Axial (XY) central slices of the analytical human head phantom used for the simulations. The phantom is represented in its voxelized form. From left to right, along the X axis, we encounter the following materials: air, skin, skull bone, healthy brain, target (iodine+brain), and again some healthy brain, the skull bone, skin, and air. The tumor was defined centered (a) or off-centered by 4 cm (b).
Left: (a) axial and (b) sagittal CT slices from the patient bearing a brain metastasis that was used for studying a SSRT treatment plan on a realistic case. Right: (a′) axial and (b′) sagittal slices of the MCNPX geometry obtained after having defined the required structures and associated materials on the CT images. From lower to higher gray levels, we defined air, soft tissues, skull bone, brain, and tumor with iodine, respectively. The represented slices are from the tumor center level.
Dose enhancement factor versus the x-ray beam energy for the tumor located in the center of the analytical phantom and loaded by 10 mg/mL of iodine (long—dashed curve), gadolinium (short—dashed curve), or gold (solid curve).
(a) Relative dose distributions obtained on the human head analytical phantom, with the tumor centered. The 25%, 50%, and 90% isodoses are drawn on the normalized dose map obtained with an 80 keV single-arc irradiation geometry. (b)–(d) Lateral (along X) normalized dose profiles computed through the central voxel of the analytical human head phantom. Only the left part of the profiles is represented for a better understanding, as they are symmetrical. 80 keV and 120 kVp irradiations are compared in (b); 80 keV and 6 MV (linac) irradiations are compared in (c); 80, 120, and 180 kVp polychromatic x-ray tube beams are compared in (d). The profiles were normalized at the center of the tumor containing 10 mg/mL of iodine.
DVHs comparing three irradiation geometries: A continuous single-arc therapy (solid curves), three noncoplanar arcs (dashed curves), and a finite number of beams (ten beams, dotted curves) equally distributed around the tumor. The curves on the right side of each plot correspond to the tumor DVHs, the curves on the left with cross marks to the brain DVHs, and the curves in the middle to the skull bone DVHs. (a) represents the axial DVHs in the central axial slice of the phantom and (b) represents the volumetric DVHs.
DVHs comparing three margins sizes: 0 (dotted curve), 1 (dashed curve), and 3 mm (solid curve). The curves on the right side of the plot correspond to the tumor DVHs, the skull bone, the curves on the left with cross marks to the brain DVHs, and the curves in the middle to the skull bone DVHs.
Lateral dose profiles (X) for a tumor centered (dashed line) and a tumor off-centered by 4 cm (solid line), irradiated with an 80 keV monochromatic beam. The dose profiles were normalized to the dose delivered to the central voxel of the centered tumor (analytical phantom irradiated with a single arc).
Axial DVHs for ten nonweighted beams (dotted curve) and ten weighted beams (solid curve) in the human head analytical phantom irradiated at 80 keV. The curves on the right side of the plot correspond to the tumor DVHs, the curves on the left with cross marks to the brain DVHs, and the curves in the middle to the skull bone DVHs.
Relative dose map obtained (a) for the nonoptimized irradiation scheme with ten equally spaced beams, (b) for the ten weighted beam irradiation scheme, and (c) for seven weighted beams. The horizontal and vertical profiles crossing the target center and used for symmetry calculation purposes are shown in dark lines in (a). The volumetric DVHs for ten nonweighted beams (dashed curve), ten weighted beams (solid curve), and seven weighted beams (dotted curve) are also represented for an irradiation with 80 keV monochromatic x-rays (d) and for a 6 MV linear accelerator x-ray beam (e). The curves on the right side of each subplot correspond to the tumor DVHs, the curves with cross marks to the brain DVHs, and the curves without marks to the skull bone DVHs.
Treatment plans performed with the patient geometry (patient specific data). The initial incidence (angle 0°) corresponds to the right lateral beam and the irradiation was performed counter clockwise. Dp means prescribed dose (i.e., the dose at the target center).
Dose enhancement factors calculated in the centered tumor loaded with 10 mg/ml of iodine, gadolinium, or gold, for different beam qualities: 80 kVp, 120 kVp, 180 kVp, and 6 MV polychromatic x-ray beams and 37, 43, 58, and 80 keV monochromatic x-rays. stands for effective energy, which is the monochromatic energy that gives the same half-value layer (in mm of Al) than the one obtained from the corresponding polychromatic beam.
Dose variability versus the margin sizes for an irradiation of a centered tumor at 80 keV.
Dose variability for centered and off-centered tumors. These values are calculated in a ROI positioned in the middle of the tumor and on the entire tumor volume with no margins.
Dose profile asymmetry values for the three irradiation geometries obtained on horizontal and vertical profiles.
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