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. 2022 Feb;32(1):74-84.
doi: 10.1016/j.zemedi.2020.09.004. Epub 2020 Nov 25.

Anthropomorphic lung phantom based validation of in-room proton therapy 4D-CBCT image correction for dose calculation

David Bondesson  1 , Arturs Meijers  2 , Guillaume Janssens  3 , Simon Rit  4 , Moritz Rabe  5 , Florian Kamp  5 , Katharina Niepel  6 , Lydia A den Otter  2 , Stefan Both  2 , Sebastien Brousmiche  3 , Julien Dinkel  7 , Claus Belka  8 , Katia Parodi  6 , Antje Knopf  9 , Christopher Kurz  10 , Guillaume Landry  11
Affiliations

Anthropomorphic lung phantom based validation of in-room proton therapy 4D-CBCT image correction for dose calculation

David Bondesson et al. Z Med Phys. 2022 Feb.

Abstract

Purpose: Ventilation-induced tumour motion remains a challenge for the accuracy of proton therapy treatments in lung patients. We investigated the feasibility of using a 4D virtual CT (4D-vCT) approach based on deformable image registration (DIR) and motion-aware 4D CBCT reconstruction (MA-ROOSTER) to enable accurate daily proton dose calculation using a gantry-mounted CBCT scanner tailored to proton therapy.

Methods: Ventilation correlated data of 10 breathing phases were acquired from a porcine ex-vivo functional lung phantom using CT and CBCT. 4D-vCTs were generated by (1) DIR of the mid-position 4D-CT to the mid-position 4D-CBCT (reconstructed with the MA-ROOSTER) using a diffeomorphic Morphons algorithm and (2) subsequent propagation of the obtained mid-position vCT to the individual 4D-CBCT phases. Proton therapy treatment planning was performed to evaluate dose calculation accuracy of the 4D-vCTs. A robust treatment plan delivering a nominal dose of 60Gy was generated on the average intensity image of the 4D-CT for an approximated internal target volume (ITV). Dose distributions were then recalculated on individual phases of the 4D-CT and the 4D-vCT based on the optimized plan. Dose accumulation was performed for 4D-vCT and 4D-CT using DIR of each phase to the mid position, which was chosen as reference. Dose based on the 4D-vCT was then evaluated against the dose calculated on 4D-CT both, phase-by-phase as well as accumulated, by comparing dose volume histogram (DVH) values (Dmean, D2%, D98%, D95%) for the ITV, and by a 3D-gamma index analysis (global, 3%/3mm, 5Gy, 20Gy and 30Gy dose thresholds).

Results: Good agreement was found between the 4D-CT and 4D-vCT-based ITV-DVH curves. The relative differences ((CT-vCT)/CT) between accumulated values of ITV Dmean, D2%, D95% and D98% for the 4D-CT and 4D-vCT-based dose distributions were -0.2%, 0.0%, -0.1% and -0.1%, respectively. Phase specific values varied between -0.5% and 0.2%, -0.2% and 0.5%, -3.5% and 1.5%, and -5.7% and 2.3%. The relative difference of accumulated Dmean over the lungs was 2.3% and Dmean for the phases varied between -5.4% and 5.8%. The gamma pass-rates with 5Gy, 20Gy and 30Gy thresholds for the accumulated doses were 96.7%, 99.6% and 99.9%, respectively. Phase-by-phase comparison yielded pass-rates between 86% and 97%, 88% and 98%, and 94% and 100%.

Conclusions: Feasibility of the suggested 4D-vCT workflow using proton therapy specific imaging equipment was shown. Results indicate the potential of the method to be applied for daily 4D proton dose estimation.

Keywords: 4D-vCT; Cone-beam; Motion; Proton therapy; Thorax; Tomography.

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Figures

Figure 1
Figure 1
Ex vivo functional porcine lung phantom used for CBCT and CT measurements.
Figure 2
Figure 2
Workflow to generate a 4D-vCT image from a 4D-CT image and CBCT projections. MA-ROOSTER uses the CBCT projections to generate a 4D-CBCT. The initial planning 4D-CT is used to generate a mid position CT as well as to extract the DVFs input to the MA-ROOSTER.
Figure 3
Figure 3
Workflow to calculate the accumulated dose for both 4D-CT and 4D-vCT. The optimized treatment plan was generated on the averaged intensity image of the 4D-CT. The treatment plan (upper left corner) was optimized on a voxel-wise image averaged along the 10 ventilation phases.
Figure 4
Figure 4
(1st to 3rd row) Display of sagittal slices from 4D-CT as well as reconstructed 4D-CBCT and 4D-vCT images (ventilation phases 2, 4, 6, 8 and 10). Dotted lines highlight the diaphragm position at maximum exhale. The ITV is depicted in yellow. (4th row) Sagittal slices of difference images (4D vCT minus 4D-CT) in HU. Red arrows highlight particular registration discrepancies occuring particularly around transitional points between tissue and the phantom. The black arrow highlights a vertical registration discrepancy at the same height as streaking artefacts produced in the CBCT reconstructions.
Figure 5
Figure 5
Dose distributions (top and middle row) and dose differences (bottom row) on phases 2, 4, 6, 8 and 10 between 4D-CT and 4D-vCT images. The ITV is displayed on the 4D-CT in yellow.
Figure 6
Figure 6
Comparison of DVH curves for the ITV and lungs (without the ITV) from 4D-vCT (solid) and 4D-CT (dotted) doses (all phases and accumulated).
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