Dose properties of a laser accelerated electron beam and prospects for clinical application

Publisher Website
Google Scholar
Abstract
Laser wakefield acceleration (LWFA) technology has evolved to where it should be evaluated for its potential as a future competitor to existing technology that produces electron and x‐ray beams. The purpose of the present work is to investigate the dosimetric properties of an electron beam that should be achievable using existing LWFA technology, and to document the necessary improvements to make radiotherapy application for LWFA viable. This paper first qualitatively reviews the fundamental principles of LWFA and describes a potential design for a 30 cm accelerator chamber containing a gas target. Electron beam energy spectra, upon which our dose calculations are based, were obtained from a uniform energy distribution and from two‐dimensional particle‐in‐cell (2D PIC) simulations. The 2D PIC simulation parameters are consistent with those reported by a previous LWFA experiment. According to the 2D PIC simulations, only approximately 0.3% of the LWFA electrons are emitted with an energy greater than 1 MeV. We studied only the high‐energy electrons to determine their potential for clinical electron beams of central energy from 9 to 21 MeV. Each electron beam was broadened and flattened by designing a dual scattering foil system to produce a uniform beam (103%>off‐axis ratio>95%) over a 25×25 cm2field. An energy windowranging from 0.5 to 6.5 MeV was selected to study central‐axis depth dose, beam flatness, and dose rate. Dose was calculated in water at a 100 cm source‐to‐surface distance using theEGS/BEAMMonte Carlo algorithm. Calculations showed that the beam flatness was fairly insensitive toHowever, since the falloff of the depth–dose curveand the dose rate both increase witha tradeoff between minimizingand maximizing dose rate is implied. Ifis constrained so thatis within 0.5 cm of its value for a monoenergetic beam, the maximum practical dose rate based on 2D PIC is approximately 0.1 Gy min−1for a 9 MeV beam and 0.03 Gy min−1for a 15 MeV beam. It was concluded that current LWFA technology should allow a table‐top terawattlaser to produce therapeutic electron beams that have acceptable flatness, penetration, and falloff of depth dose; however, the dose rate is still 1%–3% of that which would be acceptable, especially for higher‐energy electron beams. Further progress in laser technology, e.g., increasing the pulse repetition rate or number of high energy electrons generated per pulse, is necessary to give dose rates acceptable for electron beams. Future measurements confirming dosimetric calculations are required to substantiate our results. In addition to achieving adequate dose rate, significant engineering developments are needed for this technology to compete with current electron acceleration technology. Also, the functional benefits of LWFA electron beams require further study and evaluation.