Monte Carlo calculation of the dose perturbations in a dual-source HDR/PDR afterloader treatment unit

Currently, there are high dose rate afterloaders available that can drive two different radioactive sources simultaneously. The source-source and source-cable attenuations are not taken into account by current planning systems. The purpose of this work is to characterize these effects and their overall impact on clinically relevant metrics.

A (192)Ir ((169)Yb) Flexitron source is modeled within a Monte Carlo code, and its water dose distribution is evaluated. A second source (cable) is placed parallel at various distances to quantify the dose perturbations. The dual-source setup is then transposed to clinical prostate plans (n = 11). Each plan D90 is compared to the single-source D90. The worst-case scenario (two sources traveling in closely located positions) establishes the upper bounds of the deviations. Two setups are considered with sources constrained in catheter pairs or freely distributed. A metric proportional to the dwell times and R(-2) (where R is the intersource separation) helps determine the source position in each configuration.

The dose profiles vs. R(-2) (3-20 mm) show a maximal dose reduction effect of 65% (25%) for (169)Yb ((192)Ir) at small distances. A shadow region with at least 10% dose difference extends up to 10 cm. A similar study with a steel drive cable shows similar behavior with a maximal decrease of 10% (3%) under irradiation of a (169)Yb ((192)Ir) source. The relative D90 difference with the single source setup is 2.3% on average, up to 3.7%. It is obtained by superimposing the contributions from catheter pairs in the dual-source loading.

Overall deviations observed in D90 compared to a single source setup are not significant for all cases studied.