The Use of Computed
Tomography Images in Monte
Carlo Treatment Planning
Magdalena Bazalova
PhD defense
Acknowledgements
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Frank Verhaegen, PhD
Luc Beaulieu, PhD
Jean-François Carrier, PhD
Christophe Furstoss, PhD
Eric Vigneault, MD
Robin van Gils
McGill Medical Physics Unit staff and students
• Natural Sciences and Engineering Research
Council Canada
Radiation therapy
• The purpose of radiotherapy is to kill tumor cells
by delivering a prescribed dose to the tumor
while sparing the healthy tissue.
• Treatment planning is an important step of
radiotherapy and has to be performed carefully.
• The Monte Carlo method is the most accurate
technique to calculate the delivered dose during
treatment assuming the treatment machine
model and the patient anatomy are well known.
Motivation
The link between computed tomography (CT) and
Monte Carlo treatment planning (MCTP)
Mass density
ρ
PhD?
CT number
(HU)
MC geometry
Material
(bone,
tissue)
The conventional approach: (ρ,HU)
density and tissue assignment for MCTP
Mass density calibration curve
1.800
3
mass density [g/cm ]
1.600
y = 5.78E-04x + 1.05E+00
1.400
1.200
1.000
y = 8.84E-04x + 9.83E-01
0.800
0.600
y = 1.03E-03x + 1.03E+00
0.400
0.200
0.000
-1000
adipose
lung
-500
tissue
0
soft bone
500
bone
1000
HU
Metal artifact reduction
Dual-energy CT-based tissue segmentation
CT metal streaking artifacts
• Correction of CT artifacts and its influence on
Monte Carlo dose calculations
M. Bazalova, S. Palefsky, L.Beaulieu and F. Verhaegen Med. Phys. 34 21192132, 2007
- cubic spline sinogram interpolation correction method on phantoms with
steel cylinders and on a patient, MC dose calculations performed
• Monte Carlo dose calculation for phantoms with
real hip prostheses
M. Bazalova, C. Coolens, F. Cury, P. Childs, L. Beaulieu and F. Verhaegen J.
Phys. Conf. Series 102 2008
- the correction method used on phantoms with real hip prostheses, MC
dose calculations performed
corrected images
original CT images
Correction results
head phantoms
pelvic phantoms
Dose calculation results
differences from the exact distribution for
original geom.
corrected geom.
exact
18 MV photon
> 5%
< 2%
> 10%
< 2%
exact
18 MeV electron
Artifact reduction for a patient with
bilateral hip prostheses
DVH of the prostate
120
Volume (%)
100
80
60
original geometry
40
artifact corrected geometry
original geometry
20
0
0
20
40
60
80
100
120
Relative Dose
• 20% of target voxels receive zero dose in
the original geometry
• due to the incorrect assignment of some
voxels to air
• in MC dose calculation, dose to air is set to
zero
corrected geometry
tissue segmentation
Real hip prostheses
Ti-alloy
Stainless steel
Co-Cr-Mo alloy
ρ=4.48 g/cm3 ρ=6.45 g/cm3 ρ=8.20 g/cm3
Scatter and beam hardening as causes of
metal artifacts: a MC simulations study
without
beam hardening
with
beam hardening
with scatter
without scatter
HU = -80
HU = -39
HU = -78
HU = 3
CT metal streaking artifacts:
conclusions
• Sinogram interpolation correction algorithm for
metal streaking artifacts improves image quality
and makes tissue segmentation and MC dose
calculations more accurate
• Tested with three common hip prosthesis materials
• Patient study showed significant differences in DVH
of the target after artifact correction is done
• Beam hardening has a minor effect on metal
streaking artifacts compared to scatter
• In MCTP, omitting metal streaking artifact
correction leads to large dose calculation errors
Monte Carlo simulations of a CT xray tube
• M. Bazalova and F. Verhaegen Phys. Med. Biol. 52
5945-5955, 2007
•The model was used in the thesis for dual-energy CT material extraction.
•It can be used for scatter correction when metal streaking artifacts are present.
CT x-ray tube simulation results
CT x-ray tube MC simulation:
conclusions
• A Monte Carlo model of a CT x-ray tube was
developed and validated with half-value layer
and spectral measurements using a CdTe
detector.
• 100 and 140 kVp beams with no additional
filtration and with 9 mm added aluminum foil
were tested and a very good agreement was
found.
• The modeled spectra were used for dual-energy
CT-based material extraction, the next part of the
PhD. project.
Dual-energy CT imaging
• Dual-energy material extraction (DECT) is
based on
– taking CT images at two tube voltages (100
kVp and 140 kVp)
– parameterization of the linear attenuation
coefficient
• Results in the electron density (ρe) and the
atomic number (Z) values of each voxel
Material segmentation for MC dose
calculations
1 tissue type in single- energy CT?
1 tissue type in single-energy CT
DECT for MCDC
• Tissue segmentation in Monte Carlo
treatment planning: a simulation study
using dual-energy CT images
M. Bazalova, J.-F. Carrier, L. Beaulieu and F. Verhaegen Radioth. Oncol.
86 93-98, 2008
• Dual-energy CT-based material extraction
for tissue segmentation in Monte Carlo
dose calculations
M. Bazalova, J.-F. Carrier, L. Beaulieu and F. Verhaegen Phys. Med.
Biol. 53 2439-2456, 2008
• Practical aspects of dual-energy CT
DECT: Monte Carlo simulations
•BEAM and EGSnrc/DOSXYZnrc code
•Picker PQ5000
• soft spectra
100 and 140 kVp
• hard spectra
added 9mm Al filter
100 and 140 kVp
Z  (5.740,14.141)
ρe (0.292,1.692)
MC simulation results
SOFT
BEAMS
HARD
BEAMS
In order to minimize beam hardening effects, hard beams have to be used.
DECT material extraction for tissue segmentation in
Monte Carlo dose calculations
• solid water phantom with RMI cylindrical inserts, scans taken at
100 and 140 kVp with a 9 mm Al filter, Z and ρe extracted
2.8 %
1.6 %
Material segmentation using DECT
CT
DECT
exact geometry
ρe
250 kVp
18 MeV
Z
Dose calculation results
anterior 250 kVp
17 %
<1 %
lateral 18 MeV
6%
<1 %
SingleE-material segmentation
• dose calculation errors are the
largest in the soft bone tissue
equivalent material irradiated
by the 250 kVp photon beam,
up to 17%!
• the largest dose difference for
the 18 MeV electron beam is
found to be in the polyethylene
cylinder, being 6%
DualE-material segmentation
• all dose differences in all
cylinders are below 1%
Practical aspects of DECT
• Streaking artifact reduction observed in
the RMI phantom study
– US phantom with brachytherapy seeds
– permanent implant prostate patient
125I
dose calculations with MCNPX
tissues: single
dose distribution
ρsingle
70%
Ddual/Dsingle
ρdual
tissues: dual
MC dose calculation results
ρCT
ρDECT
No significant differences for this particular patient.
Dual-energy CT: conclusions
• In order to minimize beam hardening effects, dualenergy CT (DECT) should be done with hard beams
• DECT material extraction was successful for a set of
tissue-equivalent materials with a wide range of
densities and atomic numbers using 100 kVp and
140 kVp
• DECT results in more accurate MC dose calculation
results, especially for low-ρe high-Z materials and
orthovoltage and kilovoltage beams
• Patient DECT tissue segmentation is significantly
influenced by image noise and patient motion
• DECT has the potential to reduce streaking artifacts from
brachytherapy seeds, the effect on MCDC should be
studied with a larger group of patients
Overall conclusions
• The link between computed tomography images
and Monte Carlo geometry files was strengthened
by means of metal streaking artifact reduction and
dual-energy CT-based material extraction.
• The dose delivered to patients during
radiotherapy can be therefore calculated more
accurately with Monte Carlo techniques.
Future work
• Scatter correction for metal artifact removal.
• Noise reduction in DECT images.
• MC dose calculations for more permanent implant
brachytherapy patients.
Thank you!