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