IMRT_VMAT_Session 5_How to fine-tune the commissioning of a TPS.pptx

5 How to fine-tune the
commissioning of a TPS
Stephen Gardner
Medical Physicist
June 18
Henry Ford Health System (Detroit, MI, USA)
1

Outline for today’s session
1. Spot checking your commissioning data
1. Using MPPG 5A report as a reference/outline for
photon beam model validation
2. Summary of validation tests
2. Tweaking TPS: DLG and MLC transmission
• Create a new “commissioning” machine in your TPS
• Compare to measured data and perform iterations of
DLG and MLC transmission adjustments
2

Learning Objectives: After this session, you will…
o Be able to perform a comprehensive but quick spot-check of
commissioning data
o Understand the detailed, step-by-step process of how to adjust
DLG and MLC transmission in the TPS, referencing their clinic’s
data
o Be able to execute the logistics and team communications
needed to safely fine-tune commissioning (e.g., creating a
separate “machine”)

1. Spot checking your
commissioning data
4

ZOOM POLL
The dosimetric leaf gap (DLG) represents:
A. The difference in the physical leaf end and the dosimetric field edge
B. The minimum gap that is possible for an MLC system
C. The width of the sweeping gap field used during measurements
D. The width of the penumbra of an MLC-defined field
5

ZOOM POLL
The dosimetric leaf gap (DLG) represents:
A. The difference in the physical leaf end and the dosimetric field edge
B. The minimum gap that is possible for an MLC system
C. The width of the sweeping gap field used during measurements
D. The width of the penumbra of an MLC-defined field
6

References/Guides for Photon Beam
Commissioning
o MPPG 5A has proven to be an extremely
helpful guide for commissioning photon
algorithms – published in 2015
• https://doi.org/10.1120/jacmp.v16i5.5768
o The strength of this document is that it
covers many aspects of algorithm
performance in a comprehensive, clear,
and concise way
o Scope of the document – gives
recommendations for:
• Data Acquisition and Processing
• Algorithm Validation (Photon and Electron)
– Basic model validation
– Heterogeneity correction validation (not
included in this lecture)
– IMRT/VMAT dose validation
7

MPPG 5A Breakdown
o Table 3
• Basic validation of beam model – comparison in beam configuration
workspace, dose normalization is accurate
o Table 4
• More in-depth validation of 3D-CRT delivery – measurements of dose
agreement in high-dose region, penumbra, and out-of-field region
o Table 5
• Summary of dose agreement tolerances
o Table 6 (N/A for this lecture) – heterogeneity correction
validation
o Table 7
• IMRT/VMAT Validation – including TG-119/Clinical Case validation

Table 3: Basic Model Validation
o Use Table 3 from the MPPG document to perform basic validation of the model:
• (5.1) Within the physics/beam config module, compare PDD and profile for large field (calculated
vs. measured)
• (5.2) Within a test plan in the TPS - calculate a plan using absolute dose calibration conditions
and ensure that you are calculating 1 cGy/MU at the calibration point (sanity check!)
• (5.3) Quick check of PDD and output factor (OF) – within a test plan in the TPS, compare
calculated PDD and OF to measured data
o Note – these tests don’t require any new measurements! Just some re
9

Table 3: Examples –
Test 5.1
o Test 5.1 – Within
the physics module
(beam
configuration
workspace),
compare the PDD
and profile –
measured vs.
calculated dose
10

Table 3: Examples – Test 5.2
o Test 5.2 – Calculate plan using
calibration geometry and
ensure dose at calibration
depth is 1 cGy/MU.
** For our clinic, the calibration
geometry is 10x10 cm2 field
size, 100 cm SSD, with
calibration depth at dmax.
11

Table 3: Examples – Test 5.3
o Test 5.3 – Calculate a plan in
the TPS that simulates
scanning data and compare
calculated vs. measured
• This requires the user to take
a line profile at the
appropriate depth
** For our clinic, this involved
calculating a plan at 100 SSD for
the relevant field size

Table 4: Basic Photon Beam Validation Summary
o To perform this set of validation tests, measure the absolute dose at several
points for each of these fields:
• Different depths (slightly beyond dmax, mid-range/10-15 cm depth, and deep/25-30 cm
depth)
• Different off-axis positions – high dose, penumbra, and low dose
13

Table 4: Examples - Test
o Test 5.4 – small MLC-defined
field (4x4 cm2 MLC field and
5x5 cm2 jaw)
o Test 5.5 – large MLC-defined
field with extensive blocking
o Test 5.6-5.7 – use same
aperture with different SSD
** For each of these, perform
measurements at high dose,
penumbra, low dose regions at
dmax depth, 10 cm depth, and
25 cm depth.
Test 5.4
Test 5.5
Tests 5.6-5.7

Table 5: Evaluation Methods and Tolerances
o Three different regions are specified: high dose, penumbra, and low
dose tail
• For high dose and low dose tail regions – tolerance is based on dose
difference (Percent)
• For penumbra – tolerance is based on distance to agreement (DTA)

Table 7: VMAT/IMRT Summary
o The IMRT/VMAT tests are in
addition to the tests in Table
3/4 from prior slides
o In my opinion - the most
important tests from this
group are the TG-119 tests
(7.3) and Clinical tests (7.4)
• These plans will be used to
validate and adjust the TPS
model as needed to ensure
optimal IMRT/VMAT delivery

Summary – Validation Testing
(Scanning/Open Field)
o Scanning Data –
• Verify profiles using flatness and symmetry for small, mid-size, and large field sizes
at dmax, 10 cm, and 30 cm depth
– For example: 4x4, 10x10, and 30x30 cm2 field sizes (you can verify additional field
sizes if warranted)
– Typically, we want to see symmetry < 1% and then flatness consistent with past
results (within 1% of baseline if available)
– Compare measurement to calculation in TPS to verify model behavior
• Verify PDD for small, mid-size, and large field sizes by comparing measured and
calculated doses
– While you are scanning these - include MLC-defined small field PDD to satisfy
MPPG 5A test 7.1 (if you have access to a small field detector such as diode)
o Output Factors – verify agreement between calculated and measured
output factors for a variety of field sizes, ranging from 3x3 to 40x40 cm2
• While you are measuring these - include MLC-defined small field OF to satisfy
MPPG 5A test 7.2 (again if you have access to a small field detector such as diode,
micro ion chamber, etc.)

TG-119 and Clinical Case PSQA
o TG-119 Planning Guide, Reporting Form, and
Structure Sets can be found on the AAPM website:
https://www.aapm.org/pubs/tg119/default.asp
o TG-119 data set includes DICOM CT, RT Structure
files as well as planning goals and instruction
document:
• C-Shape
• Mock HN plan
• Mock Prostate
• Multi-Target
o Additional plans for testing – representative plans
from previous patients
• Replan as needed with new machine/beam model
• Should include typical disease sites encountered at your
center – HN, Prostate, Lung, Brain, etc.

2. Tweaking TPS: DLG and MLC
transmission
19

Overview for this Section
o MLC DLG and Transmission – effect on the dose distribution
for various delivery modalities (static fields, IMRT, and VMAT)
o Practical Overview/Tips on MLC Parameter Adjustment
o Creating a new ‘commissioning’ machine for your testing
o How to adjust DLG and MLC Transmission - Detailed, step-by-
step process of how to adjust these within the TPS, referencing
their clinic’s data
o Practical dry-run of adjusting model parameters with realistic
clinical data

MLC DLG and Transmission –
Effect on the Dose Distribution for
Various Delivery Modalities

MLC DLG Effect – Static Fields
0
20
40
60
80
100
0 2 4 6 8 10 12 14 16
Distance Along Profile (cm)
DLG =0.100 cm
DLG =0.200 cm

MLC Transmission Effect – Static Fields
0
20
40
60
80
100
0 2 4 6 8 10 12 14 16
MLC_trans = 1.6%
MLC_trans = 2.5%

MLC DLG Effect –
IMRT Delivery
0
20
40
60
80
100
120
0 2 4 6 8 10 12 14 16 18 20
Relative
Dose
(%)
DLG Effect - IMRT Delivery
DLG = 0.14 cm
DLG = 0.16 cm
DLG = 0.115 cm

MLC DLG Effect –
IMRT Delivery (cont’d)
0
20
40
60
80
100
120
8 9 10 11 12 13 14 15 16 17 18
Relative
Dose
(%)
DLG Effect - IMRT Delivery
DLG = 0.14 cm
DLG = 0.16 cm
DLG = 0.115 cm
Change in
penumbra width
Change in high
dose magnitude

MLC Transmission Effect –
IMRT Delivery
20
30
40
50
60
70
80
90
100
110
0 2 4 6 8 10 12 14 16 18 20
Relative
Dose
(%)
MLC Transmission Effect - IMRT Delivery
MLC_trans = 1.65%
MLC_trans = 1.85%

MLC DLG Effect (TG-119 C-Shape)
VMAT Delivery
0
20
40
60
80
100
120
0 2 4 6 8 10 12 14
Relative
Dose
(%)
DLG Effect - VMAT Delivery
DLG = 0.14 cm
DLG = 0.16 cm
DLG = 0.115 cm

MLC DLG Effect (TG-119 C-Shape)
VMAT Delivery
90
92
94
96
98
100
102
104
0 2 4 6 8 10 12 14
Relative
Dose
(%)
DLG Effect - VMAT Delivery
DLG = 0.14 cm
DLG = 0.16 cm
DLG = 0.115 cm
Smaller Change in
penumbra width
compared to IMRT
Smaller change
in high dose
compared to
IMRT

MLC Transmission Effect –
VMAT Delivery
10
20
30
40
50
60
70
80
90
100
110
0 2 4 6 8 10 12
Relative
Dose
(%)
MLC Transmission Effect - VMAT Delivery
MLC_trans = 1.65%
MLC_trans = 1.85%

How to – create a new ‘commissioning’
machine in your TPS and adjust DLG/MLC
Transmission
o General idea is to…
• Create a copy of the machine
• Create a copy of the beam model
• Adjust DLG/MLC Transmission to improve agreement

Creating a ‘Commissioning’ Machine
o For ARIA Users, the
process is:
• RT Admin Workspace
– Step 1: Select
machine to copy ->
Insert -> Export
Machine…
– Step 2: Re-import
the machine you
just exported and
name it
appropriately
– Step 3: Rename
imported machine
something like
‘Test_Physics’
– Step 4: Ensure MLC
add-on information
matches the real
machine and enter
starting DLG/MLC
transmission values
Step 1: Export
Step 2: Re-import
Step 4: MLC Add-on
Material and DLG/MLC trans
Step 3: Update
Machine Name

Creating a copy of the
beam model for testing
o For ARIA Users, the process is:
• Beam Config Workspace
– Step 1: select the machine
(Test_Physics) / energy (6x) /
algorithm (AAA_11030) and right-click
-> New Beam Data…
– Step 2: setup copy of beam model
▪ Enter appropriate therapy unit name
▪ Select ‘Copy existing data to the
calculation model’
• DO NOT select ‘assign’
• Ensure that you have selected the correct
beam model to copy and click OK
– Step 3: Match and Assign Add-Ons –
select ‘In Use’ for open field and EDW
and select Automatic Match for All.
– Step 4: Spot check values for Gamma
Error Histogram and Output Factors
to verify consistency against clinical
machine and approve test model
(right-click on the model -> Approve)
Step 3: Match and assign
Add-ons
Step 2: Setup copy of
beam model
Step 1: select
model
Step 4:
Approve Model

Practical Overview/Tips on MLC Parameter
Adjustment

How do I adjust the DLG/MLC Transmission
Values?
o For ARIA v13 and later:
• RT Admin Workspace –
– **Note – this will define these values for all beam models for this machine**
– Go to ‘Radiation and Imaging Devices’
– Select the test physics machine and go to ‘MLC’ tab
– Enter values for MLC Transmission factor and DLG within the ‘Dosimetric
Properties’ section
o For ARIA v11 and earlier:
• Beam Configuration Workspace –
– Select the test physics beam model
– Go to Beam Data -> Dosimetric Data and enter MLC Transmission factor and
DLG
o For other TPS vendors – consult with the manual for instructions on
this process

Initial
Overview –
Iterative
Tweaking
Process
• Resource for TPS beam model validation – Medical Physics Practice Guideline 5.A
• MLC Parameter/Beam Model Optimization (like any optimization process) is
iterative
• The overall process goes something like:
1. Acquire initial measurements for MLC parameters -> input to TPS
2. Calculate Beam Model
3. Generate/calculate IMRT/VMAT plans for verification
• For conventional planning – use TG-119 dataset and some previous
clinical plans if available
• For SRS/SBRT planning – critical to use representative stereotactic
plans to validate the beam model!!!
• These treatment plans should meet the relevant clinical
goals/constraints to best simulate a typical IMRT/VMAT delivery
4. Acquire point dose measurements for verification IMRT/VMAT plans
• Proper detector selection is critical – ideal chamber is a small volume
ion chamber such as CC01 or PinPoint chamber
• High dose readings to simulate the target volume!
• Low dose readings to simulate critical organs at risk!
• The point dose measurements will the primary means for MLC
parameter selection
5. Acquire planar dose measurements using Gafchromic film or array device
• Compare results once MLC parameters are finalized from point dose
measurements

MLC
Parameter
Testing –
Practical Key
Points
• Key Points for Emphasis
• Use real IMRT/VMAT plans to validate the beam model/MLC parameter values
• TG-119 data sets
• Previous clinical cases
• Make sure the intended use of the linac is included in the test cases!
• Measurements should include both point and planar dose analysis
• My clinic preference – use point dose measurements for initial tweaking of
MLC parameters
• When is the model good enough?
• TG-119 utilized confidence limits for QA results
• High Dose Point Measurement → CL = ±4.5%
• Low Dose Point Measurement → CL = ±4.7%
• Planar Dose Measurement → Gamma(3%,3mm) > 87.6%
• TG-218 proposed tolerance limits and action limits for pre-treatment QA:
• Tolerance Limit:
• Ion Chamber Measurement <2%
• Gamma(3%,2mm) > 95%
• Action Limit:
• Ion Chamber Measurement <2%
• Gamma(3%,2mm) > 95%
• Investigate outliers for additional measurements
• Aim to get average percent difference close to 0% (Mean Perc. Diff.)
• Minimize spread in QA results (Standard Deviation Of Perc. Diff.)
• Compare to other institutions/literature with similar linac and TPS
• If possible – obtain independent audit of IMRT/VMAT delivery from another
physicist/institution
• How much can you tweak the TPS values?
• My preference – tweak as little as possible to get agreement that fulfills
clinical goals of the machine

MLC Parameters –
Interpreting Results
• Which way do I need to tweak the
value?
• Increasing DLG value
→ Increase in calculated dose
• Increasing MLC Transmission value
→ Increase in calculated dose
• Example:
• If plan dose is higher than measured
dose → the next step is to increase DLG
and/or MLC Transmission and re-
calculate
Example: Plan dose is higher
than film dose → consider
decreasing DLG and/or MLC
Transmission

DLG Adjust
Details – Test
Plan Process
1. Treatment Planning – Develop a good quality plan using the test plan
structure set. Have primary treatment planning staff generate the
plan if possible!
2. QA Plan – Map the plan from step 1 onto the appropriate phantom
• Need to perform both chamber measurement and a planar
dose/fluence measurement
• Planar dose measurement can be using film, detector array,
or even the EPID
• Phantom choices include:
• Solid Water slab phantom with place for chamber/film
• Acrylic phantom with place for chamber/film
• Detector array (MapCheck, ArcCheck, Delta4, Matrixx, etc.)
• EPID measurement (Portal Dosimetry)
3. Measure QA plan and compare to predicted dose from TPS
calculation
4. Compile all test plan results (IMRT and VMAT) before making any
adjustments
• Note – IMRT and VMAT trends can differ!!

Practical dry-run of adjusting model
parameters with realistic clinical data
o The background information for this example:
• TG-119 plans and Clinical Cases (IMRT and VMAT) for 10x
energy modewere measured with ion chamber in high dose
and lose dose region
• Starting point was the initial measured DLG value of 0.115
cm and MLC Transmission = 1.65%
• Total Case Breakdown
– TG-119: 3 IMRT and 3 VMAT
– Clinical Cases: 2 IMRT and 2 VMAT

MLC Parameter
Selection
Summary
Points – 10 MV
beam modeling
process
• Decision on MLC parameters such as DLG and
Leaf Transmission depend on:
• The clinical goals for the beam model:
• Which disease sites will be treated
routinely?
• Which modality will be used more
often – IMRT or VMAT?
• The trend in the data:
• Which parameter values will minimize
the percent difference between
planned and measured doses?
the spread in the results comparing
planned and measured doses?
outliers in the data?

Plan Type
Measured Point
Dose (cGy)
Calculated Dose
(DLG = 0.115 cm)
[MLC_trans = 0.0185]
Perc. Diff.
C Shape 2.154 2.052 4.73%
H&N 2.100 2.095 0.24%
H&NSIB 2.176 2.135 1.86%
Prostate 1.988 1.957 1.54%
Prostate LN 1.968 1.946 1.11%
C Shape 2.481 2.470 0.45%
H&N 2.195 2.191 0.19%
H&NSIB 2.122 2.127 -0.22%
Prostate 1.997 1.977 1.02%
Prostate LN 1.893 1.937 -2.32%
VMAT Average 1.90%
VMAT St. Dev. 1.70%
IMRT Average -0.18%
IMRT St. Dev. 1.28%
Overall Average 0.86%
Overall St. Dev. 1.79%
Plan Type
Measured Point
Dose (cGy)
Calculated Dose
(DLG = 0.115 cm)
Perc. Diff.
C Shape 0.320 0.344 -7.51%
H&N 1.314 1.297 1.28%
H&NSIB 1.124 1.126 -0.18%
Prostate 1.322 1.279 3.22%
Prostate LN 0.890 0.874 1.80%
C Shape 0.489 0.520 -6.41%
H&N 1.316 1.331 -1.16%
H&NSIB 1.201 1.236 -2.94%
Prostate 1.650 1.630 1.18%
Prostate LN 1.136 1.163 -2.35%
VMAT Average -0.28%
VMAT St. Dev. 4.22%
IMRT Average -2.33%
IMRT St. Dev. 2.77%
Overall Average -1.31%
IMRT
Results (TG-119 Table VII)- High Dose
VMAT
IMRT
Results (TG-119 and Clinical Cases)- Low Dose
VMAT
Iteration 1 – DLG = 0.115 cm /
MLC_Trans = 1.65%
-4.0%
-3.0%
-2.0%
-1.0%
0.0%
1.0%
2.0%
3.0%
4.0%
5.0%
6.0%
0 1 2 3 4 5 6
Percent
Difference
Iteration #
VMAT - High Dose
IMRT - High Dose
VMAT - Low Dose
IMRT - Low Dose

MLC_Trans = 1.65%
Plan Type
Measured Point
Dose (cGy)
Calculated Dose
(DLG = 0.140 cm)
Perc. Diff.
C Shape 2.154 2.048 4.91%
H&N 2.100 2.087 0.62%
H&NSIB 2.176 2.128 2.19%
Prostate 1.988 1.954 1.69%
Prostate LN 1.968 1.942 1.31%
C Shape 2.481 2.448 1.34%
H&N 2.195 2.173 1.01%
H&NSIB 2.122 2.113 0.44%
Prostate 1.997 1.974 1.17%
Prostate LN 1.893 1.921 -1.47%
VMAT Average 2.15%
VMAT St. Dev. 1.65%
IMRT Average 0.49%
IMRT St. Dev. 1.15%
Plan Type
Measured Point
Dose (cGy)
Calculated Dose
(DLG = 0.140 cm)
Perc. Diff.
C Shape 0.320 0.338 -5.64%
H&N 1.314 1.287 2.04%
H&NSIB 1.124 1.117 0.62%
Prostate 1.322 1.275 3.52%
Prostate LN 0.890 0.867 2.59%
C Shape 0.489 0.496 -1.50%
H&N 1.316 1.311 0.36%
H&NSIB 1.201 1.220 -1.61%
Prostate 1.650 1.626 1.43%
Prostate LN 1.136 1.149 -1.11%
VMAT Average 0.63%
VMAT St. Dev. 3.66%
IMRT Average -0.49%
IMRT St. Dev. 1.33%
IMRT
VMAT
IMRT
VMAT
-4.0%
-3.0%
-2.0%
-1.0%
0.0%
1.0%
2.0%
3.0%
4.0%
5.0%
6.0%
0 1 2 3 4 5 6
Percent
Difference
Iteration #
VMAT - High Dose
IMRT - High Dose
VMAT - Low Dose
IMRT - Low Dose

Plan Type
Measured Point
Dose (cGy)
Calculated Dose
(DLG = 0.115 cm)
Perc. Diff.
C Shape 2.154 2.040 5.29%
H&N 2.100 2.074 1.24%
H&NSIB 2.176 2.116 2.74%
Prostate 1.988 1.945 2.14%
Prostate LN 1.968 1.932 1.82%
C Shape 2.481 2.385 3.88%
H&N 2.195 2.151 2.01%
H&NSIB 2.122 2.096 1.24%
Prostate 1.997 1.966 1.57%
Prostate LN 1.893 1.892 0.06%
VMAT Average 2.65%
VMAT St. Dev. 1.57%
IMRT Average 1.75%
IMRT St. Dev. 1.39%
Plan Type
Measured Point
Dose (cGy)
Calculated Dose
(DLG = 0.115 cm)
Perc. Diff.
C Shape 0.320 0.337 -5.33%
H&N 1.314 1.276 2.88%
H&NSIB 1.124 1.108 1.42%
Prostate 1.322 1.262 4.50%
Prostate LN 0.890 0.862 3.15%
C Shape 0.489 0.482 1.36%
H&N 1.316 1.290 1.96%
H&NSIB 1.201 1.203 -0.19%
Prostate 1.650 1.615 2.09%
Prostate LN 1.136 1.131 0.47%
VMAT Average 1.33%
VMAT St. Dev. 3.88%
IMRT Average 1.14%
IMRT St. Dev. 0.98%
IMRT
VMAT
IMRT
Results (TG-119 and Clinical Cases)- High Dose
VMAT
MLC_Trans = 1.85%
-4.0%
-3.0%
-2.0%
-1.0%
0.0%
1.0%
2.0%
3.0%
4.0%
5.0%
6.0%
0 1 2 3 4 5 6
Percent
Difference
Iteration #
VMAT - High Dose
IMRT - High Dose
VMAT - Low Dose
IMRT - Low Dose

Plan Type
Measured Point
Dose (cGy)
Calculated Dose
(DLG = 0.16 cm)
Perc. Diff.
C Shape 2.154 2.056 4.54%
H&N 2.100 2.097 0.15%
H&NSIB 2.176 2.138 1.73%
Prostate 1.988 1.960 1.39%
Prostate LN 1.968 1.949 0.96%
C Shape 2.481 2.498 -0.68%
H&N 2.195 2.192 0.14%
H&NSIB 2.122 2.127 -0.22%
Prostate 1.997 1.981 0.82%
Prostate LN 1.893 1.943 -2.64%
VMAT Average 1.75%
VMAT St. Dev. 1.67%
IMRT Average -0.52%
IMRT St. Dev. 1.31%
Plan Type
Measured Point
Dose (cGy)
Calculated Dose
(DLG = 0.16 cm)
Perc. Diff.
C Shape 0.320 0.340 -6.26%
H&N 1.314 1.296 1.36%
H&NSIB 1.124 1.123 0.09%
Prostate 1.322 1.285 2.76%
Prostate LN 0.890 0.872 2.03%
C Shape 0.489 0.508 -3.96%
H&N 1.316 1.328 -0.93%
H&NSIB 1.201 1.234 -2.77%
Prostate 1.650 1.633 1.00%
Prostate LN 1.136 1.155 -1.64%
VMAT Average 0.00%
VMAT St. Dev. 3.64%
IMRT Average -1.66%
IMRT St. Dev. 1.88%
IMRT
VMAT
IMRT
VMAT
MLC_Trans = 1.65%
-4.0%
-3.0%
-2.0%
-1.0%
0.0%
1.0%
2.0%
3.0%
4.0%
5.0%
6.0%
0 1 2 3 4 5 6
Percent
Difference
Iteration #
VMAT - High Dose
IMRT - High Dose
VMAT - Low Dose
IMRT - Low Dose

Plan Type
Measured Point
Dose (cGy)
Calculated Dose
(DLG = 0.16 cm)
Perc. Diff.
C Shape 2.154 2.059 4.40%
H&N 2.100 2.105 -0.23%
H&NSIB 2.176 2.145 1.40%
Prostate 1.988 1.963 1.24%
Prostate LN 1.968 1.954 0.70%
C Shape 2.481 2.520 -1.57%
H&N 2.195 2.209 -0.63%
H&NSIB 2.122 2.141 -0.88%
Prostate 1.997 1.984 0.67%
Prostate LN 1.893 1.960 -3.53%
VMAT Average 1.50%
VMAT St. Dev. 1.74%
IMRT Average -1.19%
IMRT St. Dev. 1.54%
Plan Type
Measured Point
Dose (cGy)
Calculated Dose
(DLG = 0.16 cm)
Perc. Diff.
C Shape 0.320 0.345 -7.83%
H&N 1.314 1.306 0.60%
H&NSIB 1.124 1.132 -0.71%
Prostate 1.322 1.289 2.46%
Prostate LN 0.890 0.879 1.24%
C Shape 0.489 0.531 -8.66%
H&N 1.316 1.348 -2.45%
H&NSIB 1.201 1.250 -4.11%
Prostate 1.650 1.637 0.76%
Prostate LN 1.136 1.173 -3.23%
VMAT Average -0.85%
VMAT St. Dev. 4.07%
IMRT Average -3.54%
IMRT St. Dev. 3.40%
IMRT
VMAT
IMRT
VMAT
MLC_Trans = 1.85%
-4.0%
-3.0%
-2.0%
-1.0%
0.0%
1.0%
2.0%
3.0%
4.0%
5.0%
6.0%
0 1 2 3 4 5 6
Percent
Difference
Iteration #
VMAT - High Dose
IMRT - High Dose
VMAT - Low Dose
IMRT - Low Dose

MLC Parameter Iteration Summary –
Graphical Analysis
-4.0%
-3.0%
-2.0%
-1.0%
0.0%
1.0%
2.0%
3.0%
4.0%
5.0%
6.0%
0 1 2 3 4 5 6
Percent
Difference
Iteration #
VMAT - High Dose
IMRT - High Dose
VMAT - Low Dose
IMRT - Low Dose
Final Value Chosen
for Planning

Summary – DLG Testing Plan Process
1. Treatment Planning – Develop a good quality plan using the test plan structure set. Have primary
treatment planning staff generate the plan if possible!
2. QA Plan – Map the plan from step 1 onto the appropriate phantom
• Need to perform both chamber measurement and a planar dose/fluence measurement
• Planar dose measurement can be using film, detector array, or even the EPID
• Phantom choices include:
• Solid Water slab phantom with place for chamber/film
• Acrylic phantom with place for chamber/film
• Detector array (MapCheck, ArcCheck, Delta4, Matrixx, etc.)
• EPID measurement (Portal Dosimetry)
3. Measure QA plan and compare to predicted dose from TPS calculation
4. Compile all test plan results (IMRT and VMAT) before making any adjustments
• Note – IMRT and VMAT trends can differ!!

Practical Tips – Adjusting Parameters on
Approved Beam Models
• If you have a beam model that is already approved and need to make adjustments for IMRT/VMAT
commissioning:
1. Use caution! Think about what could go wrong before making any adjustments. Discuss
with other physicists to make sure you have thought of everything that could come up.
2. Communicate! Once you have a plan, talk about it with relevant staff
3. Calculate! Plan out a time when you can perform the dose calculations with the
preliminary MLC DLG/Transmission values.
1. This may need to be done after-hours or on a weekend.
4. Reset! Depending on the workflow, make sure to reset the MLC parameters back to the
clinically approved values.
1. If you have multiple DLG/Transmission values you would like to test (multiple
iterations), this is the time to perform all iterations
5. Verify! Once you have reset the MLC parameters back to the original values, re-calculate a
set of test plans to verify constancy.
6. Compare! Once you have the calculations done, you can compare to the measured values
and determine optimal parameters

SRS/SBRT – You may need a
separate algorithm!
• My experience – the level of
modulation for conventional
IMRT/VMAT planning is quite
different than for SRS/SBRT
planning
• Typically, the optimal
DLG value for TG-119
planning is different
than for representative
SRS/SBRT cases
• Example at left: 6FFF
beam used for SRS/SBRT
delivery at one of our
• More data for this
example shown on
next slide

CONCLUSION
o A ‘commissioning’ machine can be created to test out the
parameters for the TPS model
o The TPS model will be adjusted based on comparison of
calculated and measured results for IMRT/VMAT plans
o A representative process for adjusting MLC parameters has
been shared for learning purposes
o The next lecture is transitions from commissioning and
adjusting the beam model to routine QA of treatment plans. The
topic is: “Patient-Specific and High-yield Machine QA for IMRT”
50

REFERENCES
o AAPM Task Group 119:
https://www.aapm.org/pubs/tg119/default.asp
o Medical Physics Practice Guidelines (MPPG) 5A:
https://doi.org/10.1120/jacmp.v16i5.5768
51
Thank you for your attention!

IMRT_VMAT_Session 5_How to fine-tune the commissioning of a TPS.pptx

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