Multigating, a 4D Optimized Beam Tracking in Scanned Ion Beam Therapy

Abstract

The treatment of moving tumors with a scanned ion beam is challenging due to interplay effects and changing beam range. We propose multigating, as a method for 4D-treatment optimization and delivery. In 3D beam tracking, tracking vectors are added during delivery to beam spot positions based on the detected motion phase. This has the disadvantage of dose errors in case of complex motion patterns and an uncertain out-of-target dose distribution. In multigating, the motion phase for each beam spot is predefined, which allows to add the tracking vector prior to beam weight optimization on all motion phases. The synchronization of delivery and target motion is assured by fast gating. The feasibility of the delivery was shown in a film experiment and required only minor software modification to the treatment planning system. In a treatment planning study in 4 lung cancer patients, target coverage could be restored to the level of a static reference plan by multigating (V95 > 99%) but not by standard beam tracking (V95 < 95%). The conformity of the multigating plans was only slightly lower than those of the static plan, with a conformity number of 72.0% (median, range 64.6–76.6%) compared to 75.8% (70.8–81.5%) in spite of target motion of up to 22 mm. In conclusion, we showed the technical feasibility of multigating, a 4D-optimization and delivery method using scanned beams that allows for conformal and homogeneous dose delivery to moving targets also in case of complex motion.

Exploratory Study of 4D versus 3D Robust Optimization in Intensity Modulated Proton Therapy for Lung Cancer

“…Only a few studies in 4D optimization have been published in particle therapy (47–50). In the aforementioned literature, phase-specific plans, or the so-called 4D raster treatment plans (RST) (51), which describe which beam spots are to be delivered to which motion phase, are generated.…”

Robust optimization in IMPT using quadratic objective functions to account for the minimum MU constraint

“…IMPT is precise but is sensitive to uncertainties caused by patient setup and range uncertainties, respiratory motion, and anatomic changes . Several robust optimization methods have been proposed and have been shown to have great advantages by achieving robust plans while maintaining high plan quality . Unfortunately, in proton therapy besides uncertainties, there are machine hardware constraints, for example, field‐size constraint, minimum–maximum energy constraint, dose‐rate constraint for synchrotron‐based proton therapy systems, and minimum–maximum monitor unit (MU) constraint.…”

“…5,6 Several robust optimization methods have been proposed and have been shown to have great advantages by achieving robust plans while maintaining high plan quality. 2,[7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] Unfortunately, in proton therapy besides uncertainties, there are machine hardware constraints, for example, field-size constraint, minimum-maximum energy constraint, dose-rate constraint for synchrotron-based proton therapy systems, and minimum-maximum monitor unit (MU) constraint. Among them, the minimum MU constraint is the most prominent one.…”

Motion management in particle therapy

“…The choice of rescanning method and the magnitude of minimization of the interplay effect are strongly dependent on the specific characteristics of the irradiation system (scanning speed, beam energy change time, beam spot size, number of beam fields etc.). Therefore, it has been shown that it is especially beneficial to combine several motion mitigation techniques such as rescanning and gating or tracking (described in the next section) and gating . Zhang et al.…”