Design of a focused collimator for proton therapy spot scanning using Monte Carlo methods

Abstract

Purpose: When designing a collimation system for pencil beam spot scanning proton therapy, a decision must be made whether or not to rotate, or focus, the collimator to match beamlet deflection as a function of off-axis distance. If the collimator is not focused, the beamlet shape and fluence will vary as a function of off-axis distance due to partial transmission through the collimator. In this work, we quantify the magnitude of these effects and propose a focused dynamic collimation system (DCS) for use in proton therapy spot scanning. Methods: This study was done in silico using a model of the Miami Cancer Institute's (MCI) IBA Proteus Plus system created in Geant4-based TOPAS. The DCS utilizes rectangular nickel trimmers mounted on rotating sliders that move in synchrony with the pencil beam to provide focused collimation at the edge of the target. Using a simplified setup of the DCS, simulations were performed at various off-axis locations corresponding to beam deflection angles ranging from 0°to 2.5°. At each offaxis location, focused (trimmer rotated) and unfocused (trimmer not rotated) simulations were performed. In all simulations, a 4 cm water equivalent thickness range shifter was placed upstream of the collimator, and a voxelized water phantom that scored dose was placed downstream, each with 4 cm airgaps. Results: Increasing the beam deflection angle for an unfocused trimmer caused the collimated edge of the beamlet profile to shift 0.08-0.61 mm from the baseline 0°simulation. There was also an increase in low-dose regions on the collimated edge ranging from 14.6% to 192.4%. Lastly, the maximum dose, D max , was 0-5% higher for the unfocused simulations. With a focused trimmer design, the profile shift and dose increases were all eliminated. Conclusions: We have shown that focusing a collimator in spot scanning proton therapy reduces dose at the collimated edge compared to conventional, unfocused collimation devices and presented a simple, mechanical design for achieving focusing for a range of source-to-collimator distances.

Innovations and the Use of Collimators in the Delivery of Pencil Beam Scanning Proton Therapy

“…The DCS has been the subject of several recent investigations about collimator design and potential use cases. From a design perspective, the placement of the collimators downstream of the range shifter reduces the lateral spread by 15% to 30%, compared to placing the collimators upstream of the range shifter [ 29 , 47 ]. Polyethylene was chosen as the range shifter material because it has the lowest scattering angle of all suitable plastics [ 60 ].…”

“…Conceptually, the DCS is a sliding-bar collimator that uses a common set of 3-cm-thick orthogonal trimmer blades placed downstream of an external 4-cm-thick polyethylene range shifter to intercept the proton beam as it scans near the target periphery [ 24 ]. To provide focused collimation, the DCS rotates the collimators to match the beam deflection angle as the collimators travel in a single plane, which can improve the lateral conformity [ 47 , 48 ]. By design, the DCS can effectively provide any aperture shape for all energy layers.…”

Experimental and Monte Carlo characterization of a dynamic collimation system prototype for pencil beam scanning proton therapy

“…In this capacity, this work also serves as an experimental basis for the continued development of dynamic collimators, which has been the subject of recent focus for the DCS using Monte Carlo methods to design focused collimators. 28 Additionally, the activation of positron-emitting nuclides is also non-negligible and should be considered when designing the encapsulation of a clinical system. While the proposed prototype is not a clinically viable device, it has provided an opportunity to benchmark the work that has previously relied solely on Monte Carlo to parameterize asymmetric proton beamlets and investigate the potential challenges that face clinical implementation of a dynamic collimation system.…”

“…Progressively, the results from this work have shown the sensitivity of the component alignment and influences of spot deflection toward a non‐focused collimator. In this capacity, this work also serves as an experimental basis for the continued development of dynamic collimators, which has been the subject of recent focus for the DCS using Monte Carlo methods to design focused collimators 28 . Additionally, the activation of positron‐emitting nuclides is also non‐negligible and should be considered when designing the encapsulation of a clinical system.…”

Characterization of penumbra sharpening and scattering by adaptive aperture for a compact pencil beam scanning proton therapy system

“…Hyer et al 12 provided a novel concept of dynamic collimation system (DCS) for penumbra reduction in spot scanning proton therapy. This work was continued by Geoghegan et al 20 who conducted an MC study regarding the design of focused DCS. They found that the focused DCS design reduced dose at the collimated edge as compared to the unfocused collimated devices.…”