Optimization of proton pencil beam positioning in collimated fields

Background The addition of static or dynamic collimator systems to the pencil beam scanning delivery technique increases the number of options for lateral field shaping. The collimator shape needs to be optimized together with the intensity modulation of spots. Purpose To minimize the proton field's lateral penumbra by investigating the fundamental relations between spot and collimating aperture edge position. Methods Analytical approaches describing the effect of spot position on the resulting spot profile are presented. The theoretical description is then compared with Monte Carlo simulations in TOPAS and in the RayStation treatment planning system, as well as with radiochromic film measurements at a clinical proton therapy facility. In the model, one single spot profile is analyzed for various spot positions in air. Further, irradiation setups in water with different energies, the combination with a range shifter, and two‐dimensional proton fields were investigated in silico. Results The further the single spot is placed beyond the collimating aperture edge (‘overscanning'), the sharper the relative lateral dose fall‐off and thus the lateral penumbra. Overscanning up to 5mm$5\,\text{mm}$ reduced the lateral penumbra by about 20% on average after a propagation of 13cm$13\,\text{cm}$ in air. This benefit from overscanning is first predicted by the analytical proofs and later verified by simulations and measurements. Corresponding analyses in water confirm the benefit in lateral penumbra with spot position optimization as observed theoretically and in air. The combination of spot overscanning with fluence modulation facilitated an additional improvement. Conclusions The lateral penumbra of single spots in collimated scanned proton fields can be improved by the method of spot overscanning. This suggests a better sparing of proximal organs at risk in smaller water depths at higher energies, especially in the plateau of the depth dose distribution. All in all, spot overscanning in collimated scanned proton fields offers particular potential in combination with techniques such as fluence modulation or dynamic collimation for optimizing the lateral penumbra to spare normal tissue.