A dosimetric study of Leipzig applicators.

PURPOSE

To obtain the absolute dose-rate distribution in liquid water for all six cup-shaped Leipzig applicators by means of an experimentally validated Monte Carlo (MC) code. These six applicators were used in high-dose-rate (HDR) afterloaders with the "classic" and v2 (192)Ir sources. The applicators have an inner diameter of 1, 2, and 3 cm, with the source traveling parallel or perpendicular to the contact surface.

METHODS AND MATERIALS

The MC GEANT4 code was used to obtain the dose-rate distribution in liquid water for the six applicators and the two HDR source models. To normalize the applicator output factors, a MC simulation for the "classic" and v2 sources in air was performed to estimate the air-kerma strength. To validate this specific application and to guarantee that realistic source-applicator geometry was considered, an experimental verification procedure was implemented in this study, in accordance with the TG43U1 recommendations. Thermolumniscent dosimeter chips and a parallel plate ionization chamber in a polymethyl methacrylate (PMMA) phantom were used to verify the MC results for the six applicators in a microSelectronHDR afterloader with the "classic" source. Dose-rate distributions dependence on phantom size has been evaluated using two different phantom sizes.

RESULTS

Percentage depth dose and off-axis profiles were obtained normalized at a depth of 3 mm along the central axis for both phantom sizes. A table of output factors, normalized to 1 U of source kerma strength at this depth, is presented. The dose measured in the PMMA phantom agrees within experimental uncertainties with the dose obtained by the MC GEANT4 code calculations. The phantom size influence on dose-rate distributions becomes significant at depths greater than 5 cm.

CONCLUSIONS

MC-detailed simulation was performed for the Nucletron Leipzig HDR applicators. The matrix data obtained, with a grid separation of 0.5 mm, can be used to build a dataset in a convenient format to model these distributions for routine use with a brachytherapy treatment planning system.