Optimizing calibration settings for accurate water equivalent path length assessment using flat panel proton radiography

Carmen Seller Oria,Stefan Both,Jeffrey Free,Arturs Meijers,Johannes A Langendijk,Antje C Knopf,Gabriel Guterres Marmitt

doi:10.1088/1361-6560/ac2c4f

Carmen Seller Oria, Stefan Both + Show 5 more

Open Access

https://doi.org/10.1088/1361-6560/ac2c4f

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Abstract

Objective: Proton range uncertainties can compromise the effectiveness of proton therapy treatments. Water equivalent path length (WEPL) assessment by flat panel detector proton radiography (FP-PR) can provide means of range uncertainty detection. Since WEPL accuracy intrinsically relies on the FP-PR calibration parameters, the purpose of this study is to establish an optimal calibration procedure that ensures high accuracy of WEPL measurements. To that end, several calibration settings were investigated. Approach: FP-PR calibration datasets were obtained simulating PR fields with different proton energies, directed towards water-equivalent material slabs of increasing thickness. The parameters investigated were the spacing between energy layers (ΔE) and the increment in thickness of the water-equivalent material slabs (ΔX) used for calibration. 30 calibrations were simulated, as a result of combining ΔE = 9, 7, 5, 3, 1 MeV and ΔX = 10, 8, 5, 3, 2, 1 mm. FP-PRs through a CIRS electron density phantom were simulated, and WEPL images corresponding to each calibration were obtained. Ground truth WEPL values were provided by range probing multi-layer ionization chamber simulations on each insert of the phantom. Relative WEPL errors between FP-PR simulations and ground truth were calculated for each insert. Mean relative WEPL errors and standard deviations across all inserts were computed for WEPL images obtained with each calibration. Main results: Large mean and standard deviations were found in WEPL images obtained with large ΔE values (ΔE = 9 or 7 MeV), for any ΔX. WEPL images obtained with ΔE ≤ 5 MeV and ΔX ≤ 5 mm resulted in a WEPL accuracy with mean values within ±0.5% and standard deviations around 1%. Significance: An optimal FP calibration in the framework of this study was established, characterized by 3 MeV ≤ ΔE ≤ 5 MeV and 2 mm ≤ ΔX ≤ 5 mm. Within these boundaries, highly accurate WEPL acquisitions using FP-PR are feasible and practical, holding the potential to assist future online range verification quality control procedures.

Highlights

Range probing and proton radiography (PR) have been proposed as tools to detect and mitigate sources of range uncertainty (Mumot et al 2010)
(Gottschalk et al 2011, Testa et al 2013, Doolan et al 2015), scintillators with charge-coupled devices (Zygmanski et al 2000, Ryu et al 2008), or flat panels (FP) (Jee et al 2017a, Zhang et al 2018), which are typically calibrated to the water equivalent path length (WEPL) experimentally or via Monte Carlo simulations (Poludniowski et al 2015, Würl et al 2020)
Mean and standard deviations are greatest for calibration settings with the largest ΔX and ΔE

Summary

Introduction

Range probing and proton radiography (PR) have been proposed as tools to detect and mitigate sources of range uncertainty (Mumot et al 2010). PR solutions, classified as list mode or integration detector configurations, were first developed in the context of double scattering proton therapy systems (Poludniowski et al 2015). List mode detector configurations are composed of upstream and/or downstream particle trackers, as well as a residual energy detector (Talamonti et al 2010, Johnson 2018). Given the growing prevalence of pencil beam scanning over double scattering systems, new PR integrating solutions compatible with pencil beam scanning were proposed (Mumot et al 2010, Telsemeyer et al 2012, Bentefour et al 2016)

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