Positioning and margin determination

Martijn Kamphuis MSc

Research Radiation Therapist IGRT

Department of Radiotherapy

Amsterdam, the Netherlands

Content of the presentation• Why do we need imaging?• Imaging modalities

– How do the work?– Why do we need them?– Pros and cons

• Summary

Why do we need imaging?y g g• How does the patient look like?

– Size and shape – Localisation of the tumour– Position of the critical structures

• To be able to model the interaction of the radiation – Size and shapep– Differences in tissue

• Density differences

Imaging modalitiesg g• Conventional

SimulatorSimulator• Radiology

CT (3D and 4D)– CT (3D and 4D)– MR

• Nuclear medicineNuclear medicine– PET

• Note: only the role in RT will discussed

Computed Tomography (CT)g y ( )

How does it work?kV-source

Detector array

http://radiographics.rsna.org

Multi slice detector

http://radiographics.rsna.org

Helical acquisition

http://radiographics rsna orghttp://radiographics.rsna.org

Multi slice acquisition

http://radiographics.rsna.org

(Filtered) back project( ) j

Attenuation of X-rayy• Photoelectric effect

Energy transfer from– Energy transfer from to elektron

– Mainly with inner Coreyshell (80%) Photon

Electron

• Atomic number• Change of

interaction• (Z)

Attenuation of the X-rayy• High contrast

imagingimaging• Hounsfield Units:

– µ: attenuation coefficientcoefficient

On the linac: dose calculation• Compton effect

dominatesdominates• Foton loses only

small part of its Core

Electron

small part of its energy

Core

Scattered

• Lineair relation with Atomic number Z

Scattered Photon

Photon

Atomic number Z• Corresponds to the

elektron density (ED)y ( )

In the TPS• Transition from houndsfield units to electron

densitydensity

Differences betweendiagnostic CT and CT simulationdiagnostic CT and CT-simulation

• Large bore• Large bore– Positioning devices– Complete patientComplete patient

contour• Flat table top• Laser markers

4D-computed tomographyg y• 3D-imaging

sufficient for manysufficient for many treatment sites

• Artifacts caused by moving targetsmoving targets causes

Pros and cons• Pro

Essential for dose• Cons

Limited anatomical– Essential for dose calculation

– Good representation

– Limited anatomical and/or pathological qualityp

of the geometry – No functional information

Positron Emission Tomography (CT)

Positron Emission Tomography (CT)

• A way of imaging– Organ function– Cellular function– Subcellular function

Labelling and injecting• PET-CT uses probes

Molecule containing a

g j g

– Molecule containing a positron emitters

– E.g fluoro-deoxy-glucose (FDG)(FDG)

• Probes takes part in normal physiology– FDG goes to cells with high

metabolism– E.g.tumour cells

How does it work?: decayy

How does it work?: detection

Why do we need it?y• Three main reasons*:

– High sensitivity for tumour tissue– Visualize biological pathways

• Hypoxia• Enable dose painting

M it t t t– Monitor treatment response• Window for Adaptive treatment

*Anca-Ligia Grosu et al., IGRT a clinical perspectiveg , p p

High sensitivity for tumour tissue• Case*

– 67 Year old male– 3 Nodules– Tumour, Infarct &

Benigne Hamartoma

• AMC– Lung NSCLCg– Cervical cancer– Anal canal– Vulva carcinoma

*http://www.wvm.petctmobile.com/zportal/portals/phys/clinical/petct_case_studies/lung/lung_case3

Delineation variation: CT versus CT + PET

CT (T2N2)SD 7 5 mm

CT + PET (T2N1)SD 3 5 mmSD 7.5 mm SD 3.5 mm

Pros and cons• Pro

Functional• Cons

Not enough data for– Functional information

– Improved tumour

– Not enough data for simulation

– Small borepdefinition – No ED data

– Acquisition time

Magnetic Resonance Imagingg g g

How does it work?

MRI signal: Hydrogen in the human body

• In water (H2O), – ~ 80% of the body

• In fatty tissue (CH2)In fatty tissue, (CH2)

How does it work? MagnetizationHydrogen (H)

Most common atom– Most common atom– Proton:

• positively charged• positively charged• proton turns around own axis

– Hydrogen also have “spin”y g p

Spin + charge = mini-magnetp g g

MRI signal: Netto-magnetisation

Outside magnetic field Volume within magnetic field (B0)g– Magnetisch moment = 0

g ( 0)– Magnetisch moment ≠ 0– spin-up & spin-down

B0

MRI signalg

B0B0

Strong magnetic field Netto magnetisationStrong magnetic field Netto-magnetisation

But…still no signal

MRI – Creating the imageg g• Major steps

– Excitation– Relaxation– Acquisition– Reconstruction

What is resonance?• Every system has its

own naturalown natural frequency

• External input can• External input can lead to larger amplitude.p

• Objects are moving in the same phase

TACOMA BRIDGETACOMA BRIDGE

MRI - Excitation

zMB B

zM0B0 B0

Mxy

xy

xy

• Netto-magnetisation– In z-direction impossible to

measure the size of B0

• Excitation using a RF-pulse– Resonance changes

• Netto-magnetisation in xy-direactionsg y– Possible to measure

MRI - RelaxationAfter RF excitation: z

• Loss of equilibrium– Relaxation to initial state

y

Mxy

– Two independent processes• T1 relaxation

xy

• T2 relaxation

MRI - T1 relaxation

t = 0 t ~ 1 secT1 relaxation

MRI - T2 relaxationz

y

Mxy

xy

Excitatie

t = 0 t ~ 100 msecT2 relaxatie

• T2 relaxation– Spins are in phase by RF pulse– Spins are running out of phase (Mxy), – T2 is tissue specific – Independent of T1 relaxation

MRI – Contrast• MR image shows the differences of

l ti t t i ti i trelaxation at a certain time point

• Depending on tissue type• Depending on tissue type– Fatty tissue– WaterWater

MRI – T1 and T2 contrast

CT T1-weighted MRI T2-weighted MRI

• Rule of thumb: T1 for anatomy, T2 for pathology

TE 30 ms; TR 566 ms TE 120 ms; TR 8400 ms

Why do we need it?y• MR containts excellent soft tissue information.

Improves delineation process– Improves delineation process

• Commonly used in• Commonly used in– Prostate

Brain– Brain

Example: prostate cancer

www.umcutrecht.nl

Pros and cons• Pro

Superior soft tissue• Cons

Acquisition time– Superior soft tissue imaging

– Spatial resolution is

– Acquisition time– Geometrical

deformationphigh

– Improved tumour d fi iti

– No ED information– Problems with table

definition– Patient safety

• Lower contrast

top and immobilization devicesLower contrast

reaction – Small bore

Putting it all togetherg g• CT-simulation

– Still the fundament of current RT treatment– Essential for dose calculation

• PET– Sensitive for tumour activityy– Decreases interobserver variability

• MRMR– Soft tissue imaging