The Use of Computed Tomography Images in Monte Carlo Treatment Planning Magdalena Bazalova PhD defense

Acknowledgements Frank Verhaegen, PhDFrank Verhaegen, PhD Luc Beaulieu, PhDLuc Beaulieu, PhD Jean-François Carrier, PhDJean-François Carrier, PhD Christophe Furstoss, PhDChristophe Furstoss, PhD Eric Vigneault, MDEric Vigneault, MD Robin van GilsRobin van Gils McGill Medical Physics Unit staff and studentsMcGill Medical Physics Unit staff and students Natural Sciences and Engineering Research Council CanadaNatural 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.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.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.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) Motivation The link between computed tomography (CT) and Monte Carlo treatment planning (MCTP) MC geometry CT number (HU) Mass density ρ Material (bone, tissue) PhD?

The conventional approach: (ρ,HU) density and tissue assignment for MCTP bone soft bone tissue adipose lung Metal artifact reduction Dual-energy CT-based tissue segmentation

Correction of CT artifacts and its influence on Monte Carlo dose calculationsCorrection of CT artifacts and its influence on Monte Carlo dose calculations M. Bazalova, S. Palefsky, L.Beaulieu and F. Verhaegen Med. Phys , 2007 M. Bazalova, S. Palefsky, L.Beaulieu and F. Verhaegen Med. Phys , 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 prosthesesMonte 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 the correction method used on phantoms with real hip prostheses, MC dose calculations performed CT metal streaking artifacts

corrected images Correction results original CT images head phantoms pelvic phantoms

Dose calculation results 18 MV photon 18 MeV electron exact exact differences from the exact distribution for original geom. corrected geom. > 5%< 2% > 10%< 2%

Artifact reduction for a patient with bilateral hip prostheses

DVH of the prostate 20% of target voxels receive zero dose in the original geometry 20% of target voxels receive zero dose in the original geometry due to the incorrect assignment of some voxels to air due to the incorrect assignment of some voxels to air in MC dose calculation, dose to air is set to zero in MC dose calculation, dose to air is set to zero tissue segmentation original geometry corrected geometry

Real hip prostheses Ti-alloy Stainless steel Co-Cr-Mo alloy ρ=4.48 g/cm 3 ρ=6.45 g/cm 3 ρ=8.20 g/cm 3

Scatter and beam hardening as causes of metal artifacts: a MC simulations study with beam hardening with scatter without scatter HU = -80 HU = -39 HU = -78 HU = 3 without beam hardening

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 accurateSinogram 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 materialsTested with three common hip prosthesis materials Patient study showed significant differences in DVH of the target after artifact correction is donePatient 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 scatterBeam hardening has a minor effect on metal streaking artifacts compared to scatter In MCTP, omitting metal streaking artifact correction leads to large dose calculation errorsIn MCTP, omitting metal streaking artifact correction leads to large dose calculation errors

Monte Carlo simulations of a CT x- ray tube M. Bazalova and F. Verhaegen Phys. Med. Biol , 2007M. Bazalova and F. Verhaegen Phys. Med. Biol , 2007 The model was used in the thesis for dual-energy CT material extraction.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.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.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.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.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 onDual-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 (ρ e ) and the atomic number (Z) values of each voxelResults in the electron density (ρ e ) 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 imagesTissue 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 , 2008 M. Bazalova, J.-F. Carrier, L. Beaulieu and F. Verhaegen Radioth. Oncol , 2008 Dual-energy CT-based material extraction for tissue segmentation in Monte Carlo dose calculationsDual-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 , 2008 M. Bazalova, J.-F. Carrier, L. Beaulieu and F. Verhaegen Phys. Med. Biol , 2008 Practical aspects of dual-energy CTPractical aspects of dual-energy CT

DECT: Monte Carlo simulations BEAM and EGSnrc/DOSXYZnrc codeBEAM and EGSnrc/DOSXYZnrc code Picker PQ5000Picker PQ5000 soft spectra soft spectra 100 and 140 kVp 100 and 140 kVp hard spectra hard spectra added 9mm Al filter added 9mm Al filter 100 and 140 kVp 100 and 140 kVp Z  (5.740,14.141) ρ e  (0.292,1.692)

SOFT BEAMS HARD BEAMS MC simulation results 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 ρ e extractedsolid water phantom with RMI cylindrical inserts, scans taken at 100 and 140 kVp with a 9 mm Al filter, Z and ρ e extracted 1.6 % 2.8 %

exact geometry CT DECT ρeρe Z Material segmentation using DECT 250 kVp 18 MeV

Dose calculation results 17 % anterior 250 kVp <1 % 6 % lateral 18 MeV <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 studyStreaking artifact reduction observed in the RMI phantom study –US phantom with brachytherapy seeds –permanent implant prostate patient

125 I dose calculations with MCNPX D dual /D single 70% dose distribution ρ dual ρ single tissues: dual tissues: single

MC dose calculation results ρ DECT ρ CT No significant differences for this particular patient.

Dual-energy CT: conclusions In order to minimize beam hardening effects, dual- energy CT (DECT) should be done with hard beamsIn order to minimize beam hardening effects, dual- energy 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 kVpDECT 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-ρ e high-Z materials and orthovoltage and kilovoltage beamsDECT results in more accurate MC dose calculation results, especially for low-ρ e high-Z materials and orthovoltage and kilovoltage beams Patient DECT tissue segmentation is significantly influenced by image noise and patient motionPatient 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 patientsDECT 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 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.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.Scatter correction for metal artifact removal. Noise reduction in DECT images.Noise reduction in DECT images. MC dose calculations for more permanent implant brachytherapy patients.MC dose calculations for more permanent implant brachytherapy patients.

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