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

PURPOSE: 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. METHODS AND MATERIALS: A Ir-192 (Yb-169) 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 D-90 is compared to the single-source D-90. 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. RESULTS: The dose profiles vs. R-2 (3-20 mm) show a maximal dose reduction effect of 65% (25%) for Yb-169 (Ir-192) 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 Yb-169 (Ir-192) source. The relative D-90 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. CONCLUSION: Overall deviations observed in D-90 compared to a single source setup are not significant for all cases studied.