Post on 03-Jun-2015
1
Chapter 12
Treatment Planning II: Patient Data, Corrections, and Set-Up
2
12.1 Acquisition of Patient Data (body contours)
Solder wire or lead wire Contour plotter
3
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.
4
12.1 Acquisition of Patient Data (internal structures)
Analog conventional tomography: along the long-axis of the body
5
12.1 Acquisition of Patient Data (internal structures)Analog transverse tomography: transverse cross-section of the body
Takahashi 1950s
6
12.1 Acquisition of Patient Data (internal structures)
Computed tomography
7
12.1 Acquisition of Patient Data (internal structures)
Computed tomography
IIt
eII
iii
ti
ii
0
0
ln
9
12.1 Acquisition of Patient Data (internal structures)
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.
11
12.1 Acquisition of Patient Data (internal structures)
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).
12
12.1 Acquisition of Patient Data (internal structures)
Magnetic Resonance Imaging (MRI)
13
12.1 Acquisition of Patient Data (internal structures)
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
14
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
16
12.3 Treatment Verification
Port image: used for treatment/set-up verification.
17
Liquid-ion chamber
12.3 Treatment VerificationElectronic Portal Imaging Device (EPID)
18
12.3 Treatment Verification
Electronic Portal Imaging Device (EPID)
Camera-based Solid-state detector
19
12.3 Treatment Verification Port image: used for treatment/set-up verification
20
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
21
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
22
12.4 Corrections for Contour Irregularities Isodose shift method
23
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
24
12.5 Corrections for Tissue Inhomogeneities
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
25
12.5 Corrections for Tissue Inhomogeneities
Power Law TAR method
P
e = 1
e = 1
e ≠ 1
d1
d2
d3
)(
)( :Factor Correction
hom
inhom
PD
PDCF
1
3
32
),(
),(
e
d
d
rdTAR
rddTARCF
rd
26
12.5 Corrections for Tissue Inhomogeneities
Generalized Power Law TAR method
P
1
n
2
d1
d2
d
)(
)( :Factor Correction
hom
inhom
PD
PDCF
rd
1,0
),(
00
1
)(1
1
dwhere
AddTARCFn
ii
ii
27
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
28
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)
29
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
30
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
31
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
32
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
33
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
34
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
35
12.5 Corrections for Tissue InhomogeneitiesAbsorbed dose within an inhomogeneity – parallel opposed beams
36
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
37
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.
38
12.6 Tissue Compensation
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).
39
12.6 Tissue Compensation