Improved accuracy in simulation of electron and x-ray beams in external beam radiotherapy

Methodology is presented with the goal to determine fluence to 1-2% for electron and x-ray beams used in external beam radiotherapy. Fluence was determined by combining accurate and detailed treatment head simulation of a modern clinical accelerator with accurate measurements done over a wide range of operating parameters. The treatment head was disassembled for confirmation of the information provided by the manufacturer and for direct measurement of treatment head geometry. Dose measurements were made on the same accelerator with methodology to improve the accuracy of the measurements. Detailed simulation of the treatment head for the different modalities and beam energies were done with two different, accurate Monte Carlo systems. Source and geometry details in the simulation were adjusted within tight constraints set by the measurements. Results give confidence the 1-2% accuracy goal is achievable for both electron and x-ray beams.