(a) Cross-sectional diagram of the proposed EGRT system. (b) Snapshot of a LOR being detected in a simplified 3D rendering of the EGRT system.
EGRT treatment scheme consisting of three major components: simulation, pretreatment, and treatment.
Calculation of the planning map from three daughter modulation maps that are built in the same sinogram space and which contain the LOR response probability values to account for the corresponding types of modulation. To obtain the modified IMRT plan map, the stack of fluence maps resulting from inverse planning optimization are interpolated and reshaped into sinogram space, and subsequently modified to further suppress dose to nearby OARs. The attenuation correction map is converted from the forward projected CT image based on Eq. (4) . Similarly, the PET activity normalization map is calculated from the projection of the diagnostic/pretreatment PET scan, excluding the PTV region, based on Eq. (3) . Note that for all modulation maps, only sinogram bins whose corresponding directions intersect the PTV are calculated.
EGRT simulation workflow for the clinical patient case (starting from the shaded module on the top left). The workflow is divided into four major segments: imaging, planning, EGRT delivery, and dose evaluation, which is different from that for the digital XCAT patient in the imaging and planning steps. In the imaging step, the emission data are simulated using GATE and the phase information is known a priori. In the planning step, the IMRT plan is optimized using MOSEK and the inverse planning algorithms as discussed in Sec. II B . Note that the PTV intersection rule is implicitly implemented in the planning scheme.
Calculation of the IMRT plan map using Pinnacle3. (a) Pinnacle3 interface for inverse planning. (b) 256-field fluence maps. (c) The central sinogram of the IMRT plan map.
Dose distribution and associated DVH comparison of 3D IMRT [(a), solid lines] and hIMRT [(b), dashed-dotted lines]. The PTV and GTV are contoured using solid lines in the dose distributions.
Comparison of 3D IMRT [(a), thin solid line], raw EGRT [(b), thin dashed line], and EGRT with planning scheme that does not include [(c), dashed-dotted line] and includes [(d), thick solid line] additional OAR modulation. Note that the heart curves for (a) and (d) are mostly overlapping.
Tumor tracking of EGRT with the complete planning scheme. Both PTV and GTV are contoured to show the dose tracking. The dose map of each phase is displayed with an individually optimized window.
Comparison of 3D IMRT [(a), dashed-dotted line] and EGRT with planning scheme [(b), solid line] for the clinical patient case.
Tumor tracking of a breast cancer lung metastasis under EGRT with the planning scheme. The PTV and GTV are contoured for positional reference and target motion delineation, respectively. The dose maps are displayed with the same window [0.5 0.85] relative to the maximum GTV dose across all phases.
A summary of major simulation parameters.
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