Dosimetry investigation of an applicator system for intraoperative radiation therapy by Monte Carlo simulations

Design and dosimetry characteristics of a commercial applicator system for intraoperative radiation therapy (IORT) utilizing ELEKTA Precise accelerator have been previously reported. The system is geometrically more complex than the standard electron applicators used for external beam therapy. Moreover, the geometry is different from the reference conditions used in various dosimetry protocols. The first purpose of this work was to validate the measured dosimetry data by Monte Carlo (MC) simulations. The second purpose was to validate the EGS5 code for the simulation of electron beams from an Elekta Precise linac. Detailed information on all components of the Elekta Precise linac head was provided by the vendor. This information was used to build the head models for the 6 MeV, 9 MeV and 12 MeV electron beams. The EGSnrc code package (which includes BEAMnrc and DOSXYZnrc codes) was used for MC simulation. BEAMnrc is an extensively validated code which can be regarded as a benchmark for MC simulations. The incident electron beam parameters (energy spectrum, FWHM) were adjusted to match the measured data (depth doses and profiles) at SSD=100 cm for 14x14 standard electron applicator. EGS5 is a general purpose MC code currently developed at KEK (Japan). EGS5 is based on the well-known EGS4 code and contains some important improvements. In EGS5 code, the complex head geometry is described using combinatorial geometry and can be visualized using CGVIEW graphical interface. Depth dose profiles and lateral dose profiles at the depth of dose maximum were obtained the 6 MeV, 9 MeV and 12 MeV electron beams for the standard 14x14 electron applicator using both codes.MC transport parameters and the incident electron beam parameters used in EGS5 simulations were selected to be the same as those used in BEAMnrc simulations. The results of the simulations were compared to the in-water dose measurements which were performed in the IBA water phantom