio but not if fewer and larger pulses are used). The influence of duration of pulses and dose-rate within each pulse is less. In tissues with the shortest half-times of repair (less than 0.5 hours) the biological effect will be greater for PDR than for CLDR. The authors show here that there are no consistent differences between half-times for tumors and for normal tissues. The effect of "office-hours" PDR is very close to that of the same size and number of pulses given with standard intervals continuously, because there will be only a few overlong (overnight) intervals compared with many more standard short intervals. Late complications will be ameliorated more than tumor cell kill by increasing the intervals between pulses without increasing their size. Probably the most valuable parts of this Handout are the Tables and the References.