Tissue inhomogeneities in Monte Carlo treatment planning for
Tissue inhomogeneities in Monte Carlo treatment
planning for proton therapy
L. Beaulieu1, M. Bazalova2,3, C. Furstoss4, F. Verhaegen2,5
(1) Centre Hospitalier Univ de Quebec, Quebec, QC, CA, (2) McGill University, Montreal, QC, CA, (3) Stanford University, Stanford, CA,
(4) Hopital Maisonneuve-Rosemont, Montreal, QC, CA, (5) Maastro Clinic, Maastricht, NL
Metal streaking artifacts: The effect of metal streaking artifacts and their
correction based on sinogram interpolation on MC proton beam dose
calculations was studied on a patient with bilateral hip prostheses. Dose
Proton therapy is gaining popularity in the treatment of cancer and the
calculations were performed for three different simulation geometries:
need for an accurate treatment planning system is obvious. Monte Carlo
considering only tissue of uniform density 1 g/cm3 (the water-only
(MC) dose calculation, despite the relatively long computation time, is the
geometry), and using a CT number to material and mass density
most accurate way to determine the dose delivered to the patient during
calibration curve with original CT images containing streaking artifacts
radiation therapy. Whereas MC dose calculations for conventional
and with artifact corrected CT images.
photon and electron radiotherapy have been studied extensively, proton
A treatment plan with two 147 MeV proton beams (45 and 315) was
beam MC dose calculations have only recently received attention. In this
simulated in the MCNPX code. First, the spread-out Bragg peak (SOBP)
work, the importance of tissue segmentation in proton therapy is
was designed using simulations in a uniform water phantom (fig 2a). It is
investigated using dual-energy CT (DECT) imaging. Another challenge in
impossible to model a modulator wheel in the MCNPX code, and
MC dose calculation treatment planning is metal streaking CT artifacts
therefore the steps of the modulator wheel were approximated by 5 mm
with the associated tissue and mass density miss-assignment. Their
thick PMMA blocks. In order to calculate the dose distribution of the
effect on MC proton beam dose calculations is studied for a prostate
SOBP in the patient in a single MC simulation, 11 PMMA blocks were
patient with bilateral hip prostheses.
inserted in the path of the (66) cm2 beams and the source particles (147
MeV protons) were sampled with their respective weights from the
Materials and Methods
volume between the blocks (fig 2b). The patient CT images with
Figure 3: The exact dose distribution (D exact) using a 200 MeV proton beam (a).
PDD with two inhomogeneities (SB3 and B200) (b), the 2% dose calculation
error is indicated by the arrow. The dose differences from D exact for Dsingle (c) and
(1.91.920) mm3 voxels were segmented into 4 materials (air, tissue,
bone and steel) using 0.1 g/cm3 mass density bins.
Tissue segmentation with dual-energy CT: CT images of a 30 cm
The dose distribution is significantly distorted in the original CT geometry
due to the artifacts (fig 4b). The apparent air between the prostheses
diameter cylindrical phantom with 9 tissue equivalent inserts (table1, fig
results in inaccurate doses with large statistical errors. Additionally due to
1a) were segmented into material and mass density maps using single-
the air, the 20% and 30% isodose lines extend by 1.5 cm in the healthy
energy CT (fig 1b) and DECT (fig 1c) material extraction. DECT tissue
tissue. This might cause problems in treatment planning and its
segmentation can distinguish materials with similar relative electron
optimization. The artifact corrected geometry produced a dose
densities e having different effective atomic numbers Zeff. The effect of
distribution similar to the water-only dose distribution (fig 4b). The true
inaccurate material segmentation for the two soft bone equivalent
dose distribution is not known.
with the commonly used single energy CT material segmentation was
Figure2: The spread-out Bragg peak for patient dose calculations (a) and the
MCNP geometry showing 0.5 cm PMMA blocks for modulation of the 147 MeV
proton beam (b).
studied. A left lateral 1616 cm2 200 MeV proton beam was simulated in
All CT geometries were converted into lattices and the dose was scored
the MCNPX code. The mass densities for (1.91.920) mm3 voxels were
using the *F8:H,P,E energy deposition tally. Protons, photons and
binned into 0.1 g/cm3 bins. The dose was calculated for the exact
electrons were transported using the la150u cross section library with
geometry (Dexact), the single energy CT geometry (Dsingle) and the dual-
energy cutoffs of 10 keV. In all simulations, 107 particles were simulated
energy CT geometry (Ddual).
in approximately 15 hours on a 3 GHz machine.
materials (B200 and CB2-10) and an adipose-equivalent material (PE)
Table 1:Relative electron densities e and effective atomic numbers Zeff for
materials used in the tissue inhomogeneity study.
Tissue segmentation with dual-energy CT: Fig 3 presents the results of
the phantom study. The exact dose distribution is shown in fig 4a and the
CT Solid Water (SW)
differences from Dsingle and Ddual are presented in fig 3b and 3c,
B200 bone mineral
CB2 - 10% CaCO3
CB2 - 30% CaCO3
CB2 - 50% CaCO3
SB3 cortical bone
Figure 4: Dose distribution for a
prostate patient calculated on the
basis of homogeneous water
geometry (a), on the basis on the
geometry with metal artifacts (b)
and using the artifact correct
images (c). The arrows indicate the
apparent range of protons due to
respectively. In both Dsingle and Ddual, the position of the Bragg peak is
shifted with respect to the true position of the Bragg peak. The shift is 0.7
cm for Dsingle and 0.7 cm or less for Ddual. This is possibly due to mass
density differences in the single energy CT and DECT geometry from the
exact geometry. Fig 3d demonstrates the dose calculation error in the
miss-assigned B200 soft bone tissue equivalent insert. The dose in the
B200 insert was by 2% lower than in the exact and DECT geometry.
Figure 1: Figure 1: The exact geometry (a), the single-energy material
segmentation (b) and the dual-energy CT material segmentation (c).
The shift in the Bragg peak demonstrates the need for careful mass
density assignment in MC dose calculations for proton beams. The dose
Metal streaking artifacts: The dose distributions calculated based on the
calculation errors using the conventional single-energy CT tissue
water-only geometry, on the original CT geometry and the artifact
segmentation below 2% suggest that the use of DECT for proton dose
corrected geometry are presented in fig 4. The shape of the 80% isodose
calculations might only have a small added benefit. The patient study
line conforms to the prostate in the water-only dose calculation (fig 4a).
shows that a metal artifact correction is necessary for patients with
bilateral hip prostheses.
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