Flat-panel imager energy-dependent proton radiography for a proton pencil-beam scanning system

Abstract

In proton-based radiotherapy, proton radiography could allow for direct measurement of the water-equivalent path length (WEPL) in tissue, which can then be used to determine relative stopping power (RSP). Additionally, proton radiographs allow for imaging in the beam's-eye-view. In this work, a proton radiography technique using a flat-panel imager and a pencil-beam scanning (PBS) system is demonstrated in phantom. Proton PBS plans were delivered on a Varian ProBeam system to a flat-panel imager. Each proton plan consisted of energy layers separated by 4.8 MeV, and a field size of 25 cm × 25 cm. All measured data is binned into a layer-by-layer delivery in post processing. To build a calibration curve correlating detector response to WEPL, the plans were delivered to slabs of solid water of increasing thickness. Pixel-by-pixel detector response in the time/energy domain is then fit as a function of WEPL. Tissue equivalent phantoms are imaged for evaluation of WEPL accuracy. A spatial resolution phantom and a head phantom are also imaged. For all experiments, the detector was run with an effective pixel size of 0.4 mm × 0.4 mm. The proposed method reconstructed RSP with mean errors of 2.65%, −0.14%, and 0.61% for lung, soft tissue, and bone, respectively. In a 40 mm thick spatial resolution phantom, a 2 mm deep pinhole with 1 mm diameter can be seen. The accuracy and spatial resolution of the method show that it could be implemented for patient position verification. Further development could lead to patient-specific verification of RSP to be used for treatment guidance.

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

“…iop.org/PMB/66/21NT02/mmedia) and figure s1 in supplementary material). For calibration settings with ΔE5 MeV and ΔX5 mm, relative WEPL errors were reduced across all inserts, although higher relative WEPL errors were found in inserts corresponding to lung equivalent tissues with respect to other inserts (see figure s1) (Harms et al 2020). Lung equivalent inserts have the lowest densities, meaning that a sub-millimeter absolute WEPL error can result in a relative WEPL error of up to −2.5%.…”

“…In this study, FP-PR fields had energies from 70 to 225 MeV, which resulted in many pencil beams stopping inside the phantom. Therefore, it is imperative to develop a methodology that excludes the lowest energy layers that would get absorbed in a patient (Huo et al 2019, Harms et al 2020.…”

“…WEPL obtaining by means of FP-PR using ERDFs was investigated in silico and verified experimentally with an electron density phantom, achieving relative stopping power accuracy below 1.5% in silico and 2.65% experimentally (Huo et al 2019, Harms et al 2020. For a head and neck phantom, FP-PR simulations were performed, and FP-PR image acquisitions were evaluated qualitatively (Huo et al 2019, Harms et al 2020. Even though WEPL accuracy relies intrinsically on the sparseness of the FP calibration dataset (Harms et al 2020), research up to date has not yet provided an optimal FP calibration procedure, which is essential for accurate WEPL assessment using FP-PR.…”

“…For a head and neck phantom, FP-PR simulations were performed, and FP-PR image acquisitions were evaluated qualitatively (Huo et al 2019, Harms et al 2020. Even though WEPL accuracy relies intrinsically on the sparseness of the FP calibration dataset (Harms et al 2020), research up to date has not yet provided an optimal FP calibration procedure, which is essential for accurate WEPL assessment using FP-PR. In this work, WEPL accuracy was assessed in silico as a function of different calibration parameters with the purpose to find an optimal setting for FP calibration.…”

Range probing as a quality control tool for CBCT‐based synthetic CTs: In vivo application for head and neck cancer patients

“…Therefore, the ability to acquire bigger RP fields would be desired. The development of larger size detectors would allow for bigger RP field acquisitions 33 …”