Comment on 'Collimated proton pencil-beam scanning for superficial targets: impact of the order of range shifter and aperture'.

We read with interest the study by Bäumer et al (2018 Phys. Med. Biol. 63 085020), in particular that their conclusions are in contrast to those of our earlier paper (Winterhalter et al 2018 Phys. Med. Biol. 63 025022), namely that positioning the collimating aperture downstream of the range shifter leads to a superior penumbra. In contrast, we found sharper penumbras for the PSI scanning Gantry when the aperture is positioned upstream of the range shifter. We have run additional Monte Carlo simulations with components derived from the paper of Bäumer et al (2018 Phys. Med. Biol. 63 085020), but without modifying the beam description. As such, we obtain a relative penumbra reduction of 13% if the aperture is positioned downstream of the ranges shifter, which lies well within the measured/calculated penumbra reductions of Bäumer et al (2018 Phys. Med. Biol. 63 085020) of 17%/11%. The conclusions of Bäumer et al (2018 Phys. Med. Biol. 63 085020) and our previous work are therefore complementary, given the differences in the thicknesses of the beam modifying devices used in the two works. In addition, our analysis implies that initial beam characteristics are less important in determining the best order of components.