An Independent Audit and Analysis of Small Field Dosimetry Quality Assurance

David FollowillIROC Houston QA Center

Outline of Presentation

• Definition of the problem• Results of audits• Possible solutions• Resources to help

Learning Objectives

• Recognize that small field dosimetry is not easy and that there are tools to help you be as accurate as possible.

Sometimes we all like to believe that all across the kingdom it’s bright and merry!

Ladies and LordsThere is a scourge across the Kingdom

Small field dosimetry

What is the truth?

From Das et al 2000

Recommendation:

The unshielded stereotactic diode and micro-chambers are the detectors of choice formeasurements in radiosurgery beams. However, werecommend comparing the measurements from atleast two different detectors listed in Table 2a, andafter applying corrections taking their average.

TG-155Small Field Dosimetry

TomoTherapy results (since Aug 2010)

0

20

40

60

80

100

120

0.9 0.91 0.92 0.93 0.94 0.95 0.96 0.97 0.98 0.99 1 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.1

# re

sults

TLD/Institution

Average: 0.970 +/- 0.024552 irradiations

But the manufacturer told me it would work, the manufacturer set the value or I sent my own measured data to the TPS company and they modeled my beams for me.

1. Accuray calibrates all of the tomotherapy units using the Exradin A1SL chamber and the user monitors constancy.

• Correction factor of 0.980 for A1SL based on Gago-Arias et al, Med Phys, 2012

Manufacturer modeling of small field dataYou make the measurementsYou send the measurementsThey create the beam model

BUTThey assume you used the correct dosimeter

They assume you made measurements correctlyThey do not know to apply additional corr. factors

RESULTIncorrect Small Beam Model

Francescon et al2011 data

Small Field Dosimetry Corrections

Situation is even worse if you consider using field sizes less then 0.5 x 0.5 cm2

Field Size (cm x cm)

Detectors 0.5x0.5 0.75x0.75 1.0x1.0 1.25x1.25 1.5x1.5 3.0x3.0

PTW 60012-60017 0.968±0.003 0.984±0.002 0.995±0.001 1.001±0.001 1.006±0.001 1.013±0.001

Sun Nuclear Dedge 0.932±0.003 0.951±0.002 0.967±0.001 0.978±0.003 0.986±0.002 1.001±0.002

IBA SFD 0.972±0.003 0.996±0.002 1.007±0.001 1.011±0.001 1.015±0.001 1.017±0.001

Exradin D1V 0.980±0.003 0.994±0.002 0.999±0.001 1.000±0.001 1.005±0.001 1.012±0.001

Exradin A16 1.112±0.018 1.044±0.007 1.020±0.001 1.007±0.001 1.002±0.001 0.999±0.001

PTW PinPoint 31014 1.128±0.018 1.053±0.007 1.024±0.002 1.010±0.001 1.005±0.001 1.000± 0.001

PTW MicroLion 1.023±0.009 0.997±0.002 0.993±0.001 0.992±0.001 0.994±0.001 0.998±0.001

Exradin W1 PSD 0.998±0.003 0.995±0.002 0.996±0.002 0.996±0.002 0.994±0.002 0.994±0.002

calculated as the mean over all the values obtained by changing the linacmodel, (PrimusTM Siemens 6 MV and Synergy® Elekta 6 MV), the radial FWHM andenergy of the electron source as presented by Francescon et al.97 These values canalso be used with caution for Varian 6 MV Linacs.

clin 10x10clin 10x10

f , fQ ,Qk

Unpublished data from P. Francescon

Gamma Knife results (since Sep 2007)

0

10

20

30

40

50

60

0.90 0.91 0.92 0.93 0.94 0.95 0.96 0.97 0.98 0.99 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10

# re

sults

TLD/Inst.

Average: 0.997 +/- 0.021266 results

Depending on the dosimeter used, the magnitude of the additional correction, down to a 0.5 x 0.5

cm2 field size can be as much as:

6%91%1%2%1% 1. 5%

2. 2%3. No correction needed4. 13%5. 8%

The answer is:

13%

The magnitude of the additional correction, down to a 0.5 x 0.5 cm2 field size can be as much as:

Francescon et al. Medical Physics, Vol. 38, No. 12, December 2011

Recommendation:

It is strongly recommended to independently verifydosimetric measurements in small fields, eitherthrough measurements carried out by a differentperson and/or though an independent external audit,such as that carried out by the RPC .

TG-155Small Field Dosimetry

RPC Measurements during onsite visitsMeasurement configuration:

depth = 10 cm, 100 cm SSD, Exradin A16Secondary jaws kept at a 10 x 10

MLC @ 10 x 10MLC @ 6 x 6MLC @ 4 x 4MLC @ 3 x 3MLC @ 2 x 2

The Problem is that our Dragon is very small!

Tables of standard small field factors

Followill et al 2012 and also will be in TG-155

We even make mistakes

Corrected Varian DataVarian 6 MV Varian 10 MV Varian 15 MV Varian 18 MV

Field Size

(cm x cm) RPC Institution RPC Institution RPC Institution RPC Institution

10 x 10 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000

6 x 6 0.938 0.945 0.959 0.964 0.963 0.964 0.969 0.971(0.005) (0.008) (0.002) (0.003) (0.005) (0.007) (0.003) (0.006)

[0.8%] [0.5%] [0.4%] [0.5%](n=127) (n=22) (n=32) (n=41)

4 x 4 0.886 0.900 0.916 0.927 0.927 0.928 0.928 0.933(0.006) (0.013) (0.006) (0.010) (0.006) (0.009) (0.002) (0.009)

[1.8%] [1.2%] [0.7%] [0.9%](n=122) (n=25) (n=32) (n=37)

3 x 3 0.851 0.870 0.880 0.894 0.892 0.894 0.884 0.891(0.007) (0.012) (0.006) (0.008) (0.006) (0.012) (0.005) (0.019)

[2.4%] [1.6%] [0.9%] [1.4%](n=123) (n=25) (n=31) (n=41)

2 x 2 0.804 0.825 0.823 0.838 0.824 0.823 0.806 0.804(0.008) (0.020) (0.005) (0.015) (0.011) (0.040) (0.007) (0.020)

[2.9%] [2.0%] [2.2%] [1.6%](n=136) (n=23) (n=33) (n=40)

 

Varian 6 MV Varian 10 MV Varian 15 MV Varian 18 MV Field Size (cm x cm) RPC Institution RPC Institution RPC Institution RPC Institution

10 x 10 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000

6 x 6 0.938

(0.937)* 0.945 0.959 0.964 0.963

(0.965) 0.964 0.969 0.971 (0.005) (0.008) (0.002) (0.003) (0.005) (0.007) (0.003) (0.006) [0.8%] [0.5%] [0.4%] [0.5%]

(n=127) (n=22) (n=32) (n=41)

4 x 4 0.886

(0.885) 0.900 0.916 0.927 0.927

(0.924) 0.928 0.928 0.933 (0.006) (0.013) (0.006) (0.010) (0.006) (0.009) (0.002) (0.009) [1.8%] [1.2%] [0.7%] [0.9%]

(n=122) (n=25) (n=32) (n=37)

3 x 3 0.851

(0.849) 0.870 0.880 0.894 0.892

(0.884) 0.894 0.884 0.891 (0.007) (0.012) (0.006) (0.008) (0.006) (0.012) (0.005) (0.019) [2.4%] [1.6%] [0.9%] [1.4%]

(n=123) (n=25) (n=31) (n=41)

2 x 2 0.804

(0.802) 0.825 0.823 0.838 0.824

(0.814) 0.823 0.806 0.804 (0.008) (0.020) (0.005) (0.015) (0.011) (0.040) (0.007) (0.020) [2.9%] [2.0%] [2.2%] [1.6%]

(n=136) (n=23) (n=33) (n=40)

Discrepancies Regarding:Number of InstitutionsReceiving rec. (n = 206)

Review QA Program 152 (74%)Photon Field Size Dependence 138 (67%)

Wedge Factor (WF) 66 (32%)Off-axis Factors (OAF)/Beam symmetry 60 (29%)

Electron Calibration 35 (17%)Photon Depth Dose 33 (16%)Electron Depth Dose 25 (12%)Photon Calibration 16 (8%)

Discrepancies Discovered (Jan. ’05 – April ’13)

On-Site Dosimetry Review Audit

This is a beam measurement issue and TPS beam modeling challenge.

There is guidance or possibly a fair maiden after the dragon is slain.

The bigger challenge is with Varian machines due to the tertiary collimation (MLC)• Varian/Pinnacle combination:

Followill et al, JACMP, vol. 13, No. 5, 2012

• Varian/Eclipse combination:Kron et al, Med Phys, vol 39 (2), 2012

Huge Resource of Published data:

239 references

5 Tables

15 Figures

13 Conclusions and recommendations

TG-155Small Field Dosimetry

New IROC Houston audit

OSLD

Commissioning and Testing

Three dosimeters were used to estimate the dose for each field size: Exradin A16Exradin D1V diodeExradin D1V diodePTW CC04Exradin W1

First Test Irradiation away from RPC

All of the following are current methods to provide a verification of your small field

dosimetry data except:

2%4%86%8%1% 1. Use of published data

2. Your 2nd set of measurements3. Rely on manufacturer’s data4. Remote audit5. Indep. meas. from another physicist

The answer is:

Reliance on the manufacturers data

All of the following are current methods to provide a verification of your small field dosimetry data except:

Ref: Das et al, AAPM TG 155 report, submitted to Med Phys

• The challenge continues for the <2 x 2 cm2 field sizes

• The correct dosimeter is essential.

• There are independent audits and standard data to help.

• We will defeat thisdragon!

Summary|Conclusion

Thank YouQuestions?

ReferencesFrancescon et al. Medical Physics, Vol. 38, No. 12, December 2011

Das et al, AAPM TG 155 report, submitted to Med Phys

Followill et al. J Appl Clin Med Phys 13(5):282-9, 2012.

Followill DS: Erratum, J Appl Clin Med Phys 15 (2): 4757 3/2014.

AcknowledgementsWork supported by PHS grant CA10953 awarded by NCI, DHHS