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