Monte Carlo dosimetric investigation of a brain tumor model irradiation using laser-driven very high energy electron beams.

Electron beams with energy up to $\sim$ 10 ${MeV}$ are commonly used in radiotherapy for treatments of superficial tumors or IORT. Laser-Plasma Acceleration (LPA) of Very High Energy Electrons (VHEE), with energy $100\mbox{--}250 {MeV}$, opens perspectives for compact, hospital compliant accelerators for new treatments of deep seated tumors. Moreover, LPA offers a promising method to produce VHEE beams with the ultra-high dose rate needed to exploit the recently discovered "FLASH effect". We present the results of a Monte Carlo study aimed at assessing the dosimetric features of a typical LPA VHEE beam, such as the one available at the ILIL laboratory of CNR-INO in Pisa, and studying the beam scanning irradiation of a deep seated tumor. Simulations show that, via an appropriately designed collimator, a $5 {mm}$ size pencil beam can be obtained, with good symmetry and flatness and a therapeutic range of about $8 {cm}$. The dose deposition in a tumor placed within a CT reconstructed human head indicates good target conformation and coverage with minimal lateral spread. A discussion of the tissue sparing due to the FLASH effect, included in the study via a parametric approach, will also be given.