9 Reporting in Brachytherapy: Dose and Volume Specification

168 Reporting in Brachytherapy

5 Reporting the Time-Dose Pattern in Brachytherapy 5.1 Description of the time-dose pattern

The description of the time-dose pattern should include the type of irradiation with the necessary data on treatment and irradiation times (49). The information on dose and time should provide the necessary data to calculate instantaneous and average dose rates. • The overall treatment time should always be recorded. • Continuous irradiation: dose rate. • Non-continuous irradiation: the total irradiation time should be recorded. • Fractionated and pulsed irradiation: the fraction size and irradiation time of each fraction, the interval between fractions, and the overall treatment time should be recorded. • When the irradiation times of the different sources are not identical, they should be recorded. For moving sources: • Stepping sources: step size, dwell location and dwell time should be recorded. Variation of the dwell times of a stepping source can be used to manipulate the dose distribution. This can be achieved either by manual adaptation of the source positions in relation to the Target Volume, or by a computer optimisation programme. If such a dose optimization is applied, this should be specified (e.g., optimization at dose points defined in the implant, or geometrical optimization (52)). • Oscillating sources: speed in different sections of the vectors should be recorded. 5.2 Biologically weighted dose For comparing applications performed using different dose rates, doses per fraction or other differences in time-dose patterns, weighting factors, W rate , must be introduced. The product of the (physical) absorbed dose, D, by these weighting factors, W rate , is the biologically weighted dose , D rate , for dose rate, dose per fraction or other differences in time-dose pattern. When evaluating the “radiobiological equivalence” between treatments performed with different time- dose patterns, the radiobiological model used (e.g., α/β, repair function, ...) as well as the numerical values of the parameters applied, must be indicated. In addition, the conditions for evaluation of radiobiological equivalence must be stated (e.g., late or early effects, type of tissue, dose and dose rate range, etc.) (35,56,66,113,115,116). For reporting, the biologically weighted dose D rate alone cannot be given, but the (physical) absorbed dose and the complete time-dose pattern should be given together with the weighted dose. This will avoid confusion when comparing treatments and will allow, eventually, re-evaluation of “radiobiological equivalence” when new and better radiobiological data becomes available. An important issue when evaluating “radiobiological equivalence” is the selection of the reference dose rate or time-dose pattern. As a general rule, and unless otherwise stated, “historical” continuous low dose rate irradiation is taken as the reference (i.e., typically with radium, 60 Gy in 6 days or about 0.5 Gy per hour, at the specification point).