Introduction
X-ray Imaging Dose from IGRT
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Image-guided radiation therapy (IGRT) has dramatically
improved the accuracy of radiotherapy
George Ding
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IGRT has emerged as the new paradigm in radiotherapy.
Department of Radiation Oncology
Vanderbilt University School of Medicine, Nashville, TN
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X-ray imaging, such as cone-beam CT (CBCT), for patient
setup can add radiation dose to patients.
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Additional imaging dose may entail biological risk
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Knowledge of x-ray imaging dose is useful in managing
additional exposure to radiosensitive organs.
WE-A-203-1
AAPM 2010 CE program
Wednesday, July 21, 2010
Philadelphia, PA
CE: AAPM 2010, 7/21/2010, Philadelphia, PA
Imaging modalities and the use in
radiotherapy
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electronic portal imaging device (EPID)
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kilovoltage digital radiography (kV DR)
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megavoltage cone-beam CT (MV-CBCT)
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kilovoltage cone-beam CT (kV-CBCT)
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CT-on-rails
CE: AAPM 2010, 7/21/2010, Philadelphia, PA
MV AP setup field (Head and Neck)
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CE: AAPM 2010, 7/21/2010, Philadelphia, PA
Electronic portal imaging device (EPID)
Flat panel
detector
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MV AP setup field
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MV AP setup field (double exposure)
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MVCT images
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MVCT on Tomotherapy unit
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MVCT on Linac unit
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MVCT images
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MVCT Tomotherapy unit
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CT-on-rail system
Images of MVCT
Tomotherapy unit
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kV CBCT system
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kV Lat setup field
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CE: AAPM 2010, 7/21/2010, Philadelphia, PA
kV CBCT system
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kV CBCT application for SBRT- Lung
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kV CBCT application for SBRT- Prostate
kV CBCT application for SBRT- spine
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CE: AAPM 2010, 7/21/2010, Philadelphia, PA
Monitoring treatment progress and ART
Feasibility of treatment target margin reduction
CE: AAPM 2010, 7/21/2010, Philadelphia, PA
CE: AAPM 2010, 7/21/2010, Philadelphia, PA
Information from 2D images
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2D projected verification images:
– Bony anatomy relative to treatment isocenter
– Seeds position relative to treatment field
– Two orthogonal project images for 3D positioning
– Treatment field shaped by MLC
Information from images
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3D images from cone-beam CT:
– Can be viewed in axial, coronal, and saggittal
reconstructions from the volumetric images
– Organ shape and position relative to isocenter
– Small lung tumor
– Can be used for monitor treatment progress and adaptive
radiotherapy (ART)
– Potential treatment target margin reduction
CE: AAPM 2010, 7/21/2010, Philadelphia, PA
CE: AAPM 2010, 7/21/2010, Philadelphia, PA
Radiation doses
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Detailed list see AAPM TG-75
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2D projected verification images:
Radiation doses cont…
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2D projected verification images:
– kV beams from OBI device ( ~ mGy/field)
– MV beams EPID ( 1-2 MU ~ 1- 2 cGy/field)
• Setup fields: two orthogonal beams
• Setup fields: two orthogonal beams
• mAs can be automatically adjusted (patient size↑ → dose↑)
• Treatment portal field
• Dose rapidly decreases as a function of depth in patient
– can be acquired during treatment (no additional dose)
– Before or after ( can be added to total dose)
• Double exposure (area larger than the treatment field)
CE: AAPM 2010, 7/21/2010, Philadelphia, PA
CE: AAPM 2010, 7/21/2010, Philadelphia, PA
Dose dependency on depth
Radiation doses cont…
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CBCT volumetric images:
– scan area is larger than the treatment field (~cGy/scan)
– Standard acquisition mode
– Low-dose acquisition mode
– For the same scan (patient size ↓ → dose ↑)
CE: AAPM 2010, 7/21/2010, Philadelphia, PA
CE: AAPM 2010, 7/21/2010, Philadelphia, PA
Dose dependency on medium for kV beam
Dose dependency on medium for MV beam
220
110
90
70
Bone slabs in water
160
Relative dose
Relative dose
180
Monte Carlo
Density corrected
80
125 kVp
200
6 MV
100
60
50
40
140
Monte Carlo
Density corrected
120
100
80
60
30
Bone slabs in water
20
40
20
10
0
0
0
2
4
6
8
10
12
14
depth in phantom /cm
CE: AAPM 2010, 7/21/2010, Philadelphia, PA
16
18
20
0
2
4
6
8
10
12
14
depth in phantom /cm
CE: AAPM 2010, 7/21/2010, Philadelphia, PA
16
18
20
Slab of water
Isodose distributions: single AP X-ray (CBCT beam) no bow-tie
20 cm thickness
A
X-rays (125 kVp)
B
125 kVp x-rays used for CBCT from a Varian Trilogy
CE: AAPM 2010, 7/21/2010, Philadelphia, PA
Isodose: single AP 6 MV beam
CE: AAPM 2010, 7/21/2010, Philadelphia, PA
Dose profiles along the line A-B on chest CT: kV vs. MV
350
A
Single AP field
Sternum
(bone)
300
6 MV beam
125 kV beam
Relative dose
250
B
200
Vertebrae
(bone)
150
100
50
0
0
A
2
4
6
8
10
12
14
Depth
/ cmin patient /cm
central-axis
depth
CE: AAPM 2010, 7/21/2010, Philadelphia, PA
16
18
B
CE: AAPM 2010, 7/21/2010, Philadelphia, PA
kV-CBCT scans
OBI 1.4
D
e
t
e
c
t
o
r
Small
x‐ray Patient
source
200 degree source rotation and the detector is centered (a) Full fan type scan Data are from: J. H. Hubbell and S. M. Seltzer, "Tables of X-Ray Mass Attenuation Coefficients and
Mass Energy-Absorption Coefficients," National Institute of Standards and Technology, Gaithersburg,
MD NISTIR 5632, 1995.
CE: AAPM 2010, 7/21/2010, Philadelphia, PA
D
e
t
e
c
t
o
r
Large Patient
x‐ray source
360 degree rotation and the detector is shifted to one side (b) Half fan type scan Figure 1 Illustration of (a) the source rotation range for full fan type scan in which the x-ray source starts
from patient’s left side and ends at 20 degrees over patient’s right side, (b) half fan type scan in which
the x-ray source starts from patient’s left side and ends at patient’s left side
CE: AAPM 2010, 7/21/2010, Philadelphia, PA
Radiation dose dependency on scan techniques: Head
OBI 1.3
OBI 1.3 Head scan: x-ray source rotated a full circle (360 degrees +10)
OBI 1.4
OBI 1.4
100
Head Scan: OBI 1.3
90
Spinal cord
80
(b)
(c)
% volume
(a)
70
Left eye
50
Brain
40
Dose distributions in color wash saggittal views resulting from
30
(a)
OBI 1.3 Head scan in which the kV x-ray source rotated 370 degrees
20
(b)
OBI 1.4 Standard Head scan in which the x-ray source rotates 200 degrees below
the patient
10
(c)
OBI 1.4 Standard Head scan in which the x-ray source rotates 200 degrees above
the patient
Right eye
0
5
Standard Dose Head: OBI 1.4
% volume
% volume
Spinal cord
60
Brain
Right eye
Bone
Body
x-ray source rotates above the patient
Right eye
70
Brain
Spinal cord
60
50
Body
40
Bone
20
10
Left eye
0
0.0
0
0.0
Left eye
30
20
10
Standard Dose Head: OBI 1.4
90
70
30
20
80
80
40
15
Head scan x-ray source rotated 200 degree above the patient
100
50
10
dose /cGy
CE: AAPM 2010, 7/21/2010, Philadelphia, PA
Head scan: x-ray source rotated 200 degrees below the patient
90
Body
0
CE: AAPM 2010, 7/21/2010, Philadelphia, PA
100
Bone
60
0.5
1.0
dose /cGy
1.5
2.0
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0.5
1.0
dose /cGy
1.5
2.0
CE: AAPM 2010, 7/21/2010, Philadelphia, PA
Radiation dose dependency on scanned length: Pelvis
Radiation dose to organs: DVH analysis
100
Pelvis: Full scan length
90
Left femur head
% volume
80
Righ femur head
70
60
50
Body
40
30
Rectum
Bladder
20
Prostate
10
Full length (17 cm)
Reduced length (5 cm)
0
0
1
2
3
dose /cGy
CE: AAPM 2010, 7/21/2010, Philadelphia, PA
CE: AAPM 2010, 7/21/2010, Philadelphia, PA
4
Radiation dose dependency on scanned length
Radiation dose dependency on scan techniques and filters: Pelvis Spot Light
Half-fan Bow-tie
Full-fan Bow-tie
Dose profiles in axial direction
dose /cGy
3
Standard scan length (17 cm)
Scan length set to 10 cm
Scan length set to 5 cm
2
1
Rectum
0
-10
-5
0
5
10
z-axis /cm
CE: AAPM 2010, 7/21/2010, Philadelphia, PA
CE: AAPM 2010, 7/21/2010, Philadelphia, PA
Radiation dose dependency on patient size and scan techniques
Thorax
Pelvis
100
Bladder
40
Rectum
30
Right femur head
Body
Left femur head
70
60
Bladder
50
40
Rectum
30
Right femur head
Body
20
20
10
0
% volume
60
50
Child
80
Left femur head
70
Low Dose Thorax: OBI 1.4
90
Child
80
% volume
100
Pelvis: OBI 1.4
90
10
Intestine
0
5
10
15
0
Intestine
0
1
2
3
4
5
Courtesy of Parham Alaei
dose /cGy
dose /cGy
CE: AAPM 2010, 7/21/2010, Philadelphia, PA
CE: AAPM 2010, 7/21/2010, Philadelphia, PA
Dose distributions resulting from MV-CBCT: pelvic scan
Courtesy of George Xu
CE: AAPM 2010, 7/21/2010, Philadelphia, PA
CE: AAPM 2010, 7/21/2010, Philadelphia, PA
Summary
Commonly used image-guided procedures
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MV imaging:
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Summary cont...
Doses from image-guided procedures
– MV-EPID: ~ 1-2 cGy /setup field
– megavoltage electronic portal imaging (MV-EPI)
– megavoltage cone-beam CT (MV-CBCT)
– megavoltage cone-beam CT (MV-CBCT)
• Linac unit: ~ 1 – 20 cGy /acquisition
• On Linac unit
• Tomotherapy unit: 2-12 cGy
• On Tomotherapy unit
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kV imaging:
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– kV-CBCT
– kilovoltage cone-beam CT (kV-CBCT)
– CT-on-rail
CE: AAPM 2010, 7/21/2010, Philadelphia, PA
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Imaging-guidance procedures are repeated daily
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kV DR imaging: high entrance dose
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exit dose (~ 5% of entrance dose)
• Soft tissue:
0.1 - 4 cGy /acquisition
• Bone:
0.3 - 8 cGy /acquisition
CE: AAPM 2010, 7/21/2010, Philadelphia, PA
Summary cont...
Imaged area is larger than the treatment field
kV imaging:
– kV DR: ~ mGy (entrance dose)
– kilovoltage digital radiography (kV DR)
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MV imaging:
Summary cont...
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MV beam imaging:
– Dose resulting from MV-CBCT is comparable to that of multiple
portal imaging acquisitions
– Negligible difference between dose to bone and dose to soft
tissues
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MV EPID imaging: exit dose (~50% of entrance dose)
kV x-ray imaging:
– Dose resulting from kV-CBCT is much larger than that of multiple
kV DR acquisitions
– Dose to bone is 2-4 times higher than the dose to soft tissues
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Compare radiation exposure from current kV-CBCT with
conventional EPID for patient positioning
Dose from kV-CBCT < Dose from EPID
CE: AAPM 2010, 7/21/2010, Philadelphia, PA
CE: AAPM 2010, 7/21/2010, Philadelphia, PA
Techniques to reduce scan exposure
Techniques to reduce scan exposure
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Rotating the x-ray source above or below the patient can result in a
factor of 3 difference in the dose to the lens of the eye
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The imaging dose to the patient can be further reduced (50%) in Pelvis
Spot Light mode if the mode is calibrated with the full fan bow-tie
filter.
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X-ray source rotating above the patient in Pelvis Spot Light mode can
minimize dose to the rectum (a 70% reduction)
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Dose to a patient for an abdominal site can be reduced significantly
(70% reduction) by selecting Low Dose Thorax scan instead of Pelvis
scan for small (e.g., pediatric) patients.
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Significant reductions in both the radiation dose (40%) and the volume
can be achieved by reducing the scanned length when only a smaller
region of interest is needed.
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
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Use latest version of scan software:
The imaging doses to patient resulting from OBI 1.4 are much
lower than OBI 1.3. For some CBCT scans, the dose is more
than 15 times less in OBI 1.4.
CE: AAPM 2010, 7/21/2010, Philadelphia, PA
CE: AAPM 2010, 7/21/2010, Philadelphia, PA
Future considerations
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Account imaging guidance 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
Acknowledgements
Parham Alaei,
University of Minnesota
Charles Coffey,
Vanderbilt University
Peter Munro,
Varian Medical Systems iLab GmbH
Jason Pawlowski,
Vanderbilt University
George Xu
Rensselaer Polytechnic Institute
ACCRE
Vanderbilt Advanced Computing Center for
Research and Education
CE: AAPM 2010, 7/21/2010, Philadelphia, PA
CE: AAPM 2010, 7/21/2010, Philadelphia, PA