Ex vivovalidation of a stoichiometric dual energy CT proton stopping power ratio calibration

Abstract

A major source of uncertainty in proton therapy is the conversion of Hounsfield unit (HU) to proton stopping power ratio relative to water (SPR). In this study, we measured and quantified the accuracy of a stoichiometric dual energy CT (DECT) SPR calibration. We applied a stoichiometric DECT calibration method to derive the SPR using CT images acquired sequentially at [Formula: see text] and [Formula: see text]. The dual energy index was derived based on the HUs of the paired spectral images and used to calculate the effective atomic number (Z ), relative electron density ([Formula: see text]), and SPRs of phantom and biological materials. Two methods were used to verify the derived SPRs. The first method measured the sample's water equivalent thicknesses to deduce the SPRs using a multi-layer ion chamber (MLIC) device. The second method utilized Gafchromic EBT3 film to directly compare relative ranges between sample and water after proton pencil beam irradiation. Ex vivo validation was performed using five different types of frozen animal tissues with the MLIC and three types of fresh animal tissues using film. In addition, the residual ranges recorded on the film were used to compare with those from the treatment planning system using both DECT and SECT derived SPRs. Bland-Altman analysis indicates that the differences between DECT and SPR measurement of tissue surrogates, frozen and fresh animal tissues has a mean of 0.07% and standard deviation of 0.58% compared to 0.55% and 1.94% respectively for single energy CT (SECT) and SPR measurement. Our ex vivo study indicates that the stoichiometric DECT SPR calibration method has the potential to be more accurate than SECT calibration under ideal conditions although beam hardening effects and other image artifacts may increase this uncertainty.

Reassessment of stopping power ratio uncertainties caused by mean excitation energies using a water‐based formalism

“…Experimental data are under acquisition by our research group and will be disclosed in future publications. However, data on experimental verifications of SPR predictions with DECT have been recently reported . In all three studies, animal tissue samples were used.…”

Theoretical and experimental analysis of photon counting detector CT for proton stopping power prediction

“…Three further studies have evaluated DECT-based SPR estimation on fresh tissue samples. [13][14][15] It should be noted that the implementation of the H€ unemohr method used in this study is not identical to Siemens syngo.via Rho\Z software, as this commercial software includes a noise reduction step, and no noise reduction was included in this study. Furthermore, the used noise levels in this study are higher than typically seen in DECT scans.…”

“…These studies include investigations on how to perform a robust calibration of the conversion from CT numbers in conventional single‐energy CT (SECT) to SPR, the influence of beam hardening (BH), the use of dual‐energy CT (DECT), as well as the use of proton CT . It has been shown in several studies that DECT gives more accurate SPR estimates than SECT using the stoichiometric method proposed by Schneider et al …”

“…Several recent studies have investigated how to improve stopping power ratio (SPR) estimations with the aim of improving the accuracy of CT-based dose calculations in proton therapy. These studies include investigations on how to perform a robust calibration of the conversion from CT numbers in conventional single-energy CT (SECT) to SPR, 1 the influence of beam hardening (BH), 2 the use of dual-energy CT (DECT), 3 as well as the use of proton CT. [4][5][6] It has been shown in several studies [7][8][9][10][11][12][13][14][15][16][17][18][19] that DECT gives more accurate SPR estimates than SECT using the stoichiometric method proposed by Schneider et al 19 . In spite of that, concerns have been raised that DECT is more influenced by image noise than SECT.…”

Comparison of single and dual energy CT for stopping power determination in proton therapy of head and neck cancer

“…Patient-specific tissue variations can also cause large SPR estimation errors when estimated based on SECT [5]. Dual-energy CT (DECT) has been proposed by several groups for improving the SPR accuracy compared to SECT [4][5][6][7][8], and DECT has been shown to be superior to SECT for organic tissue samples [9][10][11][12].…”