  The use of very high energy electrons (VHEE) for radiotherapy has been
actively studied for over two decades due to its advantageous dose
distribution, deep penetration depth and great potential of ultra-high
dose-rate irradiation. However, the high entrance dose of VHEE beams can damage
the surface skin of patients and hinder its widespread application. To address
this challenge, a novel method utilizing only two dipole magnets is presented
in this article. By adjusting the magnet strengths, the electron beams can be
guided along different angular directions towards a specific position as deep
as 20 cm inside a water phantom, creating a maximum dose over the target region
and significantly reducing the entrance dose Supported by Monte Carlo
simulations, such a beam delivery approach contains two major advantages over
previous methods: first, it is insensitive to beam energy spread, releasing the
constraints on accelerator performance, and second, the dose peak position can
be accurately controlled in both lateral and longitudinal directions. In
addition, we also show that a flattop dose peak can be generated by the
weighted sum of VHEE beams focusing at different positions. These results
demonstrate that VHEE beams can be compactly delivered into a deep-seated tumor
region in a controllable manner, thus advancing the development of the VHEE
radiotherapy towards the practical clinical applications in the near future.
