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

PURPOSE

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.

METHODS

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.

RESULTS

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.

CONCLUSIONS

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.

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

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 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.

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.