[Oral Communication] Adapted Point Kernel Collapsed Cone Convolution based on Monte Carlo Simulation Knowledge for MR-linac Dose Calculations

Adapted Point Kernel Collapsed Cone Convolution based on Monte Carlo Simulation Knowledge for MR-linac Dose Calculations

ABSTRCT

This paper describes adaptations of the original Collapsed Cone Convolution (CCC) calculation method (A. Ahnesjö, Med.Phys. 16(4) 1989) for the calculation of photon beam dose distributions in the presence of a lateral magnetic field. The presented work is based on the characteristics of the MR-linac Elekta Unity machine and focuses essentially on the restitution of the perturbation of the secondary electron trajectory in the irradiated medium i.e., considering the Electron-Return-Effect (ERE) due to the Lorentz force.

The mono-energetic point-kernels in the presence of a lateral magnetic field of 1.5 T were simulated by Monte Carlo code (GATE/Geant4) and then the spectrum of the 7 MV beam of Unity was determined in a conventional way from measured depth dose curves. The analytical formula for energy transport along each cone axes (which is the core of the original method) was revised to improve the fit of the calculated values to the Monte Carlo code data.

In the end, a significant improvement in the accuracy of the CCC calculations was obtained with, as a particular indicator, the gamma-index agreement calculated in comparison with dose distributions provided by the TPS Elekta Monaco, itself based on a Monte Carlo code (XVMC).