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

Abstract

There has been a growing interest in the development of energy-specific collimators for low-energy pencil beam scanning (PBS) to reduce the lateral penumbra. One particular device that has been the focus of several recent published works is the dynamic collimation system (DCS), which provides energy-specific collimation by intercepting the scanned proton beam as it nears to target edge with a set of orthogonal trimmer blades. While several computational studies have shown that this dynamic collimator can provide additional healthy tissue sparing, there has not been any rigorous experimental work to benchmark the theoretical models used in these initial studies. Therefore, it was the purpose of this work to demonstrate an experimental method that could integrate an experimental prototype with a clinical PBS system and benchmark the Monte Carlo methods that have been used to model the DCS. Methods: An experimental DCS prototype was designed and built in house to actively collimate individual proton beamlets during PBS within a well-characterized experimental setup. Monte Carlo methods were initially used to assess construction tolerances and later benchmarked against measurements, including integral depth dose and lateral asymmetric beamlet profiles. The experimental apparatus and measurement geometry were modeled using MCNP6 benchmarked from measurements performed at the Northwestern Chicago Proton Center. Results: Gamma analysis tests were used to evaluate the agreement between the measured and simulated profiles with a strict 1 mm/1% criteria and 5% dose threshold. Excellent agreement was observed between the simulated and measured profiles, which included 1 mm/1% gamma analysis pass rates of at least 100% and 95% for the integral depth dose (IDD) profiles and lateral profiles, respectively. Differences in the relative profile shape were observed experimentally between beamlets collimated on-and off-axis, which was attributed to the partial transmission of the beam through an unfocused collimator. Exposure rates resulting from the activation of the device were monitored with survey meter measurements and were found to agree with Monte Carlo estimates of the exposure rate to within 20%. Conclusion: A DCS prototype was constructed and integrated into a clinical dose delivery system. While the results of this work are not exhaustive, they demonstrate the effects of beam source divergence, device activation, and beamlet deflection during scanning, which were found to be successfully modeled using Monte Carlo methods and experimentally benchmarked. Excellent agreement was achieved between the simulated and measured lateral spot profiles of collimated beamlets delivered on-and off-axis in PBS. The Monte Carlo models adequately predicted the measured elevated plateau region in the integral depth-dose profiles from the low-energy scatter off the collimators.

The dosimetric enhancement of GRID profiles using an external collimator in pencil beam scanning proton therapy

“…The trimmer offsets from the mechanical axis to the radiation coordinate system were calibrated using the beamline's onboard radiograph system 33 . The negative imprint of each trimmer to the field was then related to the axis of the radiation field from which all subsequent offsets were referenced 33,34 …”

“…Simulation executables were built from the Geant4 user toolkit coded in C++ using the inherent class structures to define the simulation geometry, transport physics, and dose tallies. GRID simulations were performed locally on a quad‐core 2.8 GHz i7 processor for the IBA Universal Nozzle (UN) model from the Northwestern Medicine proton center in Geant4 using a model that was previously benchmarked using an experimental DCS prototype 34,39 . A Dynamic Collimation Monte Carlo (DCMC) simulation package 40 developed in TOPAS (Version 3.7) 37,38 was used to simulate GRID profiles delivered from an IBA dedicated nozzle (DN) system from the Miami Cancer Center (MCI) Proteus One beamline.…”

“…33 The negative imprint of each trimmer to the field was then related to the axis of the radiation field from which all subsequent offsets were referenced. 33,34 Integral depth dose (IDD) measurements and lateral profiles were acquired for uncollimated and GRID dose distributions collimated by the DCS. An image of the DCS fixed to the fully extended nozzle is shown in Figure 1.…”

“…GRID simulations were performed locally on a quad-core 2.8 GHz i7 processor for the IBA Universal Nozzle (UN) model from the Northwestern Medicine proton center in Geant4 using a model that was previously benchmarked using an experimental DCS prototype. 34,39 A Dynamic Collimation Monte Carlo (DCMC) simulation package 40 developed in TOPAS (Version 3.7) 37,38 was used to simulate GRID profiles delivered from an IBA dedicated nozzle (DN) system from the Miami Cancer Center (MCI) Proteus One beamline. The reader is referred to the work of Nelson et al 40 and Smith 35,39 for a complete discussion of the beamlet model.…”

A novel optimization algorithm for enabling dynamically collimated proton arc therapy

“…Specialized optimization algorithms have also been developed to maximize the achievable tumor dose conformity and determine the most efficient sequence pattern of the trimmers to deliver the treatment plan with minimal impact on the overall plan quality 10 , 15 . While not yet a clinical product, there have been advancements in modeling the primary and secondary radiations resulting from the DCS using Monte Carlo methods 16 – 18 and developing a suitable calculation model with a commercial treatment planning system vendor 7 .…”