Robustness of target dose coverage to motion uncertainties for scanned carbon ion beam tracking therapy of moving tumors

Abstract

Beam tracking with scanned carbon ion radiotherapy achieves highly conformal target dose by steering carbon pencil beams to follow moving tumors using real-time magnetic deflection and range modulation. The purpose of this study was to evaluate the robustness of target dose coverage from beam tracking in light of positional uncertainties of moving targets and beams. To accomplish this, we simulated beam tracking for moving targets in both water phantoms and a sample of lung cancer patients using a research treatment planning system. We modeled various deviations from perfect tracking that could arise due to uncertainty in organ motion and limited precision of a scanned ion beam tracking system. We also investigated the effects of interfractional changes in organ motion on target dose coverage by simulating a complete course of treatment using serial (weekly) 4DCTs from 6 lung cancer patients. For perfect tracking of moving targets, we found that target dose coverage was high (V̄95 was 94.8% for phantoms and 94.3% for lung cancer patients, respectively) but sensitive to changes in the phase of respiration at the start of treatment and to the respiratory period. Phase delays in tracking the moving targets led to large degradation of target dose coverage (up to 22% drop for a 15 degree delay). Sensitivity to technical uncertainties in beam tracking delivery was minimal for a lung cancer case. However, interfractional changes in anatomy and organ motion led to large decreases in target dose coverage (target coverage dropped approximately 8% due to anatomy and motion changes after 1 week). Our findings provide a better understand of the importance of each of these uncertainties for beam tracking with scanned carbon ion therapy and can be used to inform the design of future scanned ion beam tracking systems.

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.…”

A Monte‐Carlo‐based and GPU‐accelerated 4D‐dose calculator for a pencil beam scanning proton therapy system

“…During amplitude‐ or phase‐gated treatment, the patient's breathing is monitored and radiation is delivered only during specified portions of the breathing cycle . Tumor tracking is the most technically difficult method in which the tumor position is monitored in real time and the beam is actively steered to match the tumor location . For PBS delivery, layer or volume repainting/rescanning involves delivering the planned spots multiple times, with each iteration only delivering a fraction of the total number of protons to a given spot.…”

“…[22][23][24] Tumor tracking is the most technically difficult method in which the tumor position is monitored in real time and the beam is actively steered to match the tumor location. [25][26][27][28][29] For PBS delivery, layer or volume repainting/rescanning involves delivering the planned spots multiple times, with each iteration only delivering a fraction of the total number of protons to a given spot. This serves to average out local hot/cold spots by spreading spot positional errors over different positions in the breathing cycle.…”

Initial clinical evaluation of PET‐based ion beam therapy monitoring under consideration of organ motion

“…Interfractional changes of the respiratory motion, 16 as well as a limited correlation of external and internal movement [17][18][19] can lead to considerable deviations of the actually applied dose with respect to calculations based on the TP data. 20 Postirradiation PET imaging has the potential to identify fraction-specific deviations and hereby infer the actually applied treatment in the presence of target motion. The feasibility of time-resolved (4D) PET-based treatment verification has already been demonstrated in several moving phantom experiments.…”