Download simCT_RT - University of Washington

Different information
X-ray CT
PET
X-ray computed
tomography
Positron emission
tomography
Anatomy/
Form
Metabolism/
Function
PET/CT
Complementary
Information
(Wikipedia)
Simulation of X-ray CT and
radiotherapy
Robert L. Harrison
University of Washington Medical Center
Seattle, Washington, USA
Supported in part by PHS grants CA42593 and CA126593
Nuclear Medicine
Radiology
Radiotherapy
(www.imaginis.com)
(Stieber et al)
(Wikipedia)
Simulation Comparison
National lab simulations
• Simula te everything and
anything.
• Very slow.
Simulation Comparison
CT simulation
Analytic simulation
(Visible Human Project®)
• In an analytic simulation we ‘do the integrals’.
Beam-hardening
(Bushberg)
• X-ray tubes produce a range of energies.
• Lower energy photons are more easily
scattered/absorbed.
• The x-ray beam gets ‘harder’ (a higher percentage
high energy photons) as it passes through the object.
Scatter
• Collimation reduces scatter to acceptable levels.
• Scanners with larger detector areas do have
problems.
Analytic CT simulation
• For every camera line-ofresponse, step a beam
through the voxelized
object.
– Steps can be constant size
or to next boundary.
• At each step, reduce
beam strength to account
for attenuation.
– Can use the current voxel’s
attenuation or interpolate.
(De Man)
Other issues
• Partial volume (beam is
not infinitely thin, several
tissue types in voxel).
– Use finer discretization.
• Beam hardening.
– Simulate at several
energies, then sum.
• Patient motion.
– Do a series of simulations at
different positions.
(De Man)
Radiotherapy simulation
Types of radiotherapy
• Photon irradiation.
• Other particle irradiation.
• Radioactive seed implantation.
Killing a tumor
• Radiotherapy
attempts to
– maximize dose to
tumors.
– minimize dose to
normal tissues.
(Heron)
Types of radiotherapy
• Photon irradiation.
Huge doses
Organ Whole body Tumor Heart Liver
LD50
Dose
(Grays)
3-5
50-
40
30
(McGarry)
Fractionation
• Split dose into fractions to
allow tissues to heal.
– 2 Gray/day, 5 days/week, 56 weeks.
• Normal tissue recovers
more quickly.
• Most tumors have
hypoxic/necrotic (low
oxygen/dead) centers.
– Time for these areas to
develop blood supply.
(Stieber)
Dose tolerance
(McGarry)
Multiple beams
• Multiple beams
focused on tumor.
– Maximum dose
occurs where beams
converge.
– Dose to particularly
sensitive organs can
be avoided or
reduced with beam
placement.
(www.impactscan.org)
Simulate dose deposited
Typical simulator
Nucletron Oldelft
Simulix-HQ
Virtual or CT simulation
• Software simulation of radiotherapy.
– Beginning to replace physical simulation
for many situations.
– CT scan used for input data.
Radiation therapy vs. CT
(www.impactscan.org)
Large bore
Virtual simulator
• Segments patient into
organs.
• Allows user to specify beam
intensity, shape, position.
• Keeps track of
–
–
–
–
–
Total organ/tumor dose.
Maximum organ dose.
% organ over threshold dose
Minimum tumor dose.
% tumor under threshold dose.
(www.impactscan.org)
Simulation features
•
•
•
•
•
•
•
Semi-automatic anatomy/tumor definition.
3D visualization.
Beam’s eye view.
Field shaping.
Dose histogramming.
Symmetric or asymmetric dose margins.
Fast.
Take away
• Simulation of x-ray CT and radiotherapy uses analytic
simulation.
– Too many photons to track.
– Integrate % of beam escaping for CT.
– Integrate dose deposition for radiotherapy.
• X-ray CT produces density maps.
• Radiotherapy
– Treatment planning attempts to maximize the dose to tumor,
minimize the dose to normal tissue.
– Dose tolerance of organs varies widely.
– Simulation used to optimize treatment.
• Hardware or software.
References
J.T. Bushberg, The essential physics of medical imaging, Lippincott Williams & Wilkins, 2002.
B. De Man et al, Metal Streak Artifacts in X-ray Computed Tomography: Simulation Study, IEEE Transactions on
Nuclear Science, 46:3:691-696, 1999.
K.P. George et al, Brain Imaging in Neurocommunicative Disorders, in Medical speech-language pathology: a
practitioner's guide, ed. A.F. Johnson, Thieme, 1998.
D.E. Heron et al, FDG-PET and PET/CT in Radiation Therapy Simulation and Management of Patients Who Have
Primary and Recurrent Breast Cancer, PET Clin, 1:39–49, 2006.
E.G.A. Aird and J. Conway, CT simulation for radiotherapy treatment planning, British J Radiology, 75:937-949, 2002.
R. McGarry and A.T. Turrisi, Lung Cancer, in Handbook of Radiation Oncology: Basic Principles and Clinical
Protocols, ed. B.G. Haffty and L.D. Wilson, Jones & Bartlett Publishers, 2008.
R. Schmitz et al, The Physics of PET/CT Scanners, in PET and PET/CT: a clinical guide, ed. E. Lin and A. Alavi,
Thieme, 2005.
W.P. Segars and B.M.W. Tsui, Study of the efficacy of respiratory gating in myocardial SPECT using the new 4-D
NCAT phantom, IEEE Transactions on Nuclear Science, 49(3):675-679, 2002.
V.W. Stieber et al, Central Nervous System Tumors, in Technical Basis of Radiation Therapy: Practical Clinical
Applications, ed. S.H. Levitt et al, Springer, 2008.
P. Suetens, Fundamentals of medical imaging, Cambridge University Press, 2002.
www.impactscan.org/slides/impactcourse/introduction_to_ct_in_radiotherapy