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3D-CRT
Three-Dimensional Conformal Radiation Therapy
Charles Poole, I. Chetty, A. Pompoš
Objectives:
• The process of 3D-CRT and differences between 3D-CRT and 2D
• The imaging modalities that enable 3D-CRT
• The equipment and techniques used in 3D-CRT
The development of 3D-CRT
Developed at the University of Michigan, 1984 The goal of 3D-CRT is to maximize the dose tumors while minimizing doses to normal tissues 3D-CRT requires a 3D “model” of the patient for planning and delivery purposes
The evolution of planning and delivery The 2D process
GroupWise.lnk
3D Conformal Radiotherapy
Intensity Modulated Radiation Therapy (IMRT)
Courtesy W. Schlegel et al.
4D Radiation Therapy Gating or
IGRT = image guided radiation therapy
3D vs. 2D • Volume definitions (tumors and normal tissues) CT vs. simulator films 3D requires a 3D “model” of the patient
• Delivery and shaping of the radiation beams MLC vs. cerrobend blocks
• Planning and dose calculations 2D uses simplistic algorithms assuming the
patient is composed of water; calculations typically account for primary beam attenuation
3D algorithms are much more sophisticated accounting for primary and scattered radiation and patient heterogeneity
Evaluation: 3D uses DVHs; 2D uses isodose plans
The 3D-CRT Process
• Imaging / Simulation
• Planning
• Delivery
QA of all the above processes is crucial to safe and accurate delivery of 3D-CRT to patients
Consult CT Scan Virtual Sim (with Fusion)
Tx Plan
Physics QA Portal Film Tx Delivery Follow-up
Tx Planning
Physics QA Portal Imaging
Tx Delivery
The 3D-CRT process
X-Ray
PET/CT
MR Perfusion MRI-MRS NM
Multimodality Imaging
CT Scan
Prosthesis artifact in CT
MRI
T1 Weighted T2 Weighted
MRI
CT
MR
CT MRI
PTV on CT PTV on CT
with fused PET
Safety
margin
Imaging Modalities
• CT: (a) provides 3D images (volumes!!) (b) provides CT numbers (HU) which are
converted to electron densities needed for dose calculation
• MRI: soft tissue delineation, only good for
visualization of structures • PET: functional activity (uptake of radioactively
marked glucose)
3D Planning
• Fusion / Registration of images
• Volume Segmentation
• Beam design: gantry, energy, modifier
• Aperture design: BEV, MLC and Digitally Reconstructed Radiographs (DRRs)
• Dose calculation
• Plan evaluation: DVH
Fused PET
And CT images
Spyglass tool is often used
to evaluate the fusion
• PTV
• Bladder
• Rectum
• Bowel
• Outer contour (skin)
• Femurs
Contouring for
Prostate treatment
Albert Fung
Radiation treatment planning: target definitions
GTV
CTV =
GTV + 0.5 cm
PTV = CTV + 1 cm
International Commission on Radiation Units and Measurements (ICRU) report No. 50
ICRU 50 definitions
• GTV: gross tumor volume • CTV: includes microscopic disease • PTV: includes patient motion and setup
uncertainty • Field aperture: includes beam penumbra
Nomenclature of Radiation Beams
3D viewing of radiation portals
Beams Eye View (BEV), Digitally Reconstructed Radiog. (DRR)
Multi-Leaf Collimator (MLC)
Varian
MLC
3D Breast Treatment Plan
Multiple Beam Angles in 3DCRT
Dose (Gy) Differential
volume (%)
Cumulative
volume (%)
80-81 2 2
79-80 1 3
78-79 4 7
: : :
1-2 2 99
0-1 1 100
Dose Volume Histogram (DVH)
Cumulative DVH of prostate radiation
0
50
100
0 20 40 60 80
Dose (Gy)
Vo
lum
e (
%) prostate
seminal vesicle
intrapelvic nodes
bladder
rectum
sigmoid colon
penile bulb
DVH: ideal vs. realistic
PTV Critical Organs
Biological Models
• Tumor Control Probability
• Normal Tissue Complication Probability
Nomenclature • CT: Computed Tomography • MRI: Magnetic Resonance Imaging • PET: Positron Emission Tomography • GTV: Gross Tumor Volume • CTV: Clinical Target Volume • PTV: Planning Target Volume • BEV: Beam’s Eye View • MLC: Multi-Leaf Collimator • DVH: Dose Volume Histogram • DRR: Digital Reconstructed Radiograph • TCP: Tumor Control Probability • NTCP: Normal Tissue Complication
Probability
DIFFERENCES BETWEEN 2-D AND 3-D TREATMENT PLANNING
Subject 2-D 3-D
Definition of Anatomy
1. Orientation of Image Slices Only trans-axial planes Arbitrary
2. Number of Slices Typically 2, AP & Lateral Arbitrary (>100)
3. Use 3-D Structures No Yes
4. Use of MRI, Ultrasound, PET, etc No Yes
5. Use/Integration of Simulation and Treatment Portal Films
No Yes
Beam Design and Display
6. Use of BEV for beam shaping No Yes
7. Beam Orientation In axial planes Arbitrary
8. Beam Display On single axial slices 3-D divergent object
Dose Calculations
9. 3-D Shape and Densitiy of Patient No Yes
10. 3-D Divergence/Beam Geometry No Yes
11. 3-D Beam Flatness/Symmetry No Yes
12. 3-D Scatter Effects of Blocks, etc No Yes
13. 3-D Inhomogeneity Corrections No Yes
Dose Display and Plan Evaluation
14. Dose Display One slice at a time 3-D Isodose Surfaces
15. Plan Analysis Tools (DVH, etc) No Yes
16. Plan Comparison Tools No Yes