[Therapeutic ratio of fractionated high-dose-rate against continuous low-dose-rate brachytherapy analyzed by...

In order to establish the standard schedule of 192Ir-HDR brachytherapy, it is necessary to determine the optimal time-dose-fractionation. A new simulation model was used to determine the optimal schedule. This model, which is based on the LQ-model, includes the effect of incomplete repair between fractionations. The fraction surviving after irradiation is expressed as log (surviving fraction) = n (-alpha x -beta x2)-n beta x2 hn (theta), and the change in fractionation will influence the value of hn (theta). From analysis of this model, the therapeutic ratio will approach one if two fractionations (6-hour and 18-hour intervals) or three fractionations (4-hour, 4-hour and 16-hour intervals) in a day are carried out for 6 days. This simulation model will provide an optimal treatment schedule for tumor control, for example, two daily fractionations of 4.6Gy with 6-hour intervals between the daily fractionations (and 18 hours to the first fractionations of the next day), 12 fractionations to 55.2Gy in 6 days or three daily fractionations of 3.3Gy with a 4-hour interval between the daily fractionations (and 16 hours to the first fractionations of the next day), 18 fractionations to 59.4Gy in 6 days. Further clinical study should be carried out to confirm this simulation.