Investigating aperture-based approximations to model a focused dynamic collimation system for pencil beam scanning proton therapy.

Purpose. The Dynamic Collimation System (DCS) is an energy layer-specific collimation device designed to reduce the lateral penumbra in pencil beam scanning proton therapy. The DCS consists of two pairs of nickel trimmers that rapidly and independently move and rotate to intercept the scanning proton beam and an integrated range shifter to treat targets less than 4 cm deep. This work examines the validity of a single aperture approximation to model the DCS, a commonly used approximation in commercial treatment planning systems, as well as higher-order aperture-based approximations for modeling DCS-collimated dose distributions.Methods. An experimentally validated TOPAS/Geant4-based Monte Carlo model of the DCS integrated with a beam model of the IBA pencil beam scanning dedicated nozzle was used to simulate DCS- and aperture-collimated 100 MeV beamlets and composite treatment plans. The DCS was represented by three different aperture approximations: a single aperture placed halfway between the upper and lower trimmer planes, two apertures located at the upper and lower trimmer planes, and four apertures, located at both the upstream and downstream faces of each pair of trimmers. Line profiles and three-dimensional regions of interest were used to evaluate the validity and limitations of the aperture approximations investigated.Results. For pencil beams without a range shifter, minimal differences were observed between the DCS and single aperture approximation. For range shifted beamlets, the single aperture approximation yielded wider penumbra widths (up to 18%) in the X-direction and sharper widths (up to 9.4%) in the Y-direction. For the example treatment plan, the root-mean-square errors (RMSEs) in an overall three-dimensional region of interest were 1.7%, 1.3%, and 1.7% for the single aperture, two aperture, and four aperture models, respectively. If the region of interest only encompasses the lateral edges outside of the target, the resulting RMSEs were 1.7%, 1.1%, and 0.5% single aperture, two aperture, and four aperture models, respectively.Conclusions. Monte Carlo simulations of the DCS demonstrated that a single aperture approximation is sufficient for modeling pristine fields at the Bragg depth while range shifted fields require a higher-order aperture approximation. For the treatment plan considered, the double aperture model performed the best overall, however, the four-aperture model most accurately modeled the lateral field edges at the expense of increased dose differences proximal to and within the target.

Nelson NP ，Culberson WS ，Hyer DE ，Smith BR ，Flynn RT ，Hill PM ... -  《Biomedical Physics & Engineering Express》

Development and validation of the Dynamic Collimation Monte Carlo simulation package for pencil beam scanning proton therapy.

The aim of this work was to develop and experimentally validate a Dynamic Collimation Monte Carlo (DCMC) simulation package specifically designed for the simulation of collimators in pencil beam scanning proton therapy (PBS-PT). The DCMC package was developed using the TOPAS Monte Carlo platform and consists of a generalized PBS source model and collimator component extensions. A divergent point-source model of the IBA dedicated nozzle (DN) at the Miami Cancer Institute (MCI) was created and validated against on-axis commissioning measurements taken at MCI. The beamline optics were mathematically incorporated into the source to model beamlet deflections in the X and Y directions at the respective magnet planes. Off-axis measurements taken at multiple planes in air were used to validate both the off-axis spot size and divergence of the source model. The DCS trimmers were modeled and incorporated as TOPAS geometry extensions that linearly translate and rotate about the bending magnets. To validate the collimator model, a series of integral depth dose (IDD) and lateral profile measurements were acquired at MCI and used to benchmark the DCMC performance for modeling both pristine and range shifted beamlets. The water equivalent thickness (WET) of the range shifter was determined by quantifying the shift in the depth of the 80% dose point distal to the Bragg peak between the range shifted and pristine uncollimated beams. A source model of the IBA DN system was successfully commissioned against on- and off-axis IDD and lateral profile measurements performed at MCI. The divergence of the source model was matched through an optimization of the source-to-axis distance and comparison against in-air spot profiles. The DCS model was then benchmarked against collimated IDD and in-air and in-phantom lateral profile measurements. Gamma analysis was used to evaluate the agreement between measured and simulated lateral profiles and IDDs with 1%/1 mm criteria and a 1% dose threshold. For the pristine collimated beams, the average 1%/1 mm gamma pass rates across all collimator configurations investigated were 99.8% for IDDs and 97.6% and 95.2% for in-air and in-phantom lateral profiles. All range shifted collimated IDDs passed at 100% while in-air and in-phantom lateral profiles had average pass rates of 99.1% and 99.8%, respectively. The measured and simulated WET of the polyethylene range shifter was determined to be 40.9 and 41.0 mm, respectively. We have developed a TOPAS-based Monte Carlo package for modeling collimators in PBS-PT. This package was then commissioned to model the IBA DN system and DCS located at MCI using both uncollimated and collimated measurements. Validation results demonstrate that the DCMC package can be used to accurately model other aspects of a DCS implementation via simulation.

Nelson NP ，Culberson WS ，Hyer DE ，Geoghegan TJ ，Patwardhan KA ，Smith BR ，Flynn RT ，Yu J ，Rana S ，Gutiérrez AN ，Hill PM ... -  《-》

PETRA: A pencil beam trimming algorithm for analytical proton therapy dose calculations with the dynamic collimation system.

The Dynamic Collimation System (DCS) has been shown to produce superior treatment plans to uncollimated pencil beam scanning (PBS) proton therapy using an in-house treatment planning system (TPS) designed for research. Clinical implementation of the DCS requires the development and benchmarking of a rigorous dose calculation algorithm that accounts for pencil beam trimming, performs monitor unit calculations to produce deliverable plans at all beam energies, and is ideally implemented with a commercially available TPS. To present an analytical Pencil bEam TRimming Algorithm (PETRA) for the DCS, with and without its range shifter, implemented in the Astroid TPS (.decimal, Sanford, Florida, USA). PETRA was derived by generalizing an existing pencil beam dose calculation model to account for the DCS-specific effects of lateral penumbra blurring due to the nickel trimmers in two different planes, integral depth dose variation due to the trimming process, and the presence and absence of the range shifter. Tuning parameters were introduced to enable agreement between PETRA and a measurement-validated Dynamic Collimation Monte Carlo (DCMC) model of the Miami Cancer Institute's IBA Proteus Plus system equipped with the DCS. Trimmer position, spot position, beam energy, and the presence or absence of a range shifter were all used as variables for the characterization of the model. The model was calibrated for pencil beam monitor unit calculations using procedures specified by International Atomic Energy Agency Technical Report Series 398 (IAEA TRS-398). The integral depth dose curves (IDDs) for energies between 70 MeV and 160 MeV among all simulated trimmer combinations, with and without the ranger shifter, agreed between PETRA and DCMC at the 1%/1 mm 1-D gamma criteria for 99.99% of points. For lateral dose profiles, the median 2-D gamma pass rate for all profiles at 1.5%/1.5 mm was 99.99% at the water phantom surface, plateau, and Bragg peak depths without the range shifter and at the surface and Bragg peak depths with the range shifter. The minimum 1.5%/1.5 mm gamma pass rates for the 2-D profiles at the water phantom surface without and with the range shifter were 98.02% and 97.91%, respectively, and, at the Bragg peak, the minimum pass rates were 97.80% and 97.5%, respectively. The PETRA model for DCS dose calculations was successfully defined and benchmarked for use in a commercially available TPS.

Bennett LC ，Hyer DE ，Erhart K ，Nelson NP ，Culberson WS ，Smith BR ，Hill PM ，Flynn RT ... -  《-》

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

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. 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 off-axis 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. 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. 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.

Geoghegan TJ ，Nelson NP ，Flynn RT ，Hill PM ，Rana S ，Hyer DE ... -  《-》

Dosimetric delivery validation of dynamically collimated pencil beam scanning proton therapy.

Objective. Pencil beam scanning (PBS) proton therapy target dose conformity can be improved with energy layer-specific collimation. One such collimator is the dynamic collimation system (DCS), which consists of four nickel trimmer blades that intercept the scanning beam as it approaches the lateral extent of the target. While the dosimetric benefits of the DCS have been demonstrated through computational treatment planning studies, there has yet to be experimental verification of these benefits for composite multi-energy layer fields. The objective of this work is to dosimetrically characterize and experimentally validate the delivery of dynamically collimated proton therapy with the DCS equipped to a clinical PBS system.Approach. Optimized single field, uniform dose treatment plans for 3 × 3 × 3 cm3target volumes were generated using Monte Carlo dose calculations with depths ranging from 5 to 15 cm, trimmer-to-surface distances ranging from 5 to 18.15 cm, with and without a 4 cm thick polyethylene range shifter. Treatment plans were then delivered to a water phantom using a prototype DCS and an IBA dedicated nozzle system and measured with a Zebra multilayer ionization chamber, a MatriXX PT ionization chamber array, and Gafchromic™ EBT3 film.Main results. For measurements made within the SOBPs, average 2D gamma pass rates exceeded 98.5% for the MatriXX PT and 96.5% for film at the 2%/2 mm criterion across all measured uncollimated and collimated plans, respectively. For verification of the penumbra width reduction with collimation, film agreed with Monte Carlo with differences within 0.3 mm on average compared to 0.9 mm for the MatriXX PT.Significance. We have experimentally verified the delivery of DCS-collimated fields using a clinical PBS system and commonly available dosimeters and have also identified potential weaknesses for dosimeters subject to steep dose gradients.

Nelson NP ，Culberson WS ，Hyer DE ，Geoghegan TJ ，Patwardhan KA ，Smith BR ，Flynn RT ，Yu J ，Gutiérrez AN ，Hill PM ... -  《-》