Tolerance Limits and Action
Levels for Planning and
Delivery of IMRT QA
Introduction
Many sources of error and uncertainty factor into the
final dose distribution actually realized for a patient
Patient treatment planning (target delineation and dose
calculation)
setup uncertainty
interinter-fraction or intraintra-fraction organ motion
treatment delivery
Jatinder R. Palta PhD
Department of Radiation Oncology
University of Florida
Gainesville, Florida
The sharp dose gradients in IMRT amplify the
importance of these uncertainties, as even small
deviations from a planned treatment can compromise
outcome
Objectives
Process of IMRT planning and delivery
Potential sources of error in IMRT planning and
delivery
QA strategy for:
The Overall Process of IMRT
Planning and Delivery
Positioning and
Immobilization
File Transfer and
Management
Tolerance limits and action levels for planning
and delivery of IMRT
Resource requirements for IMRT QA
‘Chain’ of
IMRT Process
5
Position Verification
Plan Validation
6
IMRT Treatment
Planning
4
3
Position
Verification
7
IMRT Treatment
Delivery
8
Patient setup parameters
patient
Structure
Segmentation
IMRT Treatment
Planning and
Evaluation
IMRT Treatment
Delivery and
Verification
Plan Validation
Structure
Segmentation
Potential Sources of
Error in IMRT
Positioning
and
Immobilization
Image
Acquisition
File Transfer and
and
Management
2
1
IMRT planning
IMRT delivery
Patient specific QA
Image Acquisition
(Sim,CT,MR, …)
CT/MR/PET room
Lasers
Skin markers
Images
Radio. anatomy
Tumor
Treatment room
Lasers
Skin markers
Radio. anatomy
Tumor
Delineation
Beams
Margin
Accelerator
Planned beams
Treatment room
beam delivery data
17 steps with a lot of room for errors!
1
What is the dose distribution
received by the CTV ?
Types of Errors
17 small errors add up. Sometimes large errors
occur: when most of these 17 errors are in the
same direction
Treatment execution errors blur the cumulative dose
distribution
Errors that are made once per patient:
–
–
Treatment preparation (Planning) errors
Also called systematic errors - but stochastic in nature
Planning errors shift the cumulative dose distribution
Errors that are made for each treatment fraction:
–
–
CTV
Treatment execution errors
Also called random errors
Dose
General Elements of IMRT
Quality Assurance
Clinical QA of:
Consistent target volume and organs-at-risk
delineation
Quantitative assessment of organ motion
during imaging and treatment
Quantitative assessment of setup variation
during imaging and treatment
General Elements of IMRT
Quality Assurance
Technical QA of:
IMRT planning system
Treatment delivery equipment
Patient specific QA
Beam Modeling
IMRT Planning System Issues
(Cross beam profile with inappropriate modeling of extra-focal radiation)
Measured vs. Calculated
ADAC(4mm)
wobkmlctr
Diff
Dosimetric issues:
Computational issues:
Dose calculation matrix resolution
1.0
0.8
Relative Profile
Beam modeling (focal and extra-focal
radiation)
Small field output
Transmission through the leaves
Interleaf and leaf-end leakage
1.2
0.6
0.4
0.2
0.0
-15
-10
-5
0
5
10
15
-0.2
Off-Axis (cm)
2
Beam Modeling
Beam Modeling
(Cross beam profile with inappropriate modeling of extra-focal radiation)
(Cross beam profile with appropriate modeling of extra focal radiation)
Measured vs. Calculated
ADAC(4mm)
wobkmlctr
Diff
1.2
diff(w/bk)
diff(wobk)
wbkmlctr
wobkmlctr
1.2
1.0
1.0
0.8
Relative Profile
Relative Difference
0.8
0.6
0.6
0.4
0.4
0.2
0.2
0.0
-15
-10
-5
0
-15
-10
-5
5
10
15
-0.2
0.0
0
5
10
15
Off-Axis (cm)
-0.2
Off-Axis (cm)
Beam Modeling and IM Field
Output
Consequences of Inadequate
Beam Modeling
Segments
Segment 1
Segment 2
Segment 4-7, 9-13
Measured
Calculated
Diff(%)
Diff(abs)%
Diff(abs)cGy
1-2
40.85
40.2
1.6%
0.9%
0.65
3,8
32.41
28.5
13.7%
5.2%
3.91
29.9%
0.5%
0.39
6.7%
5.05
4-7,9-13
1.69
1.3
Total
74.95
69.9
13 segment IM Field
Segment 3
Segment 8
Leaf Transmission
Tongue and Groove Effect
MLC
Elekta
FWHM 3.3 mm
Avg. Min 0.76 mm
Window width
Following
Leaf
1
Leading
Leaf
MLC upper jaw
MLC Tertiary Collimator
1.05
1
0.95
0.6
Leaf motion
0.4
0.2
0
-0.5
0
0.5
Position relative to leaf tip (cm)
1
Discrepancies in dose
delivered increases for small
window width and large
number of MU. This is
because of approximate
nature of corrections for leaf
transmission, rounded leaf
tips, and leaf scatter
Relative Profile
Transmission
0.8
Analytic
M onte Carlo
Varian
2.0 mm
0.83 mm
0.9
0.85
0.8
0.75
0.7
-15
-10
-5
0
5
10
15
Off-Axis Distance (cm)
3
Tongue and Groove Effect
Without T&G Correction
Calculation Grid Size effect
6MV Photon step size effect for 20x20 fields
both data set use trilinear interpolation
With T&G Correction
100
0.2 cm
0.4 cm
diff(%)
80
Relative Output
60
40
20
0
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
-20
Relative OFF-Axis Distance (cm)
IMRT Planning System
Validation
IMRT Planning System
Validation
Strip intensity pattern with varying intensities
Center 100%
Meas. 183.2
Plan 180.0
Diff. 1.75%
Center 20%
Meas. 42.6
Plan 43.0
Diff. .23%
Center 0%
Meas. 11.6
Plan 8.1
Diff.
1.9%
A Pyramid Pattern with Equal MU
A strip pattern
A pyramid pattern
A well pattern
IMRT Guidance Document; Ezzell et.al.
A Pyramid Pattern with Equal MU
(The difference plot)
4
IMRT Delivery System Issues
Mechanical Alignment of MLC
(SMLC Delivery Issue)
MLC positional and leaf speed accuracy (MLC positional and
leaf speed inaccuracies can cause significant dose delivery uncertainties)
uncertainties)
Linac performance for small MU delivery (low MU per
segment and high dose rate can be an issue in IMRT dose delivery
accuracy)
MLC control issues (Large number of segments per field can be an
issue for some delivery systems)
MLC physical characteristics (leaf transmission, leafleaf-end
leakage, and interinter-leaf leakage can be an issue)
Leaf sequencing algorithms ( need to be optimized for
smoothness of intensity distribution, number of segments, mechanical
mechanical
limits of MLC, MU efficiency, leaf travel, and delivery time)
Leaf Speed Accuracy
(DMLC Delivery Issue)
No offset
0.6 mm offset
1.0 mm offset
MLC Control System Issues
Low MU and high dose rate can be a
problem for some Linac control
systems
1 MU per strip
Dose Rate: 600 MU/min
Dose Rate: 300 MU/min
Data
measured
on Varian
2100C
Sub-field MU Distribution
Patient Specific Quality Assurance
Patient Plan
Phantom Geometry
•
Most subfields have
less than 3
MU
(Data from
first 100 Head
and Neck
IMRT patients
treated at
University of
Florida)
Level 1 QA
5
Treatment Plan Verification: 2-D
Dose Distribution
Corvus Calculation
Film Measurement
Compare isodoses (film) and absolute dose (chamber)
Level 2 QA
Level 3 QA
IMRT Process Uncertainty
(Prostate)
Voxel Size
(mm)
Establishing a Rationale for
Tolerance Limits and Action
Levels
Imaging CT
X
Y
Mean
Displacement
(mm)
Z
X
Y
Item
Process
Uncertainty Uncertainty
(mm)
(mm)
Z
0.94 0.94 3.00 0.47 0.47 1.50
0.87
0.87
1.00
0.50
0.50
1.22
Isocenter
MLC
0.50 0.50 0.75 1.03
1.00
1.44
Inter-fraction
Intra-fraction
Organ motion
2.00
5.00
5.00
7.35
Planning RTP
Data Input
Calculation
Non-dosimetric
Delivery Machine
Setup
Overall Uncertainty (mm)
IMRT Process Uncertainty
(Head and Neck/CNS)
Voxel Size
(mm)
Imaging CT
X
Y
Mean
Displacement
(mm)
Z
X
Y
Item
Process
Uncertainty Uncertainty
(mm)
(mm)
7.60
Comparison of Measured and
Calculated Cross Plot
Z
0.94 0.94 1.50 0.47 0.47 1.50
0.55
0.55
1.00
0.50
0.50
1.22
Isocenter
MLC
0.50 0.50 0.75 1.03
1.00
1.44
Inter-fraction
Intra-fraction
Organ motion
0.50
1.00
0.00
1.12
Planning RTP
Data Input
Calculation
Non-dosimetric
Delivery Machine
Setup
Overall Uncertainty (mm)
2.85
6
Comparison of Measured and
Calculated Cross Plot
Comparison of Measured and
Calculated Cross Plot
10 mm DTA
difference
Data analysis
Measured with a diode-array (Map Check; Sun Nuclear Corp.)
RPC IMRT Phantom Results
Credentialing: IMRT
Anterior Posterior Profile
Anterior
Posterior
RPC IMRT Head and
Neck Phantom
TLD in the Target
and Organ-at-risk
volumes
Orthogonal
Radiochromic films TLD
8
Dose (Gy)
6
4
Cord
2
Organ
at Risk
Primary PTV
0
-4
-3
-2
-1
0
RPC Film
2
3
Posterior-Anterior Profile
8
Posterior-Anterior Profile
Anterior
9
Posterior
Central
RPC Film
8
6
d75
Dose (Gy)
Institution
Values
DMax
75%
7
Regression
Film
Regression
Inst.
6
4
50%
d50
dose (Gy)
5
d25
25%
4
2
Dmin
Organ
at Risk
Primary
PTV
0
-4
-3
-2
-1
RPC Film
0
1
Distance (cm)
Institution Values
4
Institution A
Institution Values
RPC Criteria for IMRT
Credentialing
RPC IMRT Phantom Results
Posterior
1
Distance (cm)
Target
2
3
4
Critical
Structure
3
D=DMax-Dmin
2
75%=Dmin+0.75 D
50%=Dmin+0.50 D
25%=Dmin+0.25 D
1
distance=d
Primary
PTV
d=(d75+d50+d25)/3
0
-3
Institution B
-2.5
-2
-1.5
-1
-0.5
0
Distance (cm)
7
RPC Criteria for IMRT
Credentialing
Posterior-Anterior Profile
9
Posterior
Anterior
8
7.36Gy+/-2%
7
6
Dose (Gy)
5
Institution
Values
3
Regressio
n Film
1.50
2.49Gy+/-3%
Regressio
n Inst.
Primary
PTV
1
0
-3
-2
-1
0
1
TLD (RPC/Inst)
2
1.00
2
3
4
Distance (cm)
0.50
7% for Planning Target Volume
4 mm DTA for the Organ-at-Risk
-6.0
-4.0
-2.0
0.0
2.0
Plan - Film (mm)
Segmental Multileaf Collimator
(SMLC) Delivery System
Tolerance
Limit
Action
Level
1 mm
0.2 mm
0.2 mm
2 mm
0.5 mm
0.5 mm
0.75 mm
radius
1.00 mm
radius
2%
2%
3%
3%
MLC*
Leaf position accuracy
Leaf position reproducibility
Gap width reproducibility
Gantry, MLC, and Table
Isocenter
Beam Stability
Low MU Output (<2MU)
Low MU Symmetry (<2MU)
4.0
Suggested Confidence Limit and Action
Level Values for IMRT Planning
Region
Confidence Limit*
(P=0.05)
Action Level
δ1 (high dose, small
dose gradient)
3%
5%
δ1 (high dose, large dose
gradient)
10% or 2 mm DTA
15% or 3 mm DTA
δ1 (low dose, small dose
gradient)
4%
7%
δ90-50% (dose fall off)
2 mm DTA
3 mm DTA
* Mean deviation used in the calculation of confidence limit for all regions is expressed
As a percentage of the prescribed dose according to the formula,
δi = 100% X (Dcalc – Dmeas./D prescribed)
Dynamic Multileaf Collimator
(DMLC) Delivery System
Tolerance
Limit
Action
Level
0.5 mm
0.2 mm
0.2 mm
±0.1 mm/s
1 mm
0.5 mm
0.5 mm
±0.2 mm/s
MLC*
* Measured at all four cardinal gantry angles
Leaf position accuracy
Leaf position reproducibility
Gap width reproducibility
Leaf speed
Gantry, MLC, and Table
Isocenter
Beam Stability
0.75 mm
radius
Low MU Output (<2MU)
Low MU Symmetry (<2MU)
1.00 mm
radius
3%
2%
5%
3%
IMRT Treatment Planning
(per patient treatment planning requirements for IMRT are challenging
Resource Requirements for
IMRT
IMRT requires significantly
more time for planning than
conventional 3DCRT. It also
turns out that large academic
medical centers invest more
hours per patient in IMRT
planning than community
centers. Driving the disparity
is the variation in:
Initial goals, constraints
and target definitions
Number of plans prepared
for physician review
Time spent reviewing plans
(physician and physicists,
jointly)
Additional Planning Time/Patient
12
Additional Hours/IMRT Plan
-4
Diff (mm)
-1.3
2.0
-1.0
0.0
-3.3
-3.7
-2.3
0.0
-0.7
-4.0
RPC Film
4
Critical
Structure
Inst.
1
2
3
4
5
6
7
8
9
10
10
8
800-bed AMC(2001)
400-bed AMC(1998)
6
600-bed Community(2001)
300-bed Community(1999)
4
2
0
1
Type of IMRT Facility
Data from Oncology Roundtable
8
Patient-Specific Quality
Assurance
6
5
800-bed AMC(2001)
4
600-bed AMC(1997)
3
400-bed AMC(1998)
600-bed Community(2001)
2
300-bed Community(1999)
Effort/Patient (Hour)
20
Additional QA Time/Patient
Hours
•Check dosimetry
(absolute dose)
Ion chamber
measurements in
a phantom
•Verify spatial dose
distribution
Film in a
phantom
•Check pretreatment
position
Port films taken
with initial MLC
jaw position and
matched to
corresponding
DRRs
Head & Neck IMRT Treatment
Planning Effort/Patient @ UF
Physics staff
10
Radiation Oncologist
1
0
3
1
Type of IMRT Facility
Data from Oncology Roundtable
A consensus of approximately four additional hours per patient at
program inception. However, the QA time is likely to go down as the
program matures.
6
Experience(Months)
9
12
IMRT program startup effort (approximately 20 hrs/patient) can be
significant for the physics staff, but it comes down rapidly.
Resource Requirement Analysis
(A facility treating a total of 300 RT patients that includes 40 IMRT patients)
Additional Physics/Dosimetry time (.4 FTE)
Additional machine QA (100hr)
Additional treatment planning QA (50 hr)
Patient specific QA (160 hr)
Additional time for treatment planning (200 hr)
Maximum machine workload (per 8 hr shift)
Expected to go down from 32 patients to 24
Machine uptime
Expected to go down from 99% to 95% due to
more wear and tear of delivery equipment with
IMRT
9