The Use of Computed Tomography Images in Monte Carlo Treatment Planning

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

2. Acknowledgements • 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

3. 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.

4. MotivationThe link between computed tomography (CT) and Monte Carlo treatment planning (MCTP) Mass density ρ PhD? MC geometry CT number (HU) Material (bone, tissue)

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

6. 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 2119-2132, 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. Series102 2008 - the correction method used on phantoms with real hip prostheses, MC dose calculations performed

7. head phantoms pelvic phantoms Correction results original CT images corrected images

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

9. Artifact reduction for a patient with bilateral hip prostheses

10. DVH of the prostate original geometry • 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

11. Real hip prostheses

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

13. 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

14. Monte Carlo simulations of a CT x-ray 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.

15. CT x-ray tube simulation results

16. 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.

17. 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

18. 1 tissue type in single- energy CT? 1 tissue type in single-energy CT Material segmentation for MC dose calculations

19. 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

20. 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)

21. HARD BEAMS MC simulation results SOFT BEAMS In order to minimize beam hardening effects, hard beams have to be used.

22. 2.8 % 1.6 % 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

23. CT DECT ρe 250 kVp 18 MeV Z Material segmentation using DECT exact geometry

24. Dose calculation results anterior 250 kVp lateral 18 MeV • 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% 17 % 6 % <1 % <1 %

25. Practical aspects of DECT • Streaking artifact reduction observed in the RMI phantom study • US phantom with brachytherapy seeds • permanent implant prostate patient

26. tissues: single dose distribution 70% Ddual/Dsingle tissues: dual 125I dose calculations with MCNPX ρsingle ρdual

27. ρCT ρDECT MC dose calculation results No significant differences for this particular patient.

28. Dual-energy CT: conclusions • In 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 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

29. 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.

30. Thank you!