The impact of pencil beam scanning techniques on the effectiveness and efficiency of rescanning moving targets.

Therapeutic pencil beams are typically scanned using one of the following three techniques: spot scanning, raster scanning or line scanning. While providing similar dose distributions to the target, these three techniques can differ significantly in their delivery time sequence. Thus, we can expect differences in effectiveness and time efficiency when trying to mitigate interplay effects using rescanning. At the Paul Scherrer Institute, we are able to irradiate treatment plans using either of the three delivery techniques. Hence, we can compare them directly with identical underlying machine parameters such as energy switching time or minimum/maximum beam current. For this purpose, we selected three different liver targets, optimized plans for spots, and converted them to equivalent raster and line scanning plans. In addition to the scanning technique, we varied the underlying motion curve, starting phase, prescription dose and rescanning strategy, which resulted in a total of 1584 4D dose calculations and 49 measurements. They indicate that rescanning becomes effective when achieving a high number of rescans for every dose element. Fixed minimum spot weights for spot and raster scanning machines often hamper this. By introducing adaptive scaling of the beam current within iso-energy layers for line scanning, we can flexibly lower the minimum weight whenever required and achieve higher rescanning capability. Averaged over all scenarios studied, volumetric rescanning is significantly more effective than layered provided the same number of rescans are applied. Fast lateral scanning contributes to the efficiency of rescanning. We observed that in any given time window, we can always perform more rescans using raster or line scanning compared to spot scanning irradiations. Thus, we conclude that line scanning represents a promising technique for rescanning by combining both effectiveness and efficiency.