16 chap 12 treatment planning ii

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Chapter 12

Treatment Planning II: Patient Data, Corrections, and Set-Up

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12.1 Acquisition of Patient Data (body contours)

Solder wire or lead wire Contour plotter

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12.1 Acquisition of Patient Data (body contours)

1. The contour must be taken with the patient in the treatment position.

2. A line representing the tabletop must be clearly indicated to serve as a reference.

3. Important bony landmarks and beam entry points must be indicated.

4. Check body contours during the course of treatment due to possible weight loss and change of tumor volume.

5. Take multiple body contours in the sup-inf direction if contours change significantly.

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12.1 Acquisition of Patient Data (internal structures)

Analog conventional tomography: along the long-axis of the body

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12.1 Acquisition of Patient Data (internal structures)Analog transverse tomography: transverse cross-section of the body

Takahashi 1950s

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Computed tomography

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Computed tomography

IIt

eII

iii

ti

ii

0

0

ln

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Computed tomography

CT number, Hounsfield number ( for kV photons) :

0;1000

1000

waterair

water

watertissue

HH

H

For Megavoltage photon beams, Compton scattering is the dominant event, thus electron density (rather than linear attenuation coefficient in the kilovoltage range) is more relevant for photon dose calculation.

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Magnetic Resonance Imaging (MRI)

MRI vs CT:

Advantages:

does not use ionizing radiation;

higher contrast,

better imaging of soft tissue tumors.

Disadvantage: longer scan acquisition time.

MRI is based on proton density and proton relaxation characteristics of different tissues.

Image contrast can be affected by TE (echo time) and repetition time (TR).

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Magnetic Resonance Imaging (MRI)

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Ultrasound image

Ultrasound vs CT:

Advantages:

does not use ionizing radiation;

Real time.

Better contrast in some cases (e.g. prostate)

Prostate transducer

Disadvantage:

poor image quality in some cases

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12.2 Treatment Simulation

A simulator duplicates a therapy unit in terms of its geometrical, mechanical and optical properties, but uses kilovoltage x-ray beam, for better image quality.

Varian Acuity simulator CT simulator

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12.3 Treatment Verification

Port image: used for treatment/set-up verification.

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Liquid-ion chamber

12.3 Treatment VerificationElectronic Portal Imaging Device (EPID)

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12.3 Treatment Verification

Electronic Portal Imaging Device (EPID)

Camera-based Solid-state detector

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12.3 Treatment Verification Port image: used for treatment/set-up verification

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12.4 Corrections for Contour Irregularities

A

Q

Effective SSD method

(?))(max corrA PQDD 90

80

70

60

50

90

80

70

60

50

Q’

)(')'(max dPQDDA

h

d

2

maxmax )()'(

hdSSD

dSSDQDQD

m

m

dm

2

)('

hdSSD

dSSDdPP

m

mcorr

SSD

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12.4 Corrections for Contour Irregularities TAR (TMR) method

A”

Q90

80

70

60

50

h

d

dm

SSD

),()('')(

),(),(

)('')(

max

"

max"

A

AAA

A

rhdThdPQDk

rhdTkrhdTD

hdPQDD

CFhdPPPQDD corrcorrA )(")(max

),(),( AA rhdTrdTCF

A

h

d

SSD

90

80

70

60

50

),( AA rdTkD

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12.4 Corrections for Contour Irregularities Isodose shift method

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12.5 Corrections for Tissue Inhomogeneities

• Changes in the absorption of primary beam and the associated pattern of the scattered photons

• Changes in the secondary electron fluence

• For megavoltage x-ray beams, Compton effect is predominant and we can define an effective depth (or equivalent depth, equivalent pathlength) for nonwater equivalent materials

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TAR method (equivalent pathlength)

P

e = 1

e = 1

e ≠ 1

d1

d2

d3

)(

)( :Factor Correction

hom

inhom

PD

PDCF

321

321

'

),(

),'(

dddd

dddd

rdTAR

rdTARCF

e

d

d

Equivalent depth

rd

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Power Law TAR method

P

e = 1

e = 1

e ≠ 1

d1

d2

d3

)(


hom

inhom

PD

PDCF

1

3

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),(

),(

e

d

d

rdTAR

rddTARCF

rd

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Generalized Power Law TAR method

P

1

n

2

d1

d2

d

)(


hom

inhom

PD

PDCF

rd

1,0

),(

00

1

)(1

1

dwhere

AddTARCFn

ii

ii

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Inhomogeneity correction: Generalized Batho Power Law

zA =1.

0

),(0

AzT

eD z

12

1121

),( 11

))((2

AzzTD

eeD zzz

A

z1

zA

10

)1(

1

1

1

1

),(

AzTD

ee

eDzz

z

z

Note: n is relative density to water for layer-n

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1,0

),(

00

1

)(1

0

1

zwhere

AzzTD

DCF

N

nn

n nn

23

223

),( 22

))((23

AzzTD

eDD zz

‧‧‧

A

z1

z2

zn

z3

1

11

),( 11

))((1

nn

nnn

AzzTD

eDD

nn

zznn

‧‧‧

A

z1

z2

z

Inhomogeneity correction: Generalized Batho Power Law (cont’d)

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1,0

),(

00

1

)(max1

0

1

zwhere

AdzzTD

DCF

N

nn

n nn

23

223

),( 22

))((23

AzzTD

eDD zz

‧‧‧

A

z1

z2

zn

z3

1

11

),( 11

))((1

nn

nnn

AzzTD

eDD

nn

zznn

‧‧‧

A

z1

z2

z

Inhomogeneity correction: Modified Batho Power Law

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12.5 Corrections for Tissue InhomogeneitiesEquivalent TAR method

),(

)','(

rzTAR

rzTARCF

z

dttz0

)('

kjikji

kjikjikji

w

w

,,,,

,,,,,,

~

z=0 zeffz=0

Wi,j,k are weighting factors in the vicinity of the point of calculation. The summation can be either 3D (over i,j,k), or 2D (over i,j, provided multiple slices have been coalesced and merged into a single slice at ze

ff).

~' rr )()()( rSARrrSARrw

ijk

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12.5 Corrections for Tissue InhomogeneitiesAbsorbed dose within an inhomogeneity – bone

en kerma dose :mequilibriuelectron under

soft tissue soft tissuebone

dose

kV

without bone

with bone

soft tissue soft tissuebone

dose

MV

without bone

with bone

event ricphotoelect todue

tissuesoft

en

bone

en

scatteringcompton todue

1031.31019.3 2626

tissuesoftebonee

depth depth

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12.5 Corrections for Tissue InhomogeneitiesAbsorbed dose within an inhomogeneity – soft tissue in bone

If electron fluence is the same, according to Bragg-Gray theory:

ST

B

BSTBS

DD

Under electron equilibrium, if photon fluence is the same, dose-to-bone is related to dose-to-(undisturbed) soft tissue by:

B

ST

enSTB DD

The ratio of DSTB to DST is: (in general, >1)

BSTenST

BST

STB SD

D

In clinical calculation, the change of photon fluence is estimated by TAR or TMR ratios: )(

)(

BST

BBSTSTSTB ttTMR

ttTMRDD

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12.5 Corrections for Tissue InhomogeneitiesAbsorbed dose within an inhomogeneity – soft tissue adjacent to bone

soft tissuebo

ne

Due to electron backscatter from the bone

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12.5 Corrections for Tissue InhomogeneitiesAbsorbed dose within an inhomogeneity – soft tissue adjacent to bone

soft tissuebo

ne

Up to 10 MV, initial buildup of electrons

>10 MV, increased electron fluence due to pair production in bone

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12.5 Corrections for Tissue InhomogeneitiesAbsorbed dose within an inhomogeneity – parallel opposed beams

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12.5 Corrections for Tissue InhomogeneitiesLung tissue and air cavity

Increased dose down stream from lung

lung

Increased dose in lung (for large field size)

Decreased dose in lung for small field size, especially for high energy due to loss of lateral scattering

homogeneous medium

Buildup immediately beyond the lung-soft tissue interface due to lateral loss in lung

Increased penumbra in lung due to increased lateral scattering, effect more pronounced for higher energies

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12.6 Tissue Compensation

Compensator is used to account for surface irregularity or internal inhomogeneities. Unlike bolus, the compensator is placed 15-20 cm away from the skin to keep skin-sparing effect.

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Density or thickness ratio = h’/h

Due to lateral loss in the air gap ‘d’ between the compensator and the skin surface, the density/thickness ratio is <1, otherwise, the dose to patient would be too low (over compensated).

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16 chap 12 treatment planning ii

Health & Medicine

Transcript of 16 chap 12 treatment planning ii