Quality Assurance in Stereotactic
diosurgery and Fractionated Stereotactic
Radiotherapy
David Shepard,
Shepard Ph.D.
Ph D
Swedish Cancer Institute
Seattle, WA
Timothy D. Solberg, Ph.D.
University of Texas Southwestern Medical Center
Dallas, TX
Quality Assurance in Linac SRS/SBRT
Outline
Mechanical
M
h i l aspects
• Linac
• Frames
eam data acquisition
g of TP system
y
ommissioning
nd-to-end evaluation
maging and Image Fusion
rameless
l
Radiosurgery
di
References and Guidelines
Can we hit the target?
n we putt the
th dose
d
where
h
we wantt it?
How accurate is radiosurgery?
actic Radiosurgery, AAPM Report No. 54, 1995
er sources:
MRI Distortion
Image Fusion
Relocatable frames
Dosimetric
Frames & CT
Isocentric Accuracy: The
Winston-Lutz Test
Mechanical Uncertainties
he projection of the
all centered within
the field?
the projection of the
all centered within
the field?
od results ≤ 0.5 mm
Isocentric Accuracy:
The Winston-Lutz
Test
daily Lutz test is extremely
important because:
he mechanical isocenter
an shift over time
he AMC board in Varian
ouches can fail
he cone or MLC may not be
epositioned perfectly after
ervice
ore (right) and After (left)
tively simple couch adjustment
Aft
After
Before
f
A Lutz test with the MLC is also
important because:
The Cone
Cone-based
based Lutz test does not
ell you anything about the
mechanical isocenter of the MLC
The MLC may not be repositioned
perfectly after service
Lutz test with 12 x 12 mm MLC field
End-to-end localization evaluation
Structure
C li d
Cylinder
Cube
Cone
AP
00
0.0
20.0
-35.0
Phantom Specifications
LAT
VERT
00
0.0
30 0
30.0
-17.0
40.0
-20.0
40.0
iPlan Stereotactic Coordinates
AP
LAT
VERT
10
1.0
04
0.4
30 8
30.8
20.8
-17.1
42.4
-34.6
-19.7
40.8
End-to-End Localization Accuracy
End-to-End Localization Accuracy
(Surely my vendor has checked this)
Verification of MLC
shapes and isocenter
System Accuracy
Simulate the entire
procedures: Scan,
target, plan, deliver
Phantom with
fil holder
film
h ld
Pin denotes
isocenter
Resulting film provides measure of
targeting accuracy …
Offset from
intended target
… as well as falloff for a multiple arc delivery
1
0.8
0.6
0.4
02
0.2
0
-0.5
0
0.5
1
Phantom
Hidden Target
g Test
scan,, p
plan,,
localize assess
Lucy Phantom
Imaging Uncertainties
se CT for geometric accuracy
se MR for target delineation
RI contains distortions which
pede direct correlation with CT
ta at the level required for SRS
SRS”
ereotactic Radiosurgery – AAPM Report No. 54
ther References
Sumanaweera,
S
JR Adler,
Adl
S Napel,
N
l ett al.,
l Characterization
Ch
t i ti
off
atial distortion in magnet resonance imaging and its implications
1.8 ± 0.5 mm shift of
MR images relative to
CT and
d delivered
d li
d dose
d
Shifts occur in the
f
frequency
encoding
di
direction
Due to susceptibility
artifacts between the
phantom and fiducial
markers of the
Leksell localization
box
anabe, GM Perera, RB Mooij, “Image distortion in MRI-based
er gel dosimetry of Gamma Knife stereotactic radiosurgery
equency Encoding = L/R
Frequency Encoding = A/P
anabe, GM Perera, RB Mooij, “Image distortion in MRI-based
er gel dosimetry of Gamma Knife stereotactic radiosurgery
at do we do about MR spatial distortion?
Use Image Fusion
Fusion Verification
ndard Diode
Dosimetric Uncertainty
Small field measurements
can be challenging; Diodes
and small ion chambers
are well suited to SRS
dosimetry, but their
characteristics / response
must be well understood.
Stereotactic Diode
Small field depth dose show familiar trends
0
6
12
18
24
30
36
42
60
80
100
50
100
150
200
Depth (mm)
250
300
350
400
Similar machines have similar characteristics
36A
12A
36 UNMC
12
0
50
100
150
200
Depth (cm)
250
300
350
400
Penumbra: Cones versus MMLC
3.5
Cones
MMLC
3
2.5
2
1.5
1
0.5
0
0
20
40
60
80
100
Diode Warnings!!!
Diodes exhibit enough energy dependence that
atios between large and small field measurements
re inaccurate at the level required for radiosurgery
easure output factors ≤ 2x2 cm2 using diode
≥ 2x2 and ≤ 4x4 cm2 using diode & chamber
≥ 4x4 cm2 using chamber
Diode response will drift over time
Re-measure reference between each chance in
ield size
Reference diode output to
intermediate field size
utput
actor
=
Reading (FS)diode
Reading (Ref’)diode
X
Reading (Ref
(Ref’)
)IC
Reading (Ref)IC
Reference diode output to
intermediate field size
NO!
YES!
Reading (6 mm)diode
Reading (100 mm)diode
Reading (6 mm)diode
Reading (24
mm)diode
X
Reading (24 mm)IC
Reading (100 mm)IC
adiosurgery beams exhibit a sharp decrease
in output with decreasing field size
Significant
uncertainty
Don’tt use high energy
Don
Need proof that beam data
acquisition for small
fields is difficult?
Surveyed Beam Data from 40 identical treatment units:
Percent Depth Dose
Relative Scatter Factors
Absolute Dose-to-Monitor Unit CF
Reference Condition
Applied statistical methods to compare data
Percent Deptth Dose
servations of some treatment units: 6 mm x 6 mm MLC
110
100
90
Institution A
80
InstitutionAB
Institution
Institution C
70
60
50
40
30
20
10
0
0
50
100
150
200
250
Depth (mm)
300
350
400
450
10
00
90
Institution A
Institution B
80
6.0 %
70
10.3 %
60
50
40
30
20
10
0
0
50
100
150
200
250
Depth (mm)
300
350
400
450
bservations of some treatment units: 5 mm collimator
22
11
Institution A
A
Institution
Institution B
88
66
44
22
00
00
50
50
100
150
Depth
(mm)
Depth (mm)
200
250
300
servations of some treatment units: 15 mm collimator
.2
.2
Institution A
Institution A
Institution B
11
0.8
.8
0.6
.6
0.4
.4
0.2
.2
00
00
50
50
100
100
150
150
Depth
(mm)
Depth (mm)
200
200
250
250
300
300
Relative Output Factor: 6 mm x 6 mm MLC
Outp
put Fac
ctor
~45%
Outp
put Fac
ctor
Relative Output Factor: 42 mm x 42 mm MLC
~6%
Institution
Commissioning your system: Does calculation
agree with measurement?
nd-to-end testing
simetric uncertainty
Calculation
Calculation
arc-step
= 10o
arc-step = 2o
ocenter
ocenters
4 field box
Dynamic Conformal Arcs
End-to-end testing
Dosimetric
Do
i et i
uncertainty
Absolute Dosimetry
Independent MU
Calculations
-to-end dosimetric
evaluation
RPC SRS Phantom
es not include MRI imaging
es not include image fusion
ovides only a spherical target
Hidden Target
ecludes
l d iimageimage-guided
id d positioning
iti i
int dosimetry limited by TLD accuracy
mited information on dose distribution
st results require return of phantom
Conventional
Localization
Absolute Dosimetry
Lucy Prototype
Imaging Insert
R contouring and
osimetry based on
R contours
Circular volumes
for image fusion
MR Fusion
MR based contouring,
treatment plan and dosimetry
What about “Frameless Systems?”
“frameless” stereotactic system provides
calization accuracy consistent with the safe
livery of a therapeutic dose of radiation
ven in one or few fractions, without the aid
an external reference frame, and in a
anner that is non-invasive.
ameless stereotaxis is inherently image guided
o required:
i d
mobilization – need not be linked to localization
ereo)photogrammetry the principle behind
rameless
l
technologies
t h l i
Photogrammetry is a
easurement technology
in which the threemensional coordinates
f points on an object
are determined by
easurements
t made
d in
i
two or more
photographic images
t k
taken
ffrom different
diff
t
positions
Stereophotogrammetry in Radiotherapy
Optical
otogrammetry
Spatial Resolution: 0.05 mm
Temporal Resolution: 0
0.03
03 s
Localization Accuracy:
0.2 mm
Stereophotogrammetry in Radiotherapy
X-ray Stereophotogrammetry
Calibration of Frameless SRS Systems
Establish spatial
dimensions
Specify the
isocenter
How do we know the system is targeting properly?
nd-to-end evaluation that mimics a patient procedure
X-ray
entify target & plan
DRR
Set up
p in treatment room
Irradiate
Results of Phantom Data
(
(mm)
)
L t
Lat.
L
Long.
V t
Vert.
3D vector
t
Average
-0.06
-0.01
0.05
1.11
Standard
Deviation
0.56
0.32
0.82
0.42
• Sample size = 50 trials (justified to
95% confidence level,, +/- 0.12mm)
)
Comparison in 35 SRS patients
and 565 SRT fractions
1.2
Lateral
Sup/Inf
Ant/Post
Single Fraction
1
0.8
0.6
04
0.4
0.2
0
-6.0
Single
-4.0
-2.0
0.0
2.0
(mm)
AP
Lat
Average
0.05
-0.06
0.06
4.0
6.0
8.0
Axial 3D vector
0.26
1.01
1.2
Lateral
S /I f
Sup/Inf
Ant/Post
Multiple Fractions
0
1
0.8
0.6
04
0.4
0.2
0
-6.0
ultiple
-4.0
-2.0
0.0
2.0
(mm)
AP
Lat
Average
0 17
0.17
0 17
0.17
4.0
6.0
8.0
Axial 3D vector
0 47
0.47
2 36
2.36
1.2
uperior / Inferior
Multiple Fraction
Single Fraction
1
08
0.8
0.6
0.4
0.2
0
-6.0
-4.0
-2.0
0.0
2.0
4.0
6.0
8.0
eless localization appears equivalent to frame
frame-based
based rigid fixation
Localization using
mplanted fiducials
Localization using implanted fiducials
Radiosurgery Guidelines
TRO/AANS
/
Consensus Statement on stereotactic radiosurgery
d
quality improvement, 1993
OG Radiosurgery QA Guidelines, 1993
PM Task Group Report 54, 1995
opean Quality Assurance Program on Stereotactic
Radiosurgery, 1995
N 6875-1 (Germany) Quality Assurance in Stereotactic
Radiosurgery/Radiotherapy, 2004
PM Task Group 68 on Intracranial stereotactic positioning
systems, 2005
R Practice Guidelines for the Performance of Stereotactic
Radiosurgery 2006
Radiosurgery,
R Practice Guidelines for the Performance of Stereotactic Body
Radiation Therapy, 2006
PM Task
T kG
Group 101,
101 St
Stereotactic
t ti Body
B d Radiotherapy,
R di th
2008