Patient-based Quality Assurance
for IMRT
CE-IMRT4
Dirk Verellen et al.
Verellen
- AAPM 2004AZ-VUB
Department of
Radiotherapy,
Conformal Radiation Therapy
Verellen - AAPM 2004
Creation of conformal dose distributions
& Target oriented positioning !
Outline
Î Customize
Î Target
z
z
Assess interplay between IGRT and IMRT
SM & IM assessment
Î Dose
z
z
your QA and Treatment Verification
localization - IGRT:
delivery - IMRT:
Tools: Dosimeters & Phantoms
Procedures
Î Hazard
Î Beware
& Discrepancy Analysis
of what has NOT been verified
Verellen - AAPM 2004
Customize QA procedure
Î Target
localization and IMRT:
Don’t miss the target
z Most IMRT deliveries are temporal
z
z
Real-time knowledge of the target localization
becomes an integral part of patient-related QA
for IMRT!
Î Hazard
analysis ⇒ customize procedure
Verellen - AAPM 2004
Customize QA procedure
Î How
to get comfortable?
Î Create
an efficient QA-procedure
“Don’t drown in film measurements”
z Efficient processing required
z Separate “machine specific” from “patient
specific” QA
z
Verellen - AAPM 2004
Target Localization & dose delivery
Î Conformal Dose distribution
z High dose volume is shaped to the volume occupied by
the target.
z Don’t miss the target!
∴PTV and PRV should reflect set-up accuracy!!!
Î Temporal Intensity Modulation
z Optimization based on snap-shot.
z Target displacement/movement influences dose
distribution.
∴Real-time knowledge of anatomy required!!!
Verellen - AAPM 2004
Target Delineation
Verellen - AAPM 2004
Don’t miss the target: SM & IM
Physical patient
Î
Î
Î
Î
Î
Î
Î
Î
Î
CT room coords
Lasers
Skin markers
Images
Bone
Tumor
Delineation
Margin
Planned beam
Î
Î
Î
Î
Î
Î
Î
Î
Treatment room coords
Lasers
Skin markers
Bone
Tumor
Beam
Linac
Treatment room
17 possibilities for geometrical
errors
Verellen - AAPM 2004
Van Herk et al.
Virtual patient
Don’t miss the target: SM & IM
“perfect alignment”
+
room lasers
+
+
+
+
+
+
+
skin markers
bone references
Verellen - AAPM 2004
Anatomy tracking
Don’t miss the target: SM & IM
Verellen - AAPM 2004
Reduction of SM & IM:
Patient immobilization
Verellen - AAPM 2004
Reduction of SM & IM:
Target localization
Target localization≠immobilization
Verellen - AAPM 2004
Reduction of SM & IM:
Electronic Portal Imaging
Verellen
- AAPM
2004
As a positioning tool: only
2D
information
Reduction of SM & IM:
Ultrasound Guidance
Verellen - AAPM 2004
Reduction of SM & IM:
Cone beam CT
Verellen - AAPM 2004
Jaffray et al.
Reduction of SM & IM:
Stereoscopic X-ray Imaging
Verellen - AAPM 2004
Reduction of SM & IM:
Stereoscopic X-ray Imaging
Implanted marker matching
Automated DRR fusion
Verellen - AAPM 2004
Reduction of IM
Individualized IM
Verellen - AAPM 2004
Gating
Temporal intensity modulation
Field 2
2
1
2
1
OAR
1
2
PTV
2
Field 1
Field 3
Verellen - AAPM 2004
Temporal intensity modulation
Field 2
2
1
2
1
4
3
4
1
2
6
5
6
2
Field 1
Field 3
Verellen - AAPM 2004
Temporal intensity modulation
Field 2
2
Field displacement
1
2
overdosage
Field 1 1
Field
displacement
2
3
4
5
2
3
4
3
4
5
6
4
5
3
6
4
underdosage
Verellen - AAPM 2004
1
2
Field 3
Field displacement
Temporal intensity modulation
Film moving, non-gated delivery
Film moving, gated delivery
Verellen - AAPM 2004
Coping with motion: gating
NOVALIS BODY II & III
Verellen - AAPM 2004
Verellen et al, Radiother. Oncol. 2003
Coping with motion: gating
Verellen - AAPM 2004
Coping with motion: gating
Verellen - AAPM 2004
Coping with motion: tracking
Verellen - AAPM 2004
Dose delivery
Î Create
an efficient QA-procedure
“Don’t drown in film measurements”
z Efficient processing required
z
Î Separate
“machine specific” from “patient
specific” QA
Verellen - AAPM 2004
QA for IMRT: 4 levels
Î
Pre-clinical verification of
IMRT treatment
(Patient related)
Î
Verification of fluence
maps, individual IMRT
fields on water phantom
Î
IMRT delivery specific QA
Î
Basic QA (linac, MLC)
4
3
2
1
Verellen - AAPM 2004
Level 1: Basic linac QA
Î Tests
for Validation and after every accelerator
check
z
z
z
z
z
MLC calibration and alignment
Speed Stability
Beam on/off stability
Gravity test
MLC reliability test
Î Weekly
test to find out if your MLC performs within
specs
z
e.g. Garden fence test
Verellen - AAPM 2004
Level 1: weekly QA pattern
Test Pattern with Leaf Error
Test Pattern after leaf replacement
and MLC calibration
Verellen - AAPM 2004
Solberg et al.
Level 2: IMRT delivery specific QA
Î Acceptance
Î Commissioning
z
z
e.g. alignment in tomotherapy
e.g. MLC specifications and influence on OF
Î Small
field dosimetry
amounts of MU
Î Leaf control properties
Î Small
Verellen - AAPM 2004
Level 2: Chair
Leaf transmission only
Absolute dose
Leaf transmission
&
Leaf separation
A. Van Esch et al.
Verellen - AAPM 2004
Level 3: Verification of IMRT delivery
Î Geometric
fluence profile
test profile
Î Making use of EPIDs
Î Irregular
Verellen - AAPM 2004
Level 3: Test pattern
Verellen - AAPM 2004
Level 3: Test pattern
Test Pattern with 2 mm Leaf Error
and 10% Dose Error
Test Pattern correctly delivered
Verellen - AAPM 2004
Solberg et al.
Level 4: Different philosophies
Î Every
z
day ↔ Every patient ↔ Class solutions
QA procedure largely dependent on approach
Î Top-down
↔ Bottom-up
zDetailed
zAnalysis
zTime
straightforward
consuming
zComprehensive
zDiscrepancy
analysisVerellen
complicated
- AAPM 2004
Level 4: Comprehensive Verification
➨ Verification of treatment in toto:
● absolute dose verification: thermoluminescent and alanine detectors
●
Evaluation of dose distribution: film dosimetry
Original IMRT Plan simulated for phantom
Verellen
- AAPM 2004
verification
Level 4: 2 legs to stand on
Î The
ability to create conformal dose
distributions
How to make sure it performs adequately each
time?
(machine related!)
z How to make sure this particular treatment plan
can be realized? (patient related!)
z
Î Target
z
oriented positioning
How to make sure not to miss the target each
time, and cope with motion during IMRT
Verellen - AAPM 2004
delivery?
QA in IMRT: an example
Î Non-patient
related
(Is the system performing within specs?)
z
Comprehensive test for IMRT delivery capability

Fluence map created by TPS, sequencing from TPS,
transferred and delivered
“Test Pattern”

z
e.g. every week
Regular detailed QA of linac and MLC (basic
verification)

e.g. every month
Verellen - AAPM 2004
QA in IMRT: an example
Î Patient related
(Can the system deliver this particular case?)
z
Comprehensive test for class solution (preclinical verification)
Commercial “IMRT” phantom
Anthropomorphic phantom
 Gel
 …


z
Pre-treatment verification for each patient
Independent MU calculation
Absolute dose check
Verellen - AAPM 2004
 Verification of fluence patterns


Tools
Î Dosimetry
Î Phantoms
Verellen - AAPM 2004
Dosimeters
Î
Integrating
z
z
z
z
z
z
z
Î
TLD chips
Alanine chips
MOSFET
Radiographic film (X-OMAT V, EDR-2)
Radiochromic film
EPID
Gel
Non-integrating
z
z
z
z
Ionization chamber (conventional, micro, pin-point)
diodes
diamond
Linear array detectors
Verellen - AAPM 2004
Dosimetric verification
Î Down
scaling of Monitor units
Losing small segments
z Underestimation of scatter and leakage dose
z
Î Small
field dosimetry
dose delivery:
Î Temporal
z
z
integrating dosimeters (TLD, alanine, film, gel)
non-integrating (ionization chamber, …)
Verellen - AAPM 2004
Dosimeters
Î0

z
z
z
Film
EPID
Array of TLD chips
z
Conventional
Micro
z
Î3
Diodes
MOSFET
Diamond
TLD, alanine
z
dimensional
z
Ionization chamber

Î1
Î2
dimensional
z
dimensional
z
z
z
Gel
Stack of film
3D-array of TLD chips
dimensional
z
z
Stack of TLD chips
Verellen - AAPM 2004
Linear array detectors
Phantoms
Î Generally
z



z
Internal heterogeneities are anatomically relevant
Multiple dosimeter comparison difficult
Geometric alignment cumbersome
Geometrically regular



z
3 types
Anthropomorphic
Internal construction precise
Multiple dosimeters possible
Alignment straightforward
Geometrically irregular


Create fluence map to obtain a homogeneous dose distribution
Easy for analysis
Verellen - AAPM 2004
Phantoms
Î 0D:
z
Ionization Chamber
ABSOLUTE
Î 2D:
z
Film Dosimetry
RELATIVE
Verellen - AAPM 2004
Phantoms
Î 0D:
z
z
Ionization Chamber
ABSOLUTE
TLD, alanine
ABSOLUTE-RELATIVE
Î 1D:
z
Stack of TLD
Î 21/2D:
z
Film Dosimetry
RELATIVE
Verellen - AAPM 2004
Phantoms
Î 0D:
z
TLD, alanine, MOSFET
ABSOLUTE-RELATIVE
Î 2D:
z
z
Verellen - AAPM 2004
Film Dosimetry
RELATIVE
Stack of TLD
Phantoms
Î 3D:
z
z
RELATIVE
ABSOLUTE?
De Wagter et al.
Verellen - AAPM 2004
Phantoms
Î 2D
z
Create fluence map to
generate a
homogeneous dose
distribution
Williams et al.
Verellen - AAPM 2004
A. Van Esch et al.
Phantom Verification
Î Necessary
z
z
tools:
Dose Export of a defined area or plane into file or
clipboard (ASCI)
Independent registration of measurement and calculation
needed
Verellen - AAPM 2004
Procedures: level 4
Î Fluence
profiles
Measurement and analysis
z Using measured fluence profiles to recalculate
the dose distribution
z
Î Combining
Î Absolute
z
z
phantoms and dosimeters
dose verification
Measurement
Calculation
Î 3D
Verification of dose distribution
Verellen - AAPM 2004
Fluence profiles: Film Dosimetry
rel. OD
Film Response
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
EDR2
X-Omat V
0
2
4
6
8
Dose (Gy)
Verellen - AAPM 2004
Fluence profiles: Film Dosimetry
DD < 3%, DTA < 2mm
Verellen - AAPM 2004
Ahlswede et al.
Fluence profiles: EPID
reference image
measured image
acceptance criteria:
∆ Dmax (e.g. 1 %)
DTA (e.g. 1 mm)
Verellen - AAPM 2004
A. Van Esch et al.
Fluence profiles: the other way around
Î Using
measured fluence profiles imported back into
the planning system to calculate what has been
delivered!
Verellen - AAPM 2004
Combining phantoms and dosimeters
Î Transferring
the patient’s treatment
parameters to a phantom and recalculate the
resulting dose distribution: “Mapping”

Verification with the actual treatment parameters
- Dose distribution may not be relevant
Î Simulating
the patient’s treatment on a
phantom: “Simulation”

Verification of specific treatment requirements
- Actual treatment is not verified
Verellen - AAPM 2004
Absolute dose and MU validation
Î MU
z
validation requires either
Direct measurement of dose using TPS MUs and
fluences

Time-intensive
„
„
„

z
Temporal
High dose gradients
Small field dosimetry
Currently most thorough method of validation
Independent computation of dose


Most efforts still single point
Ideally, recompute entire 3D dose
Verellen - AAPM 2004
Absolute dose measurement
Verellen - AAPM 2004
Alanine dosimetry
Calculated
(Gy)
SD
(Gy)
Measured
(Gy)
SD
(Gy)
meas/calc
case (a)
case (b)
case (c)
20.09
20.04
19.99
0.14
0. 09
0.05
20.01
19.79
19.77
0.20
0.12
0.19
1.00
0.99
0.99
Det. 1
Det. 2
Det. 3
10.73
10.85
4.32
0.14
0.12
0.07
10.74
10.77
4.31
0.13
0.11
0.04
1.00
0.99
1.00
Solitary target:
Target
surrounding OAR:
Verellen - AAPM 2004
Ionization chamber*
H&N
IMRT
Calc.
SD
Meas.
SD
(cGy)
(cGy)
(cGy)
(cGy)
166.6
4.5
165.0
0.78
201.3
3.4
200.1
0.56
5 field
Evenly
distributed
5 field
Avoiding air
cavities
*NAC 007 micro ionization chamber,Verellen
Wellhöfer
- AAPM 2004
Independent computation of dose
Î Single
ÎA
z
point approach
simple method, spread sheet – based:
Imported from TPS:


z
MU per beam
Segment shapes and weights
Not used from TPS:


Original TMR data, OF, OAR
Determination of segments that cover measuring point
Verellen - AAPM 2004
Independent computation of dose
unit normal deviate
-10
-8
-6
-4
-2
0
2
4
6
8
10
45
0.21
40
frequency
35
0.18
normal distribution
counts
30
0.15
25
0.12
20
0.09
15
0.06
10
0.03
5
0
0
-10 -9
-8
-7
-6
-5
-4
-3
-2
-1
0
1
2
3
4
5
difference (cGy)
Mean: -0.2cGy
SD: 2.0cGy
Verellen - AAPM 2004
6
7
8
9
10
Dose distribution: mapping
Original IMRT Plan
Original Plan mapped onto
Phantom
Verellen - AAPM 2004
Dose distribution: simulation
Original IMRT
simulated
for phantom
Verellenplan
- AAPM
2004
verification
Film Dosimetry : mapping
Gamma:Verellen
4% -DD
4mm DTA
AAPM /2004
Film Dosimetry : mapping
Verellen - AAPM 2004
Film Dosimetry: simulation
Verellen - AAPM 2004
Film Dosimetry: simulation
Verellen - AAPM 2004
Film Dosimetry: simulation
Gamma:
4%
DD
/ 4mm DTA
Verellen
- AAPM
2004
Phantom verification: gel measurement
Relaxation rate image
with contours along the
pixels with 90% of the
maximal dose
Transversal CT- slice
with PTV and calculated
90%-, 50%-, 20%- and
10%-isodose lines
MRT-slice
with contour along
pixels with 90% of the
maximal dose
Ahlswede et al.
Verellen - AAPM 2004
Phantom verification: gel-film
GEL
FILM - GEL
FILM
PLANNING - GEL
PLANNING
De Wagter et al.
PLANNING - FILM
Verellen - AAPM 2004
Hazard analysis
ÎIntuition/experience
from conventional
RT is lost
ÎFind weak links
ÎDefine control points
Verellen - AAPM 2004
Hazard analysis: some examples
Î Leaf
calibration
e.g. OF can change with 7% for 0.1 cm difference
in small field sizes for an Elekta linear
accelerator.
z Leaf sequence important
z
Î Tertiary
collimator
Alignment, abutting slices
z Clearance
z
Verellen - AAPM 2004
Leaf Sequencing : DMLC ↔ SMLC
100
90
80
70
60
100
90
50
40
30
20
80
70
60
10
0
0
5
10
15
20
25
50
40
30
20
10
0
0
Verellen - AAPM 2004
5
10
15
20
25
Leaf Sequencing
“Close-in” Technique
“Sweep“ or
“Sliding Window” Technique
Desired
IM - Profile
4
Trajectory
Sequence
4
3
2
3
2
1
1
Ahlswede et al.
Verellen - AAPM 2004
Sequential tomotherapy: alignment
0°
270°
90°
Verellen - AAPM 2004
Sequential tomotherapy: alignment
Verellen - AAPM 2004
Sequential tomotherapy: indexing
Hardware + Software
Verellen - AAPM 2004
Sequential tomotherapy: alignment
120
115
Dose (%)
110
alanine
TLD
film
planning
105
100
95
Second rotation
First rotation
90
0
1
2
3
4
5
6
7
8
9
10
11
Underdosage
12
13
14
Overdosage
15
postition (mm)
Verellen - AAPM 2004
Direction of table movement
Sequential tomotherapy: alignment
115
110
Dose (%)
105
alanine
film
planning
100
95
90
First rotation
85
0
1
2
Index +1mm
3
4
5
6
7
8
9
10
11
12
13
14
15 Underdosage
16 17
Second rotation
18
Overdosage
position (mm)
Verellen - AAPM 2004
Direction of table movement
Sequential tomotherapy: alignment
120
115
Dose (%)
110
105
alanine
film
planning
100
95
90
First rotation
85
0
1
2
Index -1mm
3
4
5
6
7
8
9
10
11
12
13
14
15
Underdosage
16 17
Second rotation
18
Overdosage
position (mm)
Verellen - AAPM 2004
Direction of table movement
Clearance and choice of origin
Verellen - AAPM 2004
Clearance and choice of origin
Verellen - AAPM 2004
Clearance and choice of origin
Verellen - AAPM 2004
Discrepancy analysis
Î TPS:
z
z
z
z
z
Basic beam data (PDD, OF, leaf offsets, penumbra)
Linac model
Dose calculation algorithm
Leaf sequencing algorithm
…
Î Experiment:
z
z
z
z
TLD calibration
MLC data transfer
Experimental set-up (many things can go wrong: MU,
positioning, gantry, … typically after-hours)
…
Verellen - AAPM 2004
Discrepancy analysis (cont’d)
Î Delivery
z
z
z
MLC calibration
Linac operation
…
Î Analysis
z
Incorrect registration (measurement ↔ calculation)
Down-scaling of MU
z
…
z


Losing small segments
Underestimating leakage/transmission dose
Verellen - AAPM 2004
Discrepancy analysis
R2 = 1/T2
Planning minus gel
Planning minus gel
< -5%
Verellen - AAPM 2004
De Wagter et al.
> +5%
Discrepancy analysis
0.8 cm
1.4 cm
2 x 15o
2 x 16o
Verellen - AAPM 2004
De Wagter et al.
Discrepancy analysis
The relationship between measured versus calculated dose in function of increased
constraints to the OAR while maintaining the prescribed target dose at 1.00 Gy
Case 1
Case 2
Case 3
Case 4
Prescribed
Calculated
T
(Gy)
OAR
(Gy)
T
(Gy)
OAR
(Gy)
T
(Gy)
Measured
OAR
(Gy)
T
OAR
(Gy) (Gy)
1.00
1.00
1.00
1.00
0.25
0.10
0.06
0.03
1.08
1.07
1.17
1.15
0.32
0.19
0.05
0.03
1.08
1.09
1.09
0.94
0.34
0.19
0.09
0.08
1.00
1.02
0.94
0.82
Verellen - AAPM 2004
Meas/Calc
1.04
0.98
1.83
2.92
MU/º
0.79
0.81
1.30
1.40
Inefficient use of the beam
Dose (OAR) ↓
&
Dose (Target) remains constant
⇓
The number of available ports ↓
⇓
The number of MU/° or MU/segment ↑
⇓
The contribution of leakage & scatter dose ↑
Verellen - AAPM 2004
Discrepancy analysis: Influence of leakage dose
Î
Î
Ionization chamber measurements showed a transmission of
0.5% through the vanes of the MIMiC.
This enables to calculate an estimated leakage dose based
on the total amount of MU delivered during tomotherapy
Total Leakage Calculated C + L Measured
MU
(cGy)
(cGy)
(cGy)
(cGy)
Case 1
Case 3
458
755
2.29
3.78
32.4
4.67
34.7
8.45
33.6
8.56
M/C M/(C+L)
1.04
1.83
0.97
1.01
Verellen - AAPM 2004
Discrepancy analysis
∆D = 1% DTA = 3mm
day 1 vs day 6
day 1 vs day 2
day 1 vs day 4
Verellen - AAPM 2004
A. Van Esch et al.
Discrepancy analysis
≠ Rectal filling
day 1
day 4
day 1 versus day 4
Verellen - AAPM 2004
A. Van Esch et al.
Discrepancy analysis
Wrong energy
ABSOLUTE
RELATIVE
∆D=3.3% and DTA=3mm
∆D=3.3% and DTA=3mm
MLC failure
∆D=5.5% and DTA=3mm
Verellen - AAPM 2004
A. Van Esch et al.
Beware of what has not been verified
Î Threshold
for skin contouring in TPS
dose
Î Heterogeneity correction
Î Choice of phantom
Î Target localization
Î…
Î Extra-target
Verellen - AAPM 2004
Extra-target dose
Gonick & Huffman
Verellen - AAPM 2004
WBED for a prostate case
Î Assuming
identical scatter conditions:
Hp(10) = 1.55 x 10-2 mSv/MU
Hp(70 Gy) = Hp(10) x #MU x #fractions
serial tomother. (654 MU, 5 arcs):
IMRT 1 (490 MU, 6 fields):
z IMRT 2 (128 MU, 6 fields):
z Dynamic arc (292 MU, 1 arc):
z
z
Verellen - AAPM 2004
Heterogeneity correction
Cranial
measurement
Caudal
measurement
air cavity :
target volume :
Verellen - AAPM 2004
1774 mSv
1595 mSv
417 mSv
158 mSv
Heterogeneity correction
CC Algorithm
Clarkson Algorithm
PB Algorithm
Verellen - AAPM 2004
Target volume
Heterogeneity correction
Verellen - AAPM 2004
Heterogeneity correction
BrainSCAN PB Pinnacle CCC
calc/meas
calc/meas
Balthazar I
1.05 ± 0.03
1.02 ± 0.03
1.06 ± 0.02
1.00 ± 0.02
1.04 ± 0.01
1.02 ± 0.01
(radius 2.1cm)
Balthazar II
(radius 1.4cm)
Melchior
(side: 5.9cm)
Verellen - AAPM 2004
Heterogeneity correction
80
30
60
50
40
80 95
90
100 100
5
30
50
80
80
5
30
30
60
40
20
50 80
95 95
90
80
90
50
20
5
Film – Brainscan PB
20
40
60
Film - Pinnacle CCC
80
20
40
60
80
(100%
⇔ 2Gy)
Verellen - AAPM
2004
Heterogeneity correction
# pixels / total #pixels
1
0.8
0.6
0.4
PB
film
sphere+margin
CCC
0.2
sphere only
50
100
150
200
Dose
(cGy)
Verellen
- AAPM 2004
Choice of phantom
Gamma: 4% DD / 4mm DTA
Verellen - AAPM 2004
IMRT: Patient related QA
Analysis?
Integration?
Efficiency?
Verellen - AAPM 2004
Conclusions
Î Analyze
the chain of events in your IMRT
treatment procedure
Hazard analysis: define control points
Customize QA/QC procedure
z Get comfortable with each step
z Beware of what has not been verified!
z
z
Î Complementary
dosimetry
procedure should be efficient
Î Training of personnel!
Î QA
Verellen - AAPM 2004
Acknowledgements
Special thanks to:
Ann Van Esch
(University Hospital Leuven)
Carlos De Wagter
(Ghent University Hospital)
Julia Ahlswede
(Charité, Berlin)
Tim Solberg
Verellen - AAPM 2004
(UCLA)