Shoot-through layers in upright proton arcs unlock advantages in plan quality and range verification

Upright proton therapy with compact delivery systems has the potential to reduce costs for treatments but could also lead to broadening of the beam penumbra due to energy selection close to the patient.

This study aims at combining upright static proton arcs with additional layers of shoot-through (ST) protons to sharpen the beam penumbra and improve plan quality for such systems. An additional advantage of the method is that it provides a straightforward approach for range verification with a fixed range detector opposite the fixed proton nozzle.

We examined various treatment plans for a virtual phantom: 3-beam IMPT, static arc (Arc) with/without ST (Arc+ST), and with/without collimation (+Coll). In the virtual phantom three different targets were utilized to study the effect on conformity index (CI), homogeneity index (HI), robustness and mean dose to the phantom volume. The phantom study was complemented with a head-and-neck (H&N) patient case with a similar set of plans. The delivery time for all plans was estimated using a combined model of the upright patient positioner and the proton nozzle. A range verification concept that determines residual ranges of the ST protons was studied in simulated scenarios for the H&N case.

In the phantom study, the Arc+ST plans show superior CI, HI and target robustness compared to the Arc+Coll plans. For the Arc plans without ST, the collimated plans perform better than the uncollimated plans. On the other hand, for Arc+ST, collimation has little impact on CI, HI and robustness. However, a small increase in the mean dose to the phantom volume is seen without collimation. For the H&N case, similar improvements for Arc+ST can be seen with only a marginal increase of the mean dose to the patient volume when no collimation is used. These results imply that no aperture is needed when combining arcs with ST, which in turn substantially reduces treatment times: for the H&N case the delivery time for Arc+ST is estimated to 5.4 min and for Arc+Coll to 6.5 min. The range verification simulation shows that the method is sensitive to detect systematic stopping power ratio errors, setup errors and changes in the patient anatomy.

Combining proton arcs and ST layers can enhance compact upright proton solutions by improving plan quality at the same time as delivery time is reduced. The concept is also tailored for the inclusion of a fast and straightforward residual range verification method.