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Chapter 19

Three-Dimensional Conformal Radiation Therapy

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19.1 Introduction

What is 3D-CRT:

• Based on 3D anatomic information

• Dose distribution conforms to the target, and

• Avoids critical organs and normal tissues

• May also include clinical objectives such as TCP and NTCP

Difficulties of 3D-CRT:

• Tumor (CTV) delineation

• Treatment uncertainties (setup uncertainty, organ motion, etc.)

• Lack of clinical data to verify TCP and NTCP models

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19.2 Treatment-Planning Process

3D patient Image (CT, MRI, PET…)

Target, organ delineation (segmentation)

BEV field design (beam angle, aperture)

Plan optimization (# of beams, beam angle,

energy, wedge, weight, intensity distribution)

Dose calculation

Plan evaluation (isodose display, TCP, NTCP)

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19.2 Treatment-Planning Process A. Imaging Data

Digitally Reconstructed Radiograph (DRR)

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19.2 Treatment-Planning Process A. Imaging Data

CT: attenuation coefficients (), can be converted to electron density, used for treatment planning/dose calculation. Spatial resolution ~1mm in X/Y directions, variable (1-10 mm) in Z-direction, which affects the quality of DRR

MRI: proton density, better soft tissue delineation (brain, head/neck, prostate), but insensitive to calcification and bony structures.

Spatial resolution ~1mm in all directions.PET: functional image

CT MRI PET

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19.2 Treatment-Planning Process B. Image Registration

• Point-based registration - minimizes discrepancy between corresponding point pairs

• Surface-based registration – minimizes discrepancy between two surfaces

• Image (intensity)-based registration – minimizes a similarity metric (mutual information, cross correlation, etc.) between two images.

• Deformable registration – usually image-based, point-to-point transformation to minimize a similarity metric between two images.

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19.2 Treatment-Planning Process B. Image RegistrationPoint-based registration

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19.2 Treatment-Planning Process B. Image Registration

Before registration after registration

Surface-based registration

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19.2 Treatment-Planning Process B. Image Registration

Image-based registration

after registration

before registration

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CT PET PET/CT

Lung - CTI/Siemens PET/CT

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19.2 Treatment-Planning Process B. Image Registration

deformable registration – before registration

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19.2 Treatment-Planning Process B. Image Registration

deformable registration – after registration

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19.2 Treatment-Planning Process C. Image Segmentation

Manual segmentation –

Laborious, time-consuming

Auto segmentation –

A very difficult problem

Contours drawn by physician

Contours drawn by auto-deformation from another contouranother contour

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19.2 Treatment-Planning Process D. Beam Aperture Design

Beam’s Eye View (BEV)

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target

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E. Field Multiplicity and Collimation

19.2 Treatment-Planning Process

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F. Plan Optimization and Evaluation

19.2 Treatment-Planning Process

Isodose curves Isodose surface

cord

PTV

eyes

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F. Plan Optimization and Evaluation

19.2 Treatment-Planning Process

0

25

50

75

100

Vol

ume

(%)

0 25 50 75 100Dose (Gy)

femurs

bladder

target

rectum

D=77GyV=90%

D=75GyV=30%

D=72Gy

DVH for a prostate plan

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19.3 Dose Calculation Algorithms

• Correction-based: semi-empirical, based on measured data such as TMR, OCR, etc.

• Model-based: based on phase-space data (energy spectra, angular distribution), Monte-Carlo generated dose kernels, ray-tracing 3D inhomogeneity correction.

• Monte Carlo: simulation of physical events by random sampling; commonly used codes EGS4, MCNP, FLUKA’ GEANT, etc; still too slow for routine clinical use

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19.3 Dose Calculation AlgorithmsA. Correction-based Algorithms

Based on data (PDD/TMR/TPR, OCR) measured in homogeneous phantom (water) at standard distance (e.g. SSD = 100 cm)

For individual plans, corrections needed for:

• Surface contours

• Irregular field shape/size

• Distance (inverse-square corrections)

• Non-uniform intensity (wedge, IMRT)

• Inhomogeneity correction

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19.3 Dose Calculation AlgorithmsB. Model-based Algorithms

Convolution-superposition:

'''

''''

rdrrArT

rdrrArrrD

p

p

Atten coeff Primary energy fluence

TERMA Dose kernel

Inhomogeneity correction made along the ray lines

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Point kernel convolution in homogeneous medium

Primary fluence Point kernel dose

'''' rdrrArrrD p

241r

2r

transport

interaction

E 1,1

E 2, 2

E’,’

Simulates the physical processes of particle transport and interactions

Photon

• Coherent scattering

• Photoelectric

• Compton

• Pair production

Electron

• Continuous energy loss

• Multiple scattering

• Delta ray production

• Bremsstrahlung production

• Positron annihilation

C. Monte Carlo calculation in CT grids

19.3 Dose Calculation Algorithms

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100 50709095 20

Monte Carlo Conventional

PTV GTV

Lung Treatment with 15 MV photons