Abstract
Proton therapy treatment for lungs remains challenging as images enabling the detection of inter- and intra-fractional motion, which could be used for proton dose adaptation, are not readily available. 4D computed tomography (4DCT) provides high image quality but is rarely available in-room, while in-room 4D cone beam computed tomography (4DCBCT) suffers from image quality limitations stemming mostly from scatter detection. This study investigated the feasibility of using virtual 4D computed tomography (4DvCT) as a prior for a phase-per-phase scatter correction algorithm yielding a 4D scatter corrected cone beam computed tomography image (4DCBCTcor), which can be used for proton dose calculation. 4DCT and 4DCBCT scans of a porcine lung phantom, which generated reproducible ventilation, were acquired with matching breathing patterns. Diffeomorphic Morphons, a deformable image registration algorithm, was used to register the mid-position 4DCT to the mid-position 4DCBCT and yield a 4DvCT. The 4DCBCT was reconstructed using motion-aware reconstruction based on spatial and temporal regularization (MA-ROOSTER). Successively for each phase, digitally reconstructed radiographs of the 4DvCT, simulated without scatter, were exploited to correct scatter in the corresponding CBCT projections. The 4DCBCTcor was then reconstructed with MA-ROOSTER using the corrected CBCT projections and the same settings and deformation vector fields as those already used for reconstructing the 4DCBCT. The 4DCBCTcor and the 4DvCT were evaluated phase-by-phase, performing proton dose calculations and comparison to those of a ground truth 4DCT by means of dose-volume-histograms (DVH) and gamma pass-rates (PR). For accumulated doses, DVH parameters deviated by at most 1.7% in the 4DvCT and 2.0% in the 4DCBCTcor case. The gamma PR for a (2%, 2 mm) criterion with 10% threshold were at least 93.2% (4DvCT) and 94.2% (4DCBCTcor), respectively. The 4DCBCTcor technique enabled accurate proton dose calculation, which indicates the potential for applicability to clinical 4DCBCT scans.
Highlights
Proton therapy (PT) offers a ballistic advantage over photon therapy as there is considerably less energy deposition along the trajectory to the target and a negligible amount beyond, due to the complete stopping of protons in matter at therapeutic energies
We extend the SCA of projections to a novel phase-based 4D cone beam computed tomography (4DCBCT) correction method, Figure 1. 4D computed tomography (4DCT) images of the inhale and exhale phases of the two different motions are displayed with level = −300 and window = 1600
The 4DvCT and 4DCBCTcor methods were further evaluated by comparing dose-volume histograms (DVHs) parameters (D2%, D98%, Dmean,ITV, Dmean,lung) and calculating global gamma PR using (3%, 3 mm) and (2%, 2 mm) criteria with a 10% dose threshold for lesion-specific robust proton plans versus day-of-treatment reference 4DCT for individual phases and accumulated doses
Summary
Proton therapy (PT) offers a ballistic advantage over photon therapy as there is considerably less energy deposition along the trajectory to the target and a negligible amount beyond, due to the complete stopping of protons in matter at therapeutic energies. Respiration patterns can vary markedly between different fractions (McClelland et al 2010, Zhang et al 2015), and for the treatment of moving tumours in the lung, it is of paramount importance to have daily 4D imaging. This would enable the detection of those variations and ideally allow their correction with treatment adaptation (Hoffmann et al 2017, Jakobi et al 2017, Tseng et al 2018, Albertini et al 2020), so that the full potential of IMPT can be exploited. It would be beneficial to use cone beam computed tomography (CBCT) images, which are routinely acquired in the scope of image-guided PT for patient positioning, to reconstruct the delivered fraction dose
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