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. 2021 Mar;48(3):1349-1355.
doi: 10.1002/mp.14698. Epub 2021 Feb 6.

Technical Note: CT calibration for proton treatment planning by cross-calibration with proton CT data

Paolo Farace  1   2 , Francesco Tommasino  2   3 , Roberto Righetto  1   2 , Francesco Fracchiolla  1   2 , Monica Scaringella  4 , Mara Bruzzi  4   5 , Carlo Civinini  4
Affiliations

Technical Note: CT calibration for proton treatment planning by cross-calibration with proton CT data

Paolo Farace et al. Med Phys. 2021 Mar.

Abstract

Purpose: This study explores the possibility of a new method for x-ray computed tomography (CT) calibration by means of cross-calibration with proton CT (pCT) data. The proposed method aims at a more accurate conversion of CT Hounsfield Units (HU) into proton stopping power ratio (SPR) relative to water to be used in proton-therapy treatment planning.

Methods: X-ray CT scan was acquired on a synthetic anthropomorphic phantom, composed of different tissue equivalent materials (TEMs). A pCT apparatus was instead adopted to obtain a reference three-dimensional distribution of the phantom's SPR values. After rigid registration, the x-ray CT was artificially blurred to the same resolution of pCT. Then a scatter plot showing voxel-by-voxel SPR values as a function of HU was employed to link the two measurements and thus obtaining a cross-calibrated x-ray CT calibration curve. The cross-calibration was tested at treatment planning system and then compared with a conventional calibration based on exactly the same TEMs constituting the anthropomorphic phantom.

Results: Cross-calibration provided an accurate SPR mapping, better than by conventional TEMs calibration. The dose distribution of single beams optimized on the reference SPR map was recomputed on cross-calibrated CT, showing, with respect to conventional calibration, minor deviation at the dose fall-off (lower than 1%).

Conclusions: The presented data demonstrated that, by means of reference pCT data, a heterogeneous phantom can be used for CT calibration, paving the way to the use of biological samples, with their accurate description of patients' tissues. This overcomes the limitations of conventional CT calibration requiring homogenous samples, only available by synthetic TEMs, which fail in accurately mimicking the properties of biological tissues. Once a heterogeneous biological sample is provided with its corresponding reference SPR maps, a cross-calibration procedure could be adopted by other PT centers, even when not equipped with a pCT system.

Keywords: CT; proton CT; proton therapy.

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References

REFERENCES

    1. Yang M, Zhu XR, Park PC, et al. Comprehensive analysis of proton range uncertainties related to patient stopping-power-ratio estimation using the stoichiometric calibration. Phys Med Biol. 2012;7:4095-4115.
    1. Wohlfahrt P, Möhler C, Enghardt W, et al. Refinement of the Hounsfield look-up table by retrospective application of patient-specific direct proton stopping-power prediction from dual-energy CT. Med Phys. 2020;47:1796-1806.
    1. Simard M, Bär E, Blais D, Bouchard H. Electron density and effective atomic number estimation in a maximum a posteriori framework for dual-energy computed tomography. Med Phys. 2020;47:4137-4149.
    1. De Smet V, Labarbe R, Vander Stappen F, Macq B, Sterpin E. Reassessment of stopping power ratio uncertainties caused by mean excitation energies using a water-based formalism. Med Phys. 2018;45:3361-3370.
    1. Bär E, Lalonde A, Zhang R, et al. Experimental validation of two dual-energy CT methods for proton therapy using heterogeneous tissue samples. Med Phys. 2018;45:48-59.

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