[en] The purpose of this work was to investigate the reduction in lateral dose penumbra that can be achieved when using a dynamic collimation system (DCS) for spot scanning proton therapy as a function of two beam parameters: spot size and spot spacing. This is an important investigation as both values impact the achievable dose distribution and a wide range of values currently exist depending on delivery hardware. Treatment plans were created both with and without the DCS for in-air spot sizes (σ_a_i_r) of 3, 5, 7, and 9 mm as well as spot spacing intervals of 2, 4, 6 and 8 mm. Compared to un-collimated treatment plans, the plans created with the DCS yielded a reduction in the mean dose to normal tissue surrounding the target of 26.2–40.6% for spot sizes of 3–9 mm, respectively. Increasing the spot spacing resulted in a decrease in the time penalty associated with using the DCS that was approximately proportional to the reduction in the number of rows in the raster delivery pattern. We conclude that dose distributions achievable when using the DCS are comparable to those only attainable with much smaller initial spot sizes, suggesting that the goal of improving high dose conformity may be achieved by either utilizing a DCS or by improving beam line optics. (note)

[en] Purpose: In the absence of a collimation system the lateral penumbra of spot scanning (SS) dose distributions delivered by low energy proton beams is highly dependent on the spot size. For current commercial equipment, spot size increases with decreasing proton energy thereby reducing the benefit of the SS technique. This paper presents a dynamic collimation system (DCS) for sharpening the lateral penumbra of proton therapy dose distributions delivered by SS. Methods: The collimation system presented here exploits the property that a proton pencil beam used for SS requires collimation only when it is near the target edge, enabling the use of trimmers that are in motion at times when the pencil beam is away from the target edge. The device consists of two pairs of parallel nickel trimmer blades of 2 cm thickness and dimensions of 2 cm × 18 cm in the beam's eye view. The two pairs of trimmer blades are rotated 90° relative to each other to form a rectangular shape. Each trimmer blade is capable of rapid motion in the direction perpendicular to the central beam axis by means of a linear motor, with maximum velocity and acceleration of 2.5 m/s and 19.6 m/s², respectively. The blades travel on curved tracks to match the divergence of the proton source. An algorithm for selecting blade positions is developed to minimize the dose delivered outside of the target, and treatment plans are created both with and without the DCS. Results: The snout of the DCS has outer dimensions of 22.6 × 22.6 cm² and is capable of delivering a minimum treatment field size of 15 × 15 cm². Using currently available components, the constructed system would weigh less than 20 kg. For irregularly shaped fields, the use of the DCS reduces the mean dose outside of a 2D target of 46.6 cm² by approximately 40% as compared to an identical plan without collimation. The use of the DCS increased treatment time by 1–3 s per energy layer. Conclusions: The spread of the lateral penumbra of low-energy SS proton treatments may be greatly reduced with the use of this system at the cost of only a small penalty in delivery time

[en] Treatment plans for patched-field proton therapy may not be clinically acceptable due to the dose heterogeneity introduced in the target when combining the dose distributions from two separate fields. MCNPX simulations were performed for various configurations of the Mevion S250 beamline to determine spread-out Bragg peak dose distributions and patched-field treatment plans delivered using a rotating modulator wheel to depths in the clinically relevant range between 5.0 and 30.0 cm. The dose non-uniformity (DNU) metric was defined as the difference between the maximum and minimum dose relative to the prescription observed in a patched dose distribution. The DNU was first evaluated for dose distributions from a standard delivery using constant beam current and combining through-field lateral dose profiles and with patch-field distal dose profiles. Patch-field distal dose profiles were then optimized using beam current modulation in an attempt to better complement the through-field lateral dose profiles when combined into a patched dose distribution. Using standard deliveries, DNU was 10% or less only when patching lateral profiles 12.5–17.5 cm deep. Significantly greater DNU was observed for patches outside of this range, at times exceeding 35%. Using optimized distal profiles, DNU was reduced to 10% or less for all lateral profiles deeper than 15.0 cm. Optimizing beam current modulation was found to create distal profiles with more gradual dose falloff than found in a standard delivery, allowing optimized distal dose distributions to sum more homogeneously with lateral dose distributions. The hot or cold spots that often appear in patched dose distributions from standard deliveries may therefore be mitigated by optimizing beam current. This method may also be applied to systems other than the Mevion system to further improve patched-field dose homogeneity. (paper)

[en] Neutron production is of concern for proton therapy, especially for passive scattering proton beam delivery methods. The levels of neutron dose equivalent vary significantly with system design and treatment parameters. The purpose of this study was to examine neutron dose equivalent per therapeutic dose (H/D) around the Mevion S250 proton therapy system, a novel design of proton therapy systems. The benchmark comparisons between measurement and simulation were found to be within a factor of 2 for most cases. The H/D values were evaluated as a function of various parameters. The results showed that, at a standard reference condition (10 × 10 cm"2 field size, distance 1 m detector-to-isocenter lateral to the primary proton beam direction), the H/D values range from 0.72 to 3.37 mSv Gy"−"1 for all configurations studied. The H/D values generally (1) decreased as the neutron detectors moved away from the isocenter, (2) decreased with increasing aperture field sizes, (3) increased with increasing angle from the initial beam axis and (4) were independent of treatment nozzle position. The H/D trends were consistent with other existing passive scattering proton accelerators reported in the literature. (paper)

[en] Purpose: To introduce a method to model the 3D dose distribution of laterally asymmetric proton beamlets resulting from collimation. The model enables rapid beamlet calculation for spot scanning (SS) delivery using a novel penumbra-reducing dynamic collimation system (DCS) with two pairs of trimmers oriented perpendicular to each other. Methods: Trimmed beamlet dose distributions in water were simulated with MCNPX and the collimating effects noted in the simulations were validated by experimental measurement. The simulated beamlets were modeled analytically using integral depth dose curves along with an asymmetric Gaussian function to represent fluence in the beam’s eye view (BEV). The BEV parameters consisted of Gaussian standard deviations (sigmas) along each primary axis (σ_x1,σ_x2,σ_y1,σ_y2) together with the spatial location of the maximum dose (μ_x,μ_y). Percent depth dose variation with trimmer position was accounted for with a depth-dependent correction function. Beamlet growth with depth was accounted for by combining the in-air divergence with Hong’s fit of the Highland approximation along each axis in the BEV. Results: The beamlet model showed excellent agreement with the Monte Carlo simulation data used as a benchmark. The overall passing rate for a 3D gamma test with 3%/3 mm passing criteria was 96.1% between the analytical model and Monte Carlo data in an example treatment plan. Conclusions: The analytical model is capable of accurately representing individual asymmetric beamlets resulting from use of the DCS. This method enables integration of the DCS into a treatment planning system to perform dose computation in patient datasets. The method could be generalized for use with any SS collimation system in which blades, leaves, or trimmers are used to laterally sharpen beamlets

[en] The dynamic collimation system (DCS) can be combined with pencil beam scanning proton therapy to deliver highly conformal treatment plans with unique collimation at each energy layer. This energy layer-specific collimation is accomplished through the synchronized motion of four trimmer blades that intercept the proton beam near the target boundary in the beam’s eye view. However, the corresponding treatment deliveries come at the cost of additional treatment time since the translational speed of the trimmer is slower than the scanning speed of the proton pencil beam. In an attempt to minimize the additional trimmer sequencing time of each field while still maintaining a high degree of conformity, a novel process utilizing ant colony optimization (ACO) methods was created to determine the most efficient route of trimmer sequencing and beamlet scanning patterns for a collective set of collimated proton beamlets. The ACO process was integrated within an in-house treatment planning system optimizer to determine the beam scanning and DCS trimmer sequencing patterns and compared against an analytical approximation of the trimmer sequencing time should a contour-like scanning approach be assumed instead. Due to the stochastic nature of ACO, parameters where determined so that they could ensure good convergence and an efficient optimization of trimmer sequencing that was faster than an analytical contour-like trimmer sequencing. The optimization process was tested using a set of three intracranial treatment plans which were planned using a custom research treatment planning system and were successfully optimized to reduce the additional trimmer sequencing time to approximately 60 s per treatment field while maintaining a high degree of target conformity. Thus, the novel use of ACO techniques within a treatment planning algorithm has been demonstrated to effectively determine collimation sequencing patterns for a DCS in order to minimize the additional treatment time required for trimmer movement during treatment. (paper)

[en] Patients receiving pencil beam scanning (PBS) proton therapy with the addition of a dynamic collimation system (DCS) are potentially subject to an additional neutron dose from interactions between the incident proton beam and the trimmer blades. This study investigates the secondary neutron dose rates for both single-field uniform dose (SFUD) and intensity modulated proton therapy treatments.