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Proton beam therapy has been developed to irradiate the tumor with higherprecision and dose conformity compared to conventional X-ray irradiation. Thedose conformity of this treatment modality may be further improved if narrowerproton beams are used. Still, this is limited by multiple Coulomb scattering ofprotons through tissue. The primary aim of this work was to develop techniquesto produce narrow proton beams and investigate the resulting dose profiles. Weintroduced and assessed three different proton beam shaping techniques: 1)metal collimators (100/150~MeV), 2) focusing of conventional- (100/150~MeV),and 3) focusing of high-energy (350~MeV, shoot-through) proton beams. Focusingwas governed by the initial value of the Twiss parameter $\alpha$~($\alpha_0$),and can be implemented with magnetic particle accelerator optics. The dosedistributions in water were calculated by Monte Carlo simulations using Geant4,and evaluated by target to surface dose ratio (TSDR) in addition to thetransverse beam size~($\sigma_T$) at the target. The target was defined as thelocation of the Bragg peak or the focal point. The different techniques showedgreatly differing dose profiles, where focusing gave pronouncedly higherrelative target dose and efficient use of primary protons. Metal collimatorswith radii < 2~mm gave low TSDRs (<~0.7) and large $\sigma_T$(>~3.6~mm). Incontrast, a focused beam of conventional (150~MeV) energy produced a very highTSDR (>~80) with similar $\sigma_T$ as a collimated beam. High-energy focusedbeams were able to produce TSDRs > 100 and $\sigma_T$ around 1.5~mm. From thisstudy, it appears very attractive to implement magnetically focused protonbeams in radiotherapy of small lesions or tumors in close vicinity to healthyorgans at risk. This can also lead to a paradigm change in spatiallyfractionated radiotherapy.

Sharp dose profiles for high precision proton therapy using focused proton beams