prezentacja programu powerpointindico.ictp.it/event/8651/session/0/contribution/17/... · 2019. 4....
TRANSCRIPT
![Page 1: Prezentacja programu PowerPointindico.ictp.it/event/8651/session/0/contribution/17/... · 2019. 4. 16. · Anders A. Ahnsjö, Med.Phys. 16 (4), 1989 •parameteres generated for beams](https://reader033.vdocument.in/reader033/viewer/2022052103/603e47fb9b3d3334ee641a5b/html5/thumbnails/1.jpg)
TPS:algorithms
ICTP SCHOOL ON MEDICAL PHYSICSRadiation Therapy:
Dosimetry and Treatment Planningfor Basic and Advanced Applications
ICTP, Trieste 2019
Paweł KukołowiczMedical Physics Department, Warsaw, Poland
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Paweł Kukołowicz
• Head of Medical Physics Department at Cancer-Center Instituteof Oncology in Warsaw, Poland
• Medical physicist from 1980
• Dosimetry, treatment planning, education
• I our department we have accredited SecondaryStandard Laboratory (calibration with Co60)
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Delivered dose does matter!
2/#21
10 20 30 40 50NORMAL TISSUE DOSE (Gy)
0,0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0,8
0,9
1,0
0 20 40 60 80 100 120
TARGET DOSE (Gy)
PR
OB
AB
ILIT
Y O
F E
FF
EC
T
TCP
NTCP
10 20 30 40 50
Accuracy required and achievable in radiotherapy dosimetry:
Have modern technology and techniques changed our views?
Journal of Physics: conference Series 444 (2013)
David Thwaites
3.5% - 1 st. dev.
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How many patients will receivedose smaller of 7%?
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• 7% = 2 standard deviations
• MEAN ± 2 SD covers 95% of samples• 2.5% will receive dose larger of at least 7%
• 2.5% will receive dose smaller of at least 7%
• Consequences• TCP decreases of about 7% - 14%
• (based on normalizeddose gradient concept)
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Clinical trials
5/#22
Peters L J et al. JCO 2010;28:2996-3001
Critical Impact of Radiotherapy Protocol Compliance and Qualityin the Treatment of Advanced Head and Neck Cancer: Results From TROG 02.02
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Delivered dose does matter!
6/#22
Treatment deliveryTreatment planning
Quality control of
Treatment Planning
Systems
Pre-treatment imaging
Tumour & OAR Outlining
Courtesy Liz Miles RTTQA
Input
dosimetry dataCallibration of
dosimeters
Measurements of dose
distributions
of therapeutic beams
:
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Treatment planning system
• Accuracy of dose distribution calculation
Quality control of
Treatment Planning
Systems
All our clinical decisions are made based
on the treatment plan prepared with a TPS!!!
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What are characteristics of a good TPS?
• High accuracy of dose distribution calculations
• Fast calculations
• Able to prepare plans for all contemporary techniques
• User friendly
• Robust
Varian - Eclipse
Elekta - Monaco
RaySearch – RayStation
Pinnacle
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Initial remarks
• In general• what you see is what you get
• You must use the TPS you have!
• Everything you may do is• get to know deeply the system
• to read carefully manual,
• to read papers on quality control issues,
• to have contact with other, more experienced users,
• to learn about the system limitations!!!
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How to build a good model?
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What are characteristics of a good TPS?
• High accuracy of dose distribution calculations
• Fast calculations
• User friendly
• Robust
Algorithms implemented in TPS
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Step 1 - exposure
• What radiation reachesthe absorber• fluence and energy fluence
• spectrum of energy fluence
• We call it: primary radiation
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Step 1 - exposure• Fluence – F [1/m2]
• the number dN of particles (photons) incident ona sphere of a cross-sectional area of da
• Energy fluence – ψ [J/m2]• the energy dE incident on
a sphere of cross-sectional area of da
F E
da
dNF
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Energy spectrum
• Depends on• effective accelerating potential
• target material
• flattening filter material and construction• there are flattenning filter free accelerators
• head (colimator system) material and construction
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Energy spectrum
6 MV 15 MV
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Energy spectrum calculations
• Reconstruction of spectra by iterative least squaresfitting of narrow beam transmission• it requires very precise measurements of attenuation factors
• Monte Carlo• precise knowledge of the treatment head design
• now this information is usually available
• Fiting routine• a given spectrum is used to calculate PDDs (using a database of
Monte Carlo generated Kernels) and compared with the measuredones• procedure is repeated until expected compliance is obtained
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Step 2 – Energy deposition
• Primary and secondary dose
• Primary dose• interaction of primary photon
• energy transfered to chargedparticle (mostly to electron)
• electron transfered its energy to medium
• Secondary dose• interactions of secondary photons (scattered) and so on
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Primary and secondary dosePrecise modeling of primary dose is the most important!
Sontag, Med. Phys. 1995, 22 (6)
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Energy transfered from photonsto electrons Kerma
Φ=ΔN/ ΔA
Δz
ΔA
r
F
trE
r
F
r
number of interactions
per unit mass
energy transferred
to electrons
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Energy transferred to electrons
• KERMA• Kinetic Energy Released per unit mass
F
trE
r
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Charged particle equilibrium (CPE)
Photon interaction
Electron enters Dm
Electron leavs Dm
Charged particle equilibrium exists
for the volume V
if each charged particle of a given type
and energy leaving V is replaced
by an identical particle
of the same energy entering
E1,in
E2,in
E1,out
Dm
Etr,3
Etr,2
Etr,1
Etr,1
E2,out
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Kerma Collision versus Absorbed Dose
• If CPD exists
Absorbed Dose = Kermacol
Kermacol=Kerma·(1-g)
g – fraction of energy emmited in the form of Bremstrahlung
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CPD never exists• Transient CPD exists
rela
tive e
nerg
y p
er
un
it m
ass
D
Kcolzmax
depth in medium
< 1 > 1
= 1
D=·Kcol Absorbed dose is equall to
Kerma at a little smaller depth.
D=(1+fTCPE)·Kcol
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rela
tive
ener
gy p
er u
nit m
ass
D
Kcolzmax
depth in medium
< 1 > 1
= 1
Is the CPE at dmax?
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How does the fluencedependence on the distance?
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Fluence in air – inverse square low
( 2
2
fF
FF
air
fF
air+
FF +
isocenterplane
F
F
airF
isocenterplane
F
fF
air
+F
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Uncertainty – errors in distance
• Error in distance of 1 cm approximately leads to:• 1% error
• 2% error
• 3% error
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Uncertainty – errors in distance
• Distance to isocenter = 100 cm• D = 992/1002 ≈ 0.98
• 2%
• Precise calibration of telemeter does matter!
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Fluence in water – dose in water
( dF
air
fF
water efF
F + +
FF
2
2
F
fF
water
+F
fd
( )1(
2
2
gEefF
FD tr
dF
air
fF
water
+F +
r
Primary dose – dose deposited by electrons
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Uncertainty – errors in depth
• Error in depth of 1 cm approximately leads to:• 1% error
• 3% error
• 5% error
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Difference of 1 cm of soft tissue• ≈ 0.05 1/cm
• D ≈ exp(-0.05) ≈ 0,95
• 5%
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Fluence – real situation
• Radiological depth
• In general
water
rad
hhhd
r
rrr 332211 ++
kwaterkrad hd rr
( )1(
2
2
' gEehF
FD tr
h
k
F
air
hF
Q
kkk
+F +
r
h2
h3
h1
Q’
r2
r3
r1
F
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( )1(
2
2
' gEehF
FD tr
h
k
F
air
hF
Q
kkk
+F +
r
radiologicaldepth
physicaldistance
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Another aproach to dose distribution calculation• Total energy released per unit mass
(
+F
r
hdF
airh hefF
FTERMA h
2
2
• What will happen with this released energy?• mostly it will be absorbed as primary and secondary dose
• only a little energy will escape (scattered photons, bremstrahlug)
primary energy fluence
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Convolution – monoenergetic caseTERMAh = Th
TERMA
Med.Phys. Papanikolau 1993,5,1327-1336.
Convolution kernel representing
the relative energy deposited
per unit volume for photons
of energy hv;
integral over whole medium
( ( '3''),( rdrrArThrD hvhv
( 'rrAhv r
r
'r
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Convolution – polyenergetic(real case)• Integral over space and energy spectrum
( ( dhvrdrrAdhv
rdThrD hv
hv '3''
),(
Mohan, Med.Phys, 1985, 12, 592 – 597.
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Kernels – Point Spread FunctionAnders A. Ahnsjö, Med.Phys. 16 (4), 1989
• parameteres generated for beams of spectrum typical for 4Mv, 6MV, 10MV, and 15 MV
• Ө angle with respect to the direction of impinging primary photon
• w – stands for water
2/))exp()exp((),( rrbBraArhw +
bBaA ,,,
primary scattered
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Kernels
Energy imparted per cm-3
0,4 MeV
1,25 MeV 10 MeV
The dash-dotted line first scatter terma,
calculated using the Klein-Nishina
cross sections and neglecting other process
than the Compton interaction.
Acta Oncologica, 1987, Ahnesjo
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Kernels
Energy imparted per cm-3
0,4 MeV
1,25 MeV 10 MeV
The dash-dotted line first scatter terma,
calculated using the Klein-Nishina
cross sections and neglecting other process
than the Compton interaction.
Acta Oncologica, 1987, Ahnesjo
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Convolution – polyenergetic(real case)• Integral over space and energy spectrum
( ( dhvrdrrAdhv
rdThrD hv
hv '3''
),(
Mohan, Med.Phys, 1985, 12, 592 – 597.
(
+F
r
hdF
airh hefF
FTERMA h
2
2
2/))exp()exp((),( ttbBtaAthw +
( 'rrAhv
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Approximations
• To allow calculations in a resonable time several approximations are used• treatment planning system dependent
• the same model different results
• polyenergetic monoenergetic (e.g. for mean energy)
• single energy spectrum is used
• collapse cone method
• Kernels generated for water only• scaling with density
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6 MV spectrum
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0 1 2 3 4 5 6 7
Spectrum 6 MVMean Energy 1.48 MeV
polyenergetic : monoenergetic
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Changes of spectrumlateral softenning
single energy spectrum
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Changes of spectrumlateral softenning
higher energylower energy
single energy spectrum
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Collapsed Cone Convolutionspeed-up calculations
• 30 x 30 x 30 cm3 water Phantom
• 0.3 cm grid size
• 100 x 100 x 100 calulations point = 1 000 000
• Convolution: contribution from each voxel to each voxel
1 000 000 x 1000 000 = 1 000 000 000 000
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Collapsed Cone Convolution
• CCC aproaches assumes that all the energy scattered from one voxel into small coneis absorbed along the line forming the axis of the cone
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Collapsed Cone Convolution2D illustration
8 cones
Energy is absorber in blue pixels only.
Energy desposition
decreases very quickly
with distance
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Collapsed Cone Convolution2D illustration
• According to Mackie (Teletherapy: Present and Future, Advanced medical Publishing, 1996)• 100 collapsed cones is enough
• Mobius3D – 144 collapsed cones
• Pinnacle – 80 collapsed cones
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Approximation
• Scaling depth (distance) with density
h2
h3
h1
Q’
r2
r1
r3
( ( '3''),( rdrrArThrD radhvhv
𝑑𝑟𝑎𝑑 =
𝑘
𝜌𝑘/𝜌𝑤 ⋅ ℎ𝑘
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Two spaces -worlds
• World 1• Air – fluence is scaled with distance according
to square factor• Precise
• World 2• Medium – Terma and Kernel are scaled with radiological
distance (density is taken into account)• approximation
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Convolution – monoenergetic caseTERMAh = Th
TERMA
r
( ( '3''),( rdrrArThrD radhvhv
r
'r
Square factorand scaling with
density
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Summary• Be especially careful with
• Measurements of• small beams
• OF
• Extrapolations• range of data entered into the system
• range of CT made for your patient
• Distributions in the vinicityof two much different matrials (Air – soft tissue)
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Summary
• Primary and secondary dose
• Kerma and Kolision Kerma versus Dose
• How to described Kerma by photon fluence
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Summary
• It is relatively easy to calculate the dose if• Transient CPE exist
• distance
• radiological depth
• If there is no CPE situations becoms much more difficult• transport of electrons must be considered
• interface of two dosimetrically different absorbers• air-soft tissue, lung-soft tissue, bone-soft tissue
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Summary• TCP exist
• Primary dose is at least 80% of total dose• accuracy depends on primary dose calculations
• scale fluence with inverse square factor
• depth scaled with density
• first scatter is much larger than second, third etc.
2/))exp()exp((),( ttbBtaAthw +
primary scattered
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Thank you very much for your attention!
Life is complicated but very fascinating!