Dosimetry of Small IMRT Fields

Parham Alaei, Ph.D.

Department of Radiation Oncology

University of Minnesota

NCCAAPM Symposium-October 10, 2013

1

IMRT and Small Fields

• IMRT beams typically contain “small” fields or

segments

• How “small” depends on the planner/planning

system/institutional policy

• Minimum segment size used in planning

should be equal or larger than that for which

beam data was collected

2

Beam data for planning systems

• Without IMRT:

• With IMRT:

3

4x4 cm2 40x40 cm2

40x40 cm22x2 cm2

Or smaller

Small field data challenges

• Detector size (affects profile and output factor

measurements)

• Alignment of water tank, detector, etc.

• Jaw/MLC positioning errors

• Beam modeling in TPS

• IAEA and AAPM guidelines for absolute dosimetry of

small fields

• TG-155: “Small fields and non-equilibrium condition

photon beam dosimetry”

4

Focus on: Output Factors

• Output factors needed for treatment planning system

commissioning

• Measurement field sizes/depths depend on the

planning system used:– Pinnacle: Range of field sizes (small->large), 10 cm depth, 100 cm SSD

– Eclipse: Range of field sizes (small->large), 10 cm depth, 100 cm SSD

– Xio: Range of field sizes (4x4-40x40), 10 cm depth

– Monaco: Range of field sizes (1x1-40x40), 10 cm depth, 100 cm SSD

– MasterPlan: Range of field sizes (5x5-20x20, smaller fields if needed),

10 cm depth, 100 cm SAD

5

TG 120 Recommendations

6

Med. Phys. 38(3): 131-1338 (2011)

ASTRO Recommendations

7

Practical Radiation Oncology (2011) 1, 190–195

8

9

10

Das, Ding, and Ahnesjö: Small fields: Nonequilibrium radiation dosimetry Med. Phys. 35: 206-215 (2008)

Total scatter factor Scp versus

field size measured with

various available

radiation detectors for (a) 6

MV and (b) 15 MV beams.

11

S. Dieterich and G. W. Sherouse: Comparison of seven commercial dosimetry diodes for SRS Med. Phys. 38: 4166-4173 (2011)

Focus on: Output Factors and IMRT

• If the output factors measured for input into

the treatment planning system are wrong …

• What is the consequence for “small field”

plans (IMRT, SBRT, SRS, …)?

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Small fields and IMRT

• A typical IMRT plan has 5-10 segments per beam

• There are at least ~2-3 large segments per beam

• So there may be ~ 0-7 “small” segments

• A fraction of these are < 4 cm2

– Assuming 4 cm2 is the cut-off for potential erroneous

measurements

• So the error introduced by inaccurate small field

beam data should be minimal

• This also depends on the planning system and/or

delivery system (linear accelerator)

13

14

Medical Physics, Vol. 39, No. 8, August 2012 pp. 4691-4694

Pinnacle treatment planning system

Elekta and Varian accelerators

Results may be different for other planning systems!

Method

• Two five-field IMRT plans created on TomoTherapy

“cheese” phantom (one for each linac)

• Plans optimized to deliver 200 cGy to a cylindrical

target of 2 cm diameter and 2 cm length

• Four structures used as organs-at-risk

• DMPO, minimum segment area: 2 cm2 , minimum

segment MU: 1

• Ion chamber location contoured

15

16

Output factor vs. field size (meas. @ 10 cm depth)

17

0.70

0.75

0.80

0.85

0.90

0.95

1.00

2x2 3x3 4x4 5x5 6x6 8x8 10x10

Re

lati

ve

Ou

tpu

t F

ac

tors

Field Size (cm)

Elekta

Varian

Method (Cont’d)

• Small field output factors (2x2 and 3x3 cm2) altered

by ±5, 10, and 20% and machine re-commissioned

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Altering small field output factors

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0.70

0.75

0.80

0.85

0.90

0.95

1.00

2x2 3x3 4x4 5x5 6x6 8x8 10x10

Re

lati

ve

Ou

tpu

t F

ac

tors

Field Size (cm)

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

1.05

2x2 3x3 4x4 5x5 6x6 8x8 10x10

Re

lati

ve

Ou

tpu

t F

ac

tors

Field Size (cm)

Measured

-20%

-10%

-5%

5%

10%

20%

Method (Cont’d)

• Plan re-optimized every time

• Small change in segments between plans (0-2)

• All plans were delivered to the phantom with the

ionization chamber positioned at isocenter

• In order to confirm the variations in output factors in

the planning system, single static 2×2 cm2 fields were

created for each re-commissioned machine and

delivered to the phantom

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Summary of plans

• Elekta • Varian

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Plan Name No. of segments Monitor Units

20% Decrease 538

10% Decrease 530

5% Decrease 515

Base 56 508

5% Increase 486

10% Increase 487

20% Increase 468

X: 2.0-4.5 cm

Y: 2.0-4.0 cm

40% of segments 3x3 cm2 or smaller

Plan Name No. of segments Monitor Units

20% Decrease 48 562

10% Decrease 51

5% Decrease 50

Base 53

5% Increase 53

10% Increase 50

20% Increase 55

X: 4.0-5.9 cm

Y: 4.0 cm

55% of segments 3x3 cm2 or smaller

Measured vs. Calculated dose-Elekta

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-10.00

-8.00

-6.00

-4.00

-2.00

0.00

2.00

4.00

6.00

8.00

10.00

20% decrease 10% decrease 5% decrease Base 5% increase 10% increase 20% increase

Perc

en

tag

e D

ose D

iffe

ren

ce (

cG

y)

Percent Output Factor Variation

Measured vs. Calculated dose-Varian

25

-5.00

-3.00

-1.00

1.00

3.00

5.00

20% decrease 10% decrease 5% decrease Base 5% increase 10% increase 20% increase

Perc

en

tag

e D

ose D

iffe

ren

ce (

cG

y)

Percent Output Factor Variation

Measured vs. Calculated dose-Summary

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Elekta

% Difference

(Meas. vs. Calc.)

Plan Name

20% Decrease 8.7

10% Decrease 3.6

5% Decrease 0.8

Base 0

5% Increase -2.9

10% Increase -5.4

20% Increase -8.3

Varian

% Difference

(Meas. vs. Calc.)

Plan Name

20% Decrease -0.1

10% Decrease -0.2

5% Decrease -0.1

Base 0

5% Increase -0.5

10% Increase -0.4

20% Increase 0.1

40% of segments 3x3 cm2 or smaller

Why the difference?

• Elekta

– X-diaphragm traces MLCs,

Y-jaws conform to each

segment

• Varian

– X & Y jaws fixed for the

delivery of all segments,

encompassing the largest

segment

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Why the difference?

• Elekta

– Handled as a conformal

jaw machine in Pinnacle

– MUs calculated by

employing small field

output factors as a

consequence of

conforming the collimator

jaws to each of the

segments

• Varian

– Handled as a non-conformal

jaw machine in Pinnacle

– MUs calculated by

employing output factors of

the larger-encompassing jaw

size, and modeling blocked-

field effects

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Varian segment MUs not sensitive to small field OF changes

Verifying the results using static fields

• Single, 6 MV, 2 x 2 cm2 field incident on phantom

• Same machines with modified small field OF used to

compute the dose with the TPS

• Dose calculated and measured at 3 cm depth for all

plans

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Verifying the results using static fields

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Correlation between OF change and dose/MU delivered

Single jaw/MLC-defined 2x2 cm2 field

Elekta % Difference

Plan Name Monitor Units Calculated Dose (Meas. vs. Calc.)

20% Decrease 41 28.8 26.2

10% Decrease 36 28.4 11.5

5% Decrease 34 28.8 5.6

Base 33 28.3 0.0

5% Increase 31 28.2 -4.9

10% Increase 30 28.3 -9.5

20% Increase 27 28.6 -17.2

Single jaw-defined 2x2 cm2 field

Varian % Difference

Plan Name Monitor Units Calculated Dose (Meas. vs. Calc.)

20% Decrease 41 28.8 25.2

10% Decrease 36 28.4 12.3

5% Decrease 34 28.8 5.5

Base 33 28.3 0.0

5% Increase 31 28.2 -4.9

10% Increase 30 28.2 -14.6

20% Increase 27 28.6 -16.6

Verifying the results using static fields

31

No correlation between OF change and dose/MU delivered

Single MLC-defined 2x2 cm2 field (jaws at 10x10)

Varian % Difference

Plan Name Monitor Units Calculated Dose (Meas. vs. Calc.)

20% Decrease 32 28.7 0.0

10% Decrease 0.4

5% Decrease 0.0

Base 0.0

5% Increase 0.4

10% Increase 0.0

20% Increase 0.2

Results

• Incorrect output factors do affect the dose

delivered for a conformal jaw machine (Elekta,

Siemens, Varian TrueBeam with jaw tracking)

• But they do not affect the dose delivered for a

non-conformal jaw machine (Varian)

• As applicable to Pinnacle TPS!

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Conclusion

• It is very important to have accurate output

factor data in the TPS, as it is to have accurate

beam profiles

• Inaccurate data could lead to miscalculation of

dose

• If you don’t have reliable data for small

fields/segments, don’t use them!

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Questions?

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