How to use image information? Pawe Kukoowicz Verification of - - PowerPoint PPT Presentation

IGRT2 How to use image information?

Paweł Kukołowicz

Verification of radiotherapy

 In space of dose

 Comparison of prescribed and delivered dose

(dose distribution)

 Eg. In-vivo dosimetry

 In space of location

 Portal control

 image based

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Portal control

 To minimize the set-up error  There are systematic and random errors

in patient positioning

 systematic errors deteriorate the dose delivery

much more than random errors (3x)

 The aim of portal control is to minimize the systematic

error!

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Veryfication of geometry

 Geometry

 Comparison of

 reference image and  treatment field image

Field edges Center of the beam

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Reference image

 Simultor image

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Reference image

 Simultor image

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Reference image

Digitally Reconstructed Image

 

4 4 1 1

3 3 2 2 ( d d d d

e I I

       

 

   

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DRR

digitally reconstructed radiograph

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Quality of DRR

 Depends on

 slice separation

 it is recommended to use 3 mm slice separation  but

 3 mm slice separation makes contouring very

tedious

 interpolation tools  these tools must be checked !

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Portal images

EPID Courtesy of B.Heijmen

Edges

zero of the second derivative of intensity

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Matching of reference and portal images

X Y P  12/42

Structures to be matched brain

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AP lateral

Structures to be matched H&N

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AP lateral

Structures to be matched pelvis

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AP lateral

Correction strategies

• 10
• 5

5 10 5 10 15 20 25

Fraction

Systematic and random errors Single patient, one direction

Mean value = systematic error Standard deviation = random error

AP direction

Cortesy of B.Heijmen

• 10
• 5

5 10 5 10 15 20 25

Fraction patient 2 patient 1 patient 3

Systematic and random errors For a few patients

mm

Cortesy of B.Heijmen

Systematic and random errors – 2D

head left

Cortesy of B.Heijmen

Mean group error Mx: <mi,x>  0 My: <mi,y>  0 Distribution of systematic errors x: SD(mi,x) y: SD(mi,y) Random group error

  

i 2 x , i x

N 1   

i 2 y , i y

N 1

m1 m3 m2 m4

A few patients

head left

Systemic and random errors Group of „similar” patients

Cortesy of B.Heijmen

600 prostate patients

Distribution of errors

AP

250 500 750

• 10
• 5

5 10

Random AP error (mm) N

AP

20 40 60

• 10
• 5

5 10

Systematic AP error (mm) N

Cortesy of B.Heijmen

(De Boer and Heijmen, IJROBP, 2001)

On-line protocols

• measure and correct in the same fraction

Off-line protocols

• measure during first few fractions

correct if needed

• SAL Shrinking Action Level (Amsterdam)
• NAL No Action Level (Rotterdam)
• eNAL extended NAL

Strategies

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10

• 5

5 10 15 5 15 20 25 30 35

AP displacements (mm) Prostate cancer patient Fraction

On-line correction

A few MU image Remainder MU most dose delivered with very small errors: correction of both systematic and random errors

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Data for on- off-line corrections

 2D

 EPID

 3D

 2 orthogonal iamges  CT type control

 kV cone beam CT  MV cone beam CT  CT on rails

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NAL (de Boer and Heijmen, IJROBP, 2001)

 Fraction 1,2, and 3

 set-up a patient according to protocol  portal control, (mix,miy,miz), i =1 ,2,3

 before 4th fraction

 calculate the systematic error

(mx,mean,my,mean,mz,mean)

 from 4th fraction on

 set-up a patient according to prtocol  shift couch with –(mx,mean,my,mean,mz,mean)  irradiate de Boer and Heijmen, Med. Phys. 2002

10

• 5

5 10 15 5 15 20 25 30 35

Fraction : initial error after set-up

Residual error

No NAL

No Action Level

The random error remains the same! : error after correction Residual error estimate

res  /N

(De Boer and Heijmen, IJROBP, 2001)

600 prostate patients

20 40 60

• 10
• 5

5 10

Systematic AP error (mm) N

With no correction

in

20 40 60 80 100

• 10
• 5

5 10

Systematic AP error (mm) N

With NAL

res

NAL results

(1) De Boer et al. 2002 (2) Kaatee et al. 2002 (3) De Boer et al. 2003 (4) De Boer et al. 2004

How precise may be radiotherapy?

Residual (after NAL) bony anatomy displacements [mm]:

Prostate

(1)

Cervix

(2)

LR CC AP  1.7 1.5 1.6 res 1.1 1.1 1.1  2.6 2.9 2.7 res 1.2 1.7 1.6 Lung

(3)

 2.0 2.4 2.4 res 1.3 0.6 1.2 head & neck

(4) 

1.6 1.4 res 1.1 1.2 1.6 1.0

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Why on-line verification is not recommended?

 Because

 It is time consuming.  Systemtic error influence on the margin three

times more than random error.

 However,

 It might be resonble if

 random error is large  very high accuracy is needed.

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Set Up Margin Internal Margin

Margins

 Set-up margin

 to compensate set-up errors 

errors measured with respect to external coordinate system (laser system)

 Internal margin

 to compensate movement of

the target caused by physiology (eg. breathing)



errors measured with respect to internal anatomy coordinate system ( eg. pubis symphisis)

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 

 

2 int 2 ernal up set tot

    



 

 

2 int 2 ernal up set tot

    



How to add margins?

 If set-up and internal errors may be treated

as not correlated, than we add errors in quadrature

systematic random

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tot tot

M       7 , 2

Margins

 Two formulas

tot tot

M       7 , 5 , 2

To cover the CTV for 90% of the patients with the 95% isodose (analytical solution). Herk Red, 47: 1121 - 1135, 2000 Margin size which ensures at least 95% dose is delivered to (on average) 99% of the CTV. Stroom, Red, 43: 905-919,1999

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Implementation of geometry control

 The most important task in radiotherapy

department

 „Lens of quality”

 This can’t be an incidental action

 This must be a program for the systematic

monitoring and evaluation of the various aspects

• f radiotherapy quality

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Data

 Must be collected and regurarly analysed

 feed back is a must

 in our department ones a year all results are presented

to doctors, radiation technologiests and physicists

 big errors must be analysed as quickly as possible

 conclusions must be drawn

 group systematic errors (mean of means) play

an important role in general evaluation of the quality of work and quality of equipment

 group systematic error should not be different from zero

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Breathing and related problems

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CT for planning

 Artefacts

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without breathing control with breathing control

Changes of GTV position

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In relations to bronchial tree

CT for planning

 With breathing control

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RPM system

Pattern of breathing

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Patient A Patient B

Deep-inspiration breath hold technique

 DIBH

 for patients with left breast cancer

 for some of tchem (they have to inhale and keep inhale

for some time 10 – 15 sec)

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DIBH - advantages

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Thank you for your attention!

p.kukolowicz@zfm.coi.pl