Accelerated prompt gamma estimation for clinical proton therapy simulations

Prompt-gamma track-length estimator with time tagging from proton tracking

The design of prompt-gamma detectors necessitates numerous Monte Carlo simulations to precisely develop and optimize the detection stages in proton therapy. Alongside the advancement of MC simulations, various variance reduction methods have been explored to speed-up calculations. Among these techniques, track-length estimators are interesting scoring methods for achieving both speed and accuracy in Monte Carlo simulations of rare events. This paper introduces an extension of the GATE vpgTLE module that incorporates the prompt-gamma emission time, which is tagged from the proton tracking, enhancing its utility for studies focused on detector design and optimization that rely on time measurements. The results obtained from a clinical radiotherapy plan are presented. We demonstrate that the new vpgTLE tally with time tagging is accurate, except for certain prompt-gamma lines corresponding to long mean-life nuclei.

Advanced Monte Carlo simulations of emission tomography imaging systems with GATE

Built on top of the Geant4 toolkit, GATE is collaboratively developed for more than 15 years to design Monte Carlo simulations of nuclear-based imaging systems. It is, in particular, used by researchers and industrials to design, optimize, understand and create innovative emission tomography systems. In this paper, we reviewed the recent developments that have been proposed to simulate modern detectors and provide a comprehensive report on imaging systems that have been simulated and evaluated in GATE. Additionally, some methodological developments that are not specific for imaging but that can improve detector modeling and provide computation time gains, such as Variance Reduction Techniques and Artificial Intelligence integration, are described and discussed.

Accuracy of using high-energy prompt gamma to verify proton beam range with a Compton camera: A Monte Carlo simulation study

Prompt gamma (PG) rays emitted during proton therapy has been used for proton range verification. Because high-energy PG emission is well correlated to the Bragg peak (BP), high-energy PG rays are well-suited for proton range verification. However, the low production and detection of high-energy PG rays often lead to inaccurate BP position estimates. The aim of this study is to improve the BP position estimates obtained from high-energy PG rays. We propose a BP position estimation method based on the local maximum closest to the distal fall-off region. We present the results of Monte Carlo simulations in which a water phantom was irradiated with a proton beam. Our results show that the BP position estimated from the 6.13 MeV PG rays can be improved using the proposed position estimation method. Moreover, the 6.92 and 7.12 MeV PG rays can be used for predicting the BP position. However, the accuracy of the BP position estimation decreases with decreasing tissue oxygen levels. We also found that the subtraction of the PG images of 6.13 MeV from those of 6.92 and 7.12 MeV can be used to predict the BP position with a mean accuracy of < 2 mm. The accurate estimation of the BP position can be achieved using different high-energy PG rays, but factors including position estimation, irradiated tissue and event selection should be carefully taken into account.

Neutron track length estimator for GATE Monte Carlo dose calculation in radiotherapy

The out-of-field dose in radiation therapy is a growing concern in regards to the late side-effects and secondary cancer induction. In high-energy x-ray therapy, the secondary neutrons generated through photonuclear reactions in the accelerator are part of this secondary dose. The neutron dose is currently not estimated by the treatment planning system while it appears to be preponderant for distances greater than 50 cm from the isocenter. Monte Carlo simulation has become the gold standard for accurately calculating the neutron dose under specific treatment conditions but the method is also known for having a slow statistical convergence, which makes it difficult to be used on a clinical basis. The neutron track length estimator, a neutron variance reduction technique inspired by the track length estimator method has thus been developped for the first time in the Monte Carlo code GATE to allow a fast computation of the neutron dose in radiotherapy. The details of its implementation, as well as the comparison of its performances against the analog MC method, are presented here. A gain of time from 15 to 400 can be obtained by our method, with a mean difference in the dose calculation of about 1% in comparison with the analog MC method.

Use of a LYSO-based Compton camera for prompt gamma range verification in proton therapy

PURPOSE

A Compton camera (CC), which measures prompt gammas (PGs) emitted during proton therapy, is a potentially useful imaging device for proton range verification. The aim of this study was to evaluate how well the reconstructed PG images obtained from various two-stage CC configurations reproduce the distal falloff of the PG emission.

METHODS

We conducted Monte Carlo simulations to evaluate different two-stage CCs positioned orthogonal to a proton pencil beam irradiating a water phantom. The results were compared with those obtained for a three-stage CC. In particular, all detectors were made of lutetium-yttrium orthosilicate (LYSO) crystals.

RESULTS

We found that: (a) the position resolution of the detector led to more uncertainty in predicting the depth of maximum emission and distal falloff positions than did the energy resolution of the detector; (b) reducing the thickness of the absorber detector reduces the effect of position resolution on the quality of reconstructed images and improves falloff position estimates; (c) incomplete absorption of PGs can be filtered by restricting incident gamma energies to known PG energy spectral peaks; and (d) there is greater bias and less accuracy in predicting distal falloff positions with the three-stage CC compared with the two-stage CC.

CONCLUSIONS

This study demonstrates the feasibility of using various CC designs and event selection methods to improve the imaging of PG rays. In our designed two-stage CCs, the thin LYSO-based absorber can provide better predictions of the distal falloff positions than the thick one. Compared to three-stage CCs, two-stage CCs are less biased and provide more accurate range verification.