  The two dominant radiotherapy methods are either simplified in terms of beam
generation and handling, which compromises the energy deposition curve in
tissues (photon therapy), or require extensive accelerator facilities and
complex beam delivery systems to provide a favorable shape of the energy
deposition curve (hadron therapy). The advantages of both of these methods,
such as the low cost of the apparatus, ease of beam generation, and a suitable
shape of the energy deposition curve in tissues, can potentially be achieved by
using a very high-energy electron beam (beam energy in the order of a few
hundreds of MeV) focused on the area of the tumor lesion. However, focusing of
the beam is usually done with the use of quadrupole magnets which makes the
beam delivery system complex and challenging from the engineering point of
view. In this thesis, the feasibility of an alternative solution is explored,
where focusing is performed by a bent silicon crystal with an appropriate shape
of its exit face. Such a crystal lens can be a very light object (mass in the
order of grams), allowing for much simpler beam delivery systems of
radiotherapy facilities. As a result of this feasibility study, a simulation of
a bent silicon crystal with profiled exit was prepared in Geant4. The outcome
obtained from the simulation proved the focusing ability of such profiled
crystal. However, the focusing strength of the crystal is not strong enough.
Technical requirements needed to enable an improvement in focusing efficiency
were identified and understood. Moreover, alternative solutions providing
stronger focusing while using profiled crystals were proposed.
