Application of the "bioeffects" algorithm of a treatment planning system

In this study, both a four-field box and two-field AP/PA treatment plan are combined with two insertions of Cs-137 in a tandem and ovoids setup, to evaluate the bioeffects program of a treatment planning system. External beam energies studied are 18 and 6 MV. It is shown that there is a slight difference in the 50-70 time dose fractionation (TDF) isolines when comparing 6 MV and 18 MV, for the AP/PA setup. There is practically no difference for TDF isoline values larger than 80 for both energies with either the four-field or the two-field setup. This is because the brachytherapy contributed the majority of the dose to the regions near the applicator and the TDF values reflect the higher dose delivered by the brachytherapy relative to the external beams in that region. For this simple evaluation of the bioeffects program, the combination of the external beam plan and the brachytherapy plan does not give us enhanced information on the effectiveness of the plan.