Influence of heterogeneous media on Very High Energy Electron (VHEE) dose penetration and a Monte Carlo-based comparison with existing radiotherapy modalities

In this paper we investigate the dosimetric effects target inhomogeneities on VHEE (Very High Energy Electron) beam dose profiles. Dose profile sensitivity to high and low density (0.001 -2.2 g/cm(3)) embedded media in water was measured for 156 MeV VHEE beams. To the knowledge of the authors, this is the first systematic experiment on dose penetration in inhomogeneous targets at this high energy electron beams. The results of the dosimetry experiments were compared with simulation results acquired with the TOPAS/GEANT4 Monte Carlo codes and were compared with the sensitivity of photon and proton beams. Dosimetry experiments with VHEE beams at 156 MeV were performed by embedding various density inserts in a cuboid 30 x 30 x 10 cm(3) water phantom. This setup was then irradiated with a narrow (sigma = 1.2 mm) Gaussian VHEE beams. Transverse dose profiles were recorded at various depths within the water phantom using radosensitive EBT-XD Gafchromic films. Simulations were performed using the TOPAS user wrap for the GEANT4 Monte Carlo toolkit with the measured beam and target geometry parameters from the CLEAR facility and compared with sensitivity of photon and protons beams to inhomogeneities in water phantoms. Less than 8% dose deviation was shown for all measured dose data across all recorded depths (0-20 cm). Monte Carlo simulations within the whole water phantom volume showed a 15% dose deviation. Simulation studies indicated that using proton and photon beams within a therapeutic energy range in MC simulations of this experimental setup resulted in dose change in the central plane up to 100% and 74% of the dose maximum in the central plane respectively. The dosimetry results of these experiments and TOPAS/GEANT4 simulations compare well. Dose profiles of VHEE beams were found to be relatively insensitive (< 15% dose difference across the central plane) to embedded high and low density geometries compared to simulation results with therapeutic proton beams (up to 100% of dose maximum in the plane). Therefore, VHEE is robust to anatomical changes and has the potential to be a reliable mode of radiotherapy for treating tumors in highly inhomogeneous and mobile regions such as the lung.