Post on 16-Jul-2020
1
IMRT Delivery System QA
Thomas LoSasso, PhD
Memorial Sloan Kettering Cancer Center
IMRT Dose Delivery
• Acceptance testing • Commissioning • Quality assurance • Verification
QA
Static IMRT Leaf affects borders only affects dose within target position 1-2 mm OK 0.2 mm OK
Why: specific tests for IMRT?
Accuracy of leaf positioning (gaps)
Gap error Dose error
0.0
5.0
10.0
15.0
20.0
0 1 2 3 4 5
Nominal gap (cm)
% D
ose
erro
rRange of gap width
2.01.0
0.50.2
Gap error (mm)
Acceptance testing
• MLC alignment • MLC leaf / gap calibration • Timing errors • Beam characteristics at small MU
A60
A31B30
B1
B60
B31A30
A1
MLC Alignment
A60
A31B30
B1
A1
A30B31
B60
Film 1
Film 2
Field B90°
Field B270°
Field A90°
Field A90°
2
Film 1
Film 20.5 mm misalignments
perfect alignment
Doubly - exposed films MLCtable.txt corrections
00.20.40.60.8
11.21.41.61.8
-10 -8 -6 -4 -2 0 2 4 6 8 10
Distance from central axis (cm)
Corr
ectio
n (m
m)
"precise"
Varian
MLCtablemeasurement
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
-20 -10 0 10 20
Distance from axis (cm)
Gap
at i
soce
nter
upperlower
Gap measurement - mechanical DMLC - Symmetry and Flatness
0.90
0.95
1.00
1.05
-15 -10 -5 0 5 10 15
Distance from axis (cm)
Rela
tive
outp
ut
open5mm5mm/open
12 cm
14 cm
0.90
0.95
1.00
1.05
-15 -10 -5 0 5 10 15
Distance from axis (cm)
Rela
tive
outp
ut
transmissiongap only
0.90
0.95
1.00
1.05
-15 -10 -5 0 5 10 15
Distance from axis (cm)
Rela
tive
outp
ut
open5mm5mm/open
DMLC - Symmetry and Flatness
y = 0.009945x + 0.017685R2 = 0.999977
0.00
0.10
0.20
0.30
-4 -2 0 2 4 6 8 10 12 14 16 18 20
1.80
6 MV
y = 0.009954x + 0.018705R2 = 0.999984
0.00
0.10
0.20
0.30
-4 -2 0 2 4 6 8 10 12 14 16 18 20
1.90
15 MV
Gap correction - measurement
Rel
ativ
e ou
tput
Nominal gap (mm)
3
1.4
1.5
1.6
1.7
1.8
1.9
2
2.1
2.2
2.3
CAX 5cm off 10cm offSeries4 Series5 Series6
445
6X 15X
444
6X 15X
Phelps
6X 15X
442
6X
245
6X 15X
+/- 1%
15 MV 6 MV
Mark 1 Mark 2 Millenium
Effe
ctiv
e ga
p of
fset
(mm
)
Effective gap offset variation
1.0
1.2
1.4
1.6
1.8
2.0
-25 -20 -15 -10 -5 0 5 10 15 20 25
Distance from axis (cm)
Eff g
ap o
ffset
(mm
@ is
o)
Millennium 120
2–4% Dose
Off-axis variation - "cupping"
with “cupping”correction
with individual leafcorrections
Off-axis variation - corrected
1.0
1.2
1.4
1.6
1.8
2.0
-25 -20 -15 -10 -5 0 5 10 15 20 25
Distance from axis (cm)
Eff g
ap o
ffset
~1% dose
1.0
1.2
1.4
1.6
1.8
2.0
-25 -20 -15 -10 -5 0 5 10 15 20 25
Distance from axis (cm)
Eff g
ap o
ffset
S = 0.047 mm
S = 0.027 mm
Frequency of deviations (%) # GlobalField Room Angle logged - prescribed leaf positions @ iso (mm) checks RMS
<0.05 0.05-1.0 1.0-2.0 2.0-3.0 3.0-4.0
LAO 245 0 49 50 0.5 0.04 12528 0.26225 50 50 0.5 0.04 12528 0.26
445 0 52 48 0.5 0.02 12492 0.26225 48 51 0.6 0.01 12510 0.26
LPO 245 0 59 40 0.9 0.09 0.03 9918 0.26285 59 40 1.0 0.09 0.04 9918 0.28
445 0 61 38 0.9 0.10 9810 0.22285 58 41 1.0 0.10 9810 0.26
PA 245 0 50 49 0.4 0.01 0.01 18228 0.140 50 50 0.4 0.01 0.01 18198 0.14
445 0 52 47 0.4 0.02 18180 0.140 52 47 0.4 0.01 18144 0.14
RPO 245 0 62 36 1.4 0.10 0.03 10116 0.2475 63 36 1.4 0.11 0.03 10116 0.24
445 0 63 35 1.4 0.12 10152 0.2275 63 35 1.4 0.12 10062 0.22
RAO 245 0 47 53 0.3 0.02 14490 0.14135 48 52 0.3 0.01 14508 0.14
445 0 49 50 0.3 0.01 14526 0.14135 50 49 0.3 0.01 14436 0.14
# checks = # moving leaves * BOT / 55 msec
Dose differences (log-dva)Overlay
Log file Leaf sequence file dose distribution
Inf
Post
Sup
Ant
jaws
2 cm
4
Delivered with dose mode at 400MU/min(CL2300, 6MV, 100 SSD, 1.5 cm depth)
-100
-50
0
50
100
150
200
Diff
eren
ce (%
)
# Segment0.25 MU/seg
4 MU/seg
1 MU/seg
16 MU/seg 25 MU/seg
-15-10
-505
1015
4 MU/seg
16 MU/seg 25 MU/seg
Courtesy of Ping Xia, UCSF
Timing error – SMLC (Varian)
400 mu/min
Courtesy of Gary Ezzell and the Journal of Applied Clinical Medical Physics.
Timing error – SMLC (Varian)
14 MU8X
28 MU4X
110 MU 55 MU2X
110 MU
Dose differences (log-dva)Overlay
-2.0-1.5-1.0-0.50.00.51.01.52.0
0 10 20 30 40 50 60 70 80 90 100MU (%)
Leaf
erro
r (lo
gged
- pr
escr
ibed
)
14B
14A
-2.0-1.5-1.0-0.50.00.51.01.52.0
-50 -40 -30 -20 -10 0 10 20 30 40 50Distance from axis (mm)
Leaf
erro
r (lo
gged
- pr
escr
ibed
)
14A
14B
150 MU
Timing / communication errors
-2.0-1.5-1.0-0.50.00.51.01.52.0
0 10 20 30 40 50 60 70 80 90 100MU (%)
Leaf
erro
r (lo
gged
- pr
escr
ibed
)
14B
14A
-2.0-1.5-1.0-0.50.00.51.01.52.0
0 10 20 30 40 50 60 70 80 90 100MU (%)
Leaf
erro
r (lo
gged
- pr
escr
ibed
)
14B
14A
-2.0-1.5-1.0-0.50.0
0.51.01.52.0
-50 -40 -30 -20 -10 0 10 20 30 40 50Distance from axis (mm)
Leaf
erro
r (lo
gged
- pr
escr
ibed
)
14A
14B
-2.0-1.5-1.0-0.50.00.51.01.52.0
-50 -40 -30 -20 -10 0 10 20 30 40 50Distance from axis (mm)
Leaf
erro
r (lo
gged
- pr
escr
ibed
)
14A
14B
60 MU150 MU
Timing / communication errors
Overlayplanlog
Difference(log - plan)
Log file Leaf sequence file dose distribution
150 MU prescribed / 60 MU delivered
A
GF
ECD BA
GF
ECD B
Comparison of prescribed and delivered doses
5
0.85
0.90
0.95
1.00
1.05
1.10
1.15
0 30 60 90 120 150MU delivered
Rel
ativ
e do
se (c
orre
cted
to 1
50
MU
)
ABCDEFG
PrescribedMU = 150
MU decrease dose errors increase Timing errorIs there an analogous problem for DMLC?
• Trailing leaves (B-side) compensate for the leading leaves (A-side)
• Dose (MU) error = position error (B) – position errorleaf speed (B) leaf speed (A)
• However, some leaf sequencers actively utilize the beam hold-off to control leaf speed
Beam Stability: Flatness, Symmetry
• Stability of flatness and symmetry affects dose rate for small fields directed off the central axis.
• For an open 20x20cm2 field, measure profiles for irradiations ranging from 1 to 100 MU.
– Sun Nuclear Profiler (46 diodes, 10 profiles/sec).
• Flatness is +/-3% if more than 5MU delivered.• Symmetry +/-3% if more than 4MU delivered.
Courtesy of M. Sharpe, William Beaumont Hospital
Mark 2 (442)
0
0.01
0.02
0.03
0.04
-10 -5 0 5 10
ABaverage
Millenium (444)
0
0.01
0.02
0.03
0.04
-10 -5 0 5 10
ABaverage
Mark 2 (245)
0
0.01
0.02
0.03
0.04
-10 -5 0 5 10
ABaverage
Mark 1 (445)
0
0.01
0.02
0.03
0.04
-10 -5 0 5 10
ABaverage
Transmission profiles across leaves
Distance from axis (cm)
Tran
smis
sion
Parallel
Perpendicular
X
Y
Respiratory gating
Leaf motion
Splitting large fields
6
Routine QA
MLC QAMLC specificMechanical stabilityAccuracy of leaf positioning (gaps)
VerificationPatient / field specificTx plan is transferred and delivered correctly
Quality assurance
Motor fatigue Film pattern (semi-weekly)
MLC calibration Dosimetry (monthly)
Backlash Dosimetry (monthly)
Grease Alcohol (semi-annually)
0
10
20
30
40
50
60
70
Jan-92
Jan-93
Jan-94
Jan-95
Jan-96
Jan-97
Jan-98
Jan-99
Jan-00
Jan-01
Jan-02
Jan-03
Date
# 445 245 442
Room
42
70
21
1st MLC TxMLC installed
1st dMLC Tx
Motors replacedMotor replacement history MLC motors replaced445 245 442B A B A B A
26 26 2625 25 2524 2 24 2423 23 2322 22 2221 21 2120 20 2019 19 1918 18 1817 3 17 2 1716 2 16 2 1615 5 15 1514 14 1413 13 1312 12 1211 2 2 11 1110 2 10 109 2 9 98 2 8 87 7 76 6 65 5 54 2 4 43 2 3 32 2 21 1 1
6a Field Size Defined by MLC at 100 cm SAD by Light Field: Date: __________ Initials: __________ Tolerance: 0.5 mm
Digital setting (cm) A carriage B carriage A leaves B leaves 20.0 16.0 12.0 8.0 4.0 0.5 -4.0 -8.0 -12.0 -16.0
Note: If leaf positions exceed tolerance, reinitialize and recheck. If this does
not correct the problem, verify that the correction parameters in files MLCTABLE.TXT and MLCXCAL.TXT have not changed.
4 cm
Leaf / carriage positioning vs off-axis distance
7
← - 0.5 mm
→ + 0.5 mm
← - 0.2 mm
→ + 0.2 mm
errors introducedLeaf positioning
1 mm bands 17b DMLC Output (Gap width) Check
15 MV x-rays, File: DMLC_QA2.dva, Dose = 100 MU, Dose rate = 240 MU/min SAD: 100cm, Depth = 3.1cm, Backup = 10cm, C.S. = 10x10cm2 Tolerance: (The ratios should be within 2% (< 0.2 mm gap) of the annual calibration.)
Measure the output for the scanned field before and after reinitializing the MLC. Measure the output for a 10x10cm2 reference field size defined by the jaws (MLC retracted). Calculate the ratio of the scanned field to the reference field outputs.
Reference10x10
DMLC - before reinitialization
DMLC - after reinitialization
Reference 10x10
Average Annual Ratio
17c Beam Holdoff Functionality
Date: Initials
Same setup as 17b, Dose = 5 MU Record the output and the Tx time. Check if the holdoff is being applied. Reading Functioning
DMLC Output vs Time
0.158
0.160
0.162
0.164
0.166
0.168
Jan-98 Jan-99 Jan-00 Jan-01 Jan-02 Jan-03 Jan-04Date
DM
LC /
10x1
0
445 245 442 444 241441 244
0.2 mm ~1%
17d Output vs Gantry and Collimator Angles
Date: Initials
15 MV x-rays, File: DMLC_QA2.dva, Dose = 100 MU, Dose rate = 240 MU/min SAD: 100cm, Spokas in air w/buildup, C.S. = 10x10cm2 Tolerance: (The ratios should be within 3% (< 0.2 mm gap) of each other.) Position a cylindrical chamber in air at isocenter.
For each gantry angle, measure the open field and the scanned field outputs. At gantry angles of 90° and 270°, also measure output for collimator angles of 90°. Calculate the ratio of the scanned field to the reference field outputs.
Collimator angle
Gantry Reference DMLC DMLC / Reference angle 0° 0° 90° 0° 90°
0° 90°
270° 180° 0°
0.16
0.165
0.17
0.175
0.18
1998 1999 2000 2001 2002
Millennium (444)
0.16
0.165
0.17
0.175
0.18
1998 1999 2000 2001 2002
0-090-0270-0180-090-90270-90
Millennium (241)
± 0.2 mm(± 1%)
DMLC output vs gantry and collimator angle
Year
DM
LC /
10x1
0
0.16
0.165
0.17
0.175
0.18
1998 1999 2000 2001 2002
Mark 2 (245)
0.16
0.165
0.17
0.175
0.18
1998 1999 2000 2001 2002
Mark 1 (445)
DMLC output vs gantry angle(diode array)
0.97
0.98
0.99
1.00
1.01
1.02
1.03
-15 -10 -5 0 5 10 15
Distance from axis (cm)
Rela
tive
outp
ut
0-0
90-0
270-0
0.2 mm
8
Summary1. Acceptance testing
• MLC capable of IMRT delivery
2. Commissioning• MLC modeled accurately in TPS
3. Routine MLC QA• leaves and carriages - mechanics
4. Patient specific QA• independent verification
IMRTDose Accuracy
Overlay Difference
PlanFilm Film - Plan
2 cm
Improved calculations needed for IMRT fields
~25%
2 cm
Film - Plan
Dosimetry (film – plan)
T&G Source MLC scatter
Corrections (plan – plan)
Lung
PAfield
x10
Tongue and Groove
-1 0 1 2
9
Tongue and groove
01020304050607080
-7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5
Distance from CAX (cm)
Rel
ativ
e do
se
-1 cm 0 cm 1 cm 2 cm
DMLC output vs gantry and collimator angle
0.950.960.970.980.991.001.011.021.031.041.05
1998 1999 2000 2001 2002
DMLC
/ 10
x10
(nor
m to
0-0
)
0-0 180-090-0 90-90270-0 270-90Mark 1 (445)
adju
sted
bea
rings
repl
aced
bea
rings
adju
sted
driv
e be
lts
adju
sted
bea
rings
adju
sted
bea
rings
adju
sted
bea
rings