Introduction
• Image-guided radiation therapy (IGRT) significantly
improves the accuracy of radiotherapy.
X-ray imaging dose to
therapy patients
• It plays an essential role in the accurately delivery of
highly confirmed dose to target.
George Ding,
Ding PhD
• IGRT is the new paradigm in radiotherapy.
Department of Radiation Oncology
Vanderbilt University School of Medicine, Nashville, TN
• X-ray imaging procedures for patient setup add
radiation dose to patients.
ACMP 2011, Saturday, April 30, 2011
Chattanooga, TN
• Additional imaging dose may entail risk to patients.
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Introduction
2
Talk Outline
Commonly used x-ray image devices
• Compares the amount of radiation exposure to organs
resulting from different image guidance procedures
• MV electronic portal imaging device (EPID)
• Presents a perspective view on the imaging dose
related to the therapeutic dose
o 2D images: portal images
o 3D images: MV-CBCT
• Suggests techniques to reduce the imaging dose in
clinical applications including kV x-ray and MV xray imaging
• kV x-ray devices integrated to treatment unit
o 2D images: digital radiography
o 3D images: kV-CBCT
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4
A typical MV setup field
Electronic portal imaging device (EPID)
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6
1
MV CBCT on Linac
Examples of dose MV setup fields
An Anterior and a Rt lat fields (2 MUs for each setup field)
100
EPID
90
Brain
% volu
ume
80
70
Brain Stem
60
Left eye
50
40
Right eye
30
20
10
0
0
1
2
3
4
5
6
dose /cGy
7
Dose from MVCT
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Images of MVCT
9
MVCT on Tomo unit
10
Dose form MVCT on Tomo unit
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12
2
kV x-ray devices on treatment unit
Dose form MVCT on Tomo unit
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14
kV x-ray devices on treatment unit
kV x-ray devices on treatment unit
2D images: digital radiograph
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Dose dependency on depth between kV and MV
Dose dependency on medium for MV beam
Single beam incident from right
Single beam incident from right
110
220
6 MV
100
90
180
60
50
40
30
140
Monte Carlo
Density corrected
120
0
100
80
60
Bone slabs in water
20
40
20
10
0
0
0
2
4
6
8
10
12
14
depth in phantom /cm
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Bone slabs in water
160
Relative d
dose
Relative dose
70
125 kVp
200
Monte Carlo
Density corrected
80
16
18
20
0
2
4
6
8
10
12
14
16
18
20
depth in phantom /cm
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3
Dose dependency on medium for kV beam
Dose distributions: a single Anterior beam
Slab of water
6 MV beam
20 cm thickness
A
B
MV: ~ 1- 3 cGy
A
110 kVp beam
A
B
MV: ~ 0.01 cGy
What are the dose profilesBalong the line AB
between MV and kV beams?
125 kVp x-rays used for CBCT from a Varian Trilogy
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Dose distributions: a single Anterior beam
6 MV beam
275
A
% 6 MV beam
% 110 kVp beam
250
225
% dose
B
A
A
B
175
125
250
200
175
dose to vertebrae
(bone)
150
275
225
dose to sternum
(bone)
200
kV x-ray medium dependency: soft tissues vs. bone
150
125
100
100
75
75
50
50
25
25
0
0
0
5
10
depth (from A to B) /cm
15
B MV: exit dose 40%
kV: exit dose 4%
Data are from: J. H. Hubbell and S. M. Seltzer, "Tables of X-Ray Mass Attenuation Coefficients and Mass EnergyAbsorption Coefficients," National Institute of Standards and Technology, Gaithersburg, MD NISTIR 5632, 1995.
110 kVp beam
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3D images: kV CBCT
Radiation dose dependency on scan techniques: Head
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4
Dose dependency on scan techniques and filters: Pelvis Spot Light
Radiation dose dependency on scanned length: Pelvis
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Dose dependency on scan techniques and filters: Pelvis Spot Light
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Radiation dose dependency on patient size and scan techniques
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Perspective view: EPID ( 4 cGy) vs. kV-CBCT (~ 0.3 cGy)
100
70
% volume
Brain Stem
Left eye
40
30
Stardard Head
80
60
50
Perspective view: kV CBCT dose continue to decrease
kV-CBCT
90
Brain
80
% volume
100
EPID
90
70
60
Right eye
40
20
20
Brain Stem
50
30
Right eye
Brain
Left eye
10
10
kV-CBCT from Trilogy unit
0
0
0
1
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2
3
dose /cGy
4
5
6
0.0
0.1
0.2
0.3
dose /cGy
0.4
0.5
kV-CBCT from TrueBeam unit
0.6
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30
5
Perspective view: EPID vs. kV-CBCT (Trilogy)
Perspective view: EPID vs. kV-CBCT
4 cGy x 35 Fractions = 140 cGy
100
100
EPID
Left femur head
Rectum
80
70
Prostate
70
60
Bladder
50
Righ femur head
40
Skin
60
Left femur head
Rectum
50
Bladder
40
30
30
Righ femur head
EPID
Two 6 MV setup fields
20
20
Skin
10
0
Pelvis: kV-CBCT
90
80
% volume
% volume
90
0
1
10
2
3
4
5
1.5 cGy x 35 Fractions = 52 cGy
6
0
Prostate
0
1
2
3
4
5
6
dose /cGy
dose /cGy
kV-CBCT
from Trilogy unit OBI 1.4
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Summary
Perspective view: MV orthogonal setup fields dose ( 2-5 cGy)
Doses from image-guided procedures
z
MV imaging:
–
–
z
MV-EPID: ~ 4-6 cGy from two orthogonal setup fields
megavoltage cone-beam CT (MV-CBCT)
• Linac unit: ~ 1 – 20 cGy
y /acquisition
q
• Tomotherapy unit: 2-12 cGy
kV imaging:
–
–
kV DR: ~ 0.01 cGy
kV-CBCT
•
•
kV-CBCT from Trilogy unit
Two 6 MV setup fields
Soft tissue:
Bone:
0.1 - 3 cGy /acquisition
0.3 - 6 cGy /acquisition
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Summary
z
Summary
Conventional MV setup fields
z
Imaged area is larger than the treatment field
z
Imaging-guidance procedures are more frequent than diagnostic
imaging
z
Repeated imaging procedures can sum up significant dose to
radiosensitive organs
0.1 - 2 cGy (soft tissues), 0.3- 5 cGy (bone)
z
MV EPID imaging: exit dose ( ~ 50% of entrance dose)
For 30 fractions: 3 – 60 cGy (soft tissues) and 90 – 150 cGy (bone)
z
kV DR imaging: very low dose (also low exit dose ~ 5% of
entrance dose)
4-6 cGy from two orthogonal setup fields
For 30 factions: 100 – 200 cGy additional dose to patient
z
kV imaging: Single
Si l kV-CBCT
kV CBCT
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Techniques to reduce the imaging dose
Summary
z
z
MV imaging:
– The progress is continually being made by manufacturers.
– Dose resulting from MV-CBCT is comparable to that of multiple
portal imaging acquisitions
z
Use imaging guidance efficiently:
– Choose the procedure and the frequency that is most suitable for the
purpose
– Develop protocols for using image guidance procedures
– Pay attention to pediatric patients and imaged volume
– Negligible difference between dose to bone and dose to soft tissues
z
Improve imaging technology (manufacturers)
kV imaging:
– Dose resulting from kV-CBCT is much larger than that of multiple kV
DR acquisitions
z
– Dose to bone is 2-4 times higher than the dose to soft tissues
Account imaging dose for radiotherapy patients
– Calculate organ doses resulting from image guided procedures
– Account them as part of total dose to patients in radiotherapy treatment
planning systems
– AAPM TG-180
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Acknowledgements
Peter Munro,
Varian Medical Systems iLab GmbH
Fitz-William Taylor,
visiting student from US Military Academy at West Point
Matthew Deeley,
student at Vanderbilt University
Jason Pawlowski,
student from Vanderbilt University
Charles Coffey,
Vanderbilt University
Arnold Malcolm,
Vanderbilt University
ACCRE
Vanderbilt Advanced Computing Center for
Research and Education
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