Pre-clinical Evaluation of the New Xoft-AxxentTM Electronic Brachytherapy System

PrePre--clinical Evaluation of the New clinical Evaluation of the New XoftXoft--AxxentAxxentTMTM Electronic Electronic Brachytherapy SystemBrachytherapy System

J Turian, D Bernard, Z Hu, A Dickler, J ChuJ Turian, D Bernard, Z Hu, A Dickler, J ChuRush University Medical Center, Chicago, IL 60612Rush University Medical Center, Chicago, IL 60612

WEWE--CC--224C224C--0808

9/5/2006 WE-C-224C-08 2

OutlineOutlineOutline

Introduction

System description

Testing methodology

Tests performed/Results

Conclusions

IntroductionIntroduction

System descriptionSystem description

Testing methodologyTesting methodology

Tests performed/ResultsTests performed/Results

ConclusionsConclusions

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IntroductionIntroductionIntroduction

Why EBT– Dosimetric characteristic similar to existing radionuclides

– Ability to switch the source on/off

– Adjustable radiation output

– Less regulatory requirements

– Less shielding

– Elimination of radioactive waste

Why EBTWhy EBT–– Dosimetric characteristic similar to existing Dosimetric characteristic similar to existing radionuclidesradionuclides

–– Ability to switch the source on/offAbility to switch the source on/off

–– Adjustable radiation outputAdjustable radiation output

–– Less regulatory requirementsLess regulatory requirements

–– Less shieldingLess shielding

–– Elimination of radioactive wasteElimination of radioactive waste

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ObjectivesObjectivesObjectives

Investigate the Xoft-Axxent EB system in a clinical setting using an existing framework

Provide feedback on the system’s performance and suggest improvements

Evaluate and improve a proposed set of QA procedures

Investigate the Investigate the XoftXoft--AxxentAxxent EB system in a clinical EB system in a clinical setting using an existing frameworksetting using an existing framework

Provide feedback on the systemProvide feedback on the system’’s performance s performance and suggest improvementsand suggest improvements

Evaluate and improve a proposed set of QA Evaluate and improve a proposed set of QA proceduresprocedures

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System DescriptionSystem DescriptionSystem Description

Cooling connections

HV connection

Micro-miniature X-ray source

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System description: X-ray sourceSystem description: XSystem description: X--ray sourceray source

Miniature X-ray source inserted into a “flexible” cooling catheter

High vacuum x-ray tube technology50 kV operating potentialOutput: ~18 Gy/min @1cm in tissueWater cooled

X-Ray Tube HV Cable

X-Ray Source Tip Detail

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System Description: X-ray SourceSystem Description: XSystem Description: X--ray Sourceray Source

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System Description: ApplicatorSystem Description: ApplicatorSystem Description: Applicator

Multiple BalloonShapes and Sizes

HDR X-ray SourceLumen

Balloon Inflation Valve

Multi-Lumen Shaft

Drainage Port Valve

Drainage Holes

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Testing MethodologyTesting MethodologyTesting Methodology

AAPM Task Group Reports:– TG-43,TG-43U Brachytherapy Dosimetry Formalism

– TG-56 Code of Practice for Brachytherapy Physics

– TG-59 High Dose Rate Tx Delivery

System log files examination

Measurements using equipment:– Provided by Xoft

– General therapy equipment available in the clinic

AAPM Task Group Reports:AAPM Task Group Reports:–– TGTG--43,TG43,TG--43U Brachytherapy 43U Brachytherapy DosimetryDosimetry FormalismFormalism

–– TGTG--56 Code of Practice for Brachytherapy Physics56 Code of Practice for Brachytherapy Physics

–– TGTG--59 High Dose Rate 59 High Dose Rate TxTx DeliveryDelivery

System log files examination System log files examination

Measurements using equipment:Measurements using equipment:–– Provided by Provided by XoftXoft

–– General therapy equipment available in the clinic General therapy equipment available in the clinic

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Tests PerformedTests PerformedTests Performed

Controller’s GUI functionality

Beam stability

Output stability

Timer accuracy / End effects

Source transit time

Positional accuracy

Treatment planning & delivery

Safety characteristics

Some general design constraints

ControllerController’’s GUI functionalitys GUI functionality

Beam stabilityBeam stability

Output stability Output stability

Timer accuracy / End effectsTimer accuracy / End effects

Source transit timeSource transit time

Positional accuracyPositional accuracy

Treatment planning & delivery Treatment planning & delivery

Safety characteristicsSafety characteristics

Some general design constraintsSome general design constraints

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Controller FunctionalityController FunctionalityController Functionality

Hope you had a chance to see the unit (Xoft booth)

Setup and delivery:– Enter patient data

– Import dwell time file

– Perform source calibration

– Treatment delivery

– Treatment interrupt recovery

– Treatment log files

Status indicator lights

Emergency off

Hope you had a chance to see the unit (Hope you had a chance to see the unit (XoftXoft booth)booth)

Setup and delivery:Setup and delivery:–– Enter patient dataEnter patient data

–– Import dwell time fileImport dwell time file

–– Perform source calibrationPerform source calibration

–– Treatment deliveryTreatment delivery

–– Treatment interrupt recoveryTreatment interrupt recovery

–– Treatment log filesTreatment log files

Status indicator lightsStatus indicator lights

Emergency offEmergency off

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Beam Parameters StabilityBeam Parameters StabilityBeam Parameters Stability

Generator kVp, mA– Direct measurement was

impractical (use the log files)

Beam current (µA) –from log files

Startup dose– Magnitude

– Beam quality

– How to account for it

Generator kVp, Generator kVp, mAmA–– Direct measurement was Direct measurement was

impractical (use the log impractical (use the log files)files)

Beam current (Beam current (µµAA) ) ––from log filesfrom log files

Startup doseStartup dose–– MagnitudeMagnitude

–– Beam qualityBeam quality

–– How to account for itHow to account for it

20s DwT exposure; kVp, filament current, beam current vs time

0.001

0.01

0.1

1

10

100

0:000:000:000:000:000:000:000:000:000:020:060:100:140:180:22

time(m:ss)

VOLTAGE(kV) FILAMENT(A)Beam(mA)

20s DwT exposure; kVp, filament current, beam current vs time

0.001

0.01

0.1

1

10

100

0:000:000:000:000:000:000:000:000:000:020:060:100:140:180:22

time(m:ss)

VOLTAGE(kV) FILAMENT(A)Beam(mA)

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Beam Parameters StabilityBeam Parameters StabilityBeam Parameters Stability

voltagevoltage 0.26%0.26% 0.17%0.17% 0.23%0.23% 0.20%0.20%

currentcurrent 0.62%0.62% 0.43%0.43% 0.42%0.42% 0.39%0.39%

Single tube over different days (1σ)

Different tube on same day (1σ)

SNSN 27232723 26892689 27582758 29042904voltagevoltage 0.19%0.19% 0.17%0.17% 0.04%0.04% 0.05%0.05%

currentcurrent 0.18%0.18% 0.39%0.39% 0.26%0.26% 0.15%0.15%

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Output Constancy of Four SourcesOutput Constancy of Four SourcesOutput Constancy of Four Sources

Standard Imaging Well chamber100 sec collection time

tube Calibration current (nA)

Corrected collection

charge (nC) 2689 106.3 1.415 2723 100.5 1.446 2758 104.5 1.451 2904 85.6 1.403

tube Calibration current (nA)

Corrected collection

charge (nC) 2689 106.3 1.415 2723 100.5 1.446 2758 104.5 1.451 2904 85.6 1.403

σ=1.6%

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Time Adjustment Factor FTime Adjustment Factor Time Adjustment Factor FF

elecSK

k

PINSF××

= 06.1

I - measured current corrected for temp and pressure

Pelec – electrometer correction factor

1.06 – balloon correction factor

Sk – treatment planning AKS (nominal value)

NSK – AKS ion chamber coefficient NSK=R·C

C – ion chamber calibration factor for I-125 (6711)

R – ratio of Xoft/6711 (for 50 kVp: R=1.939)

I I -- measured current corrected for temp and pressuremeasured current corrected for temp and pressure

PPelecelec –– electrometer correction factorelectrometer correction factor

1.06 1.06 –– balloon correction factorballoon correction factor

SSkk –– treatment planning AKS (nominal value)treatment planning AKS (nominal value)

NNSKSK –– AKS ion chamber coefficient NAKS ion chamber coefficient NSKSK=R=R··CC

C C –– ion chamber calibration factor for Iion chamber calibration factor for I--125 (6711)125 (6711)

R R –– ratio of Xoft/6711 (for 50 kVp: R=1.939)ratio of Xoft/6711 (for 50 kVp: R=1.939)

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Output Constancy versus TimeOutput Constancy versus TimeOutput Constancy versus Time

Standard Imaging Well chamber100 sec collection time

tube Calibration current (nA)

Corrected collection

charge (nC) 1/30 102.1 1.463 1/31 100.5 1.446 2/1 101.5 1.464 2/2 102.4 1.445

tube Calibration current (nA)

Corrected collection

charge (nC) 1/30 102.1 1.463 1/31 100.5 1.446 2/1 101.5 1.464 2/2 102.4 1.445

σ=0.8% σ=0.7%

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End EffectsEnd EffectsEnd Effects

Materials– QA phantom provided by Xoft

– PTW TN34013 (0.0053cm3) soft X-ray ion chamber

– CNMC electrometer

Methods– Measure the charge collected for

different exposure times

Including start up

During stable operation

MaterialsMaterials–– QA phantom provided by QA phantom provided by XoftXoft

– PTW TN34013 (0.0053cm3) soft Xsoft X--ray ion chamberray ion chamber

–– CNMC electrometerCNMC electrometer

MethodsMethods–– Measure the charge collected for Measure the charge collected for

different exposure timesdifferent exposure times

Including start upIncluding start up

During stable operationDuring stable operation

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Timer Error and LinearityTimer Error and LinearityTimer Error and Linearity

Obtained from:– 20,40,80,160,320 sec

exposures

– charge collected using the PTW chamber

– Regression results

Timer accuracy 2.0 sec

Timer linearity 0.32%

2s may be clinically significant and should be taken into account

Obtained from:– 20,40,80,160,320 sec

exposures

– charge collected using the PTW chamber

– Regression results

Timer accuracy 2.0 sec

Timer linearity 0.32%

2s may be clinically 2s may be clinically significant and should be significant and should be taken into accounttaken into account

Timer Linearity / Accuracy

y = 0.9968x - 1.9923R2 = 1

0.0050.00

100.00150.00200.00250.00300.00350.00400.00450.00

0.00 100.00 200.00 300.00 400.00 500.00

Xoft Dwell Time (s)M

easu

red

Dw

ell T

ime

(s)

Timer Linearity / Accuracy

y = 0.9968x - 1.9923R2 = 1

0.0050.00

100.00150.00200.00250.00300.00350.00400.00450.00

0.00 100.00 200.00 300.00 400.00 500.00

Xoft Dwell Time (s)M

easu

red

Dw

ell T

ime

(s)

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Start Up DoseStart Up DoseStart Up Dose

Source Ramp-Up Sequence

0 4 8 12 16 20 24 28 32

Time (seconds)

High Voltage (kV)

Filament Current (A)

Beam Current (uA)20 kV

50 kV

1.1 A1.5 A

300 uA

100 uA

Source Ramp-Up Sequence

0 4 8 12 16 20 24 28 32

Time (seconds)

High Voltage (kV)

Filament Current (A)

Beam Current (uA)20 kV

50 kV

1.1 A1.5 A

300 uA

100 uA

Up to 10% of the prescribed dose

Beam quality is reduced (37.5 for 50 kV beam)

Corrections should be applied for the first dwell position

(courtesy of Xoft)

Up to 10% of the prescribed dose

Beam quality is reduced (37.5 for 50 kV beam)

Corrections should be applied for the first dwell position

(courtesy of Xoft)

Dose During Source Turn-On Sequence(Axxent Source in Water)

0.01

0.10

1.00

10.00

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Radius (cm)

Del

iver

ed D

ose

(Gy)

Turn-On3.0 sec at 50 kV5.7 sec at 37.5 kV

Dose During Source Turn-On Sequence(Axxent Source in Water)

0.01

0.10

1.00

10.00

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Radius (cm)

Del

iver

ed D

ose

(Gy)

Turn-On3.0 sec at 50 kV5.7 sec at 37.5 kV

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Source Transit TimeSource Transit TimeSource Transit Time

Catheter length: 25 cmSource travel distance: 7.5 cm– HDR sources:

Nucletron-12 cm for 0.25 cm step size; 24 cm for 0.5 cm step sizeVarisource-5 cm and 10 cm at 0.25 and 0.5 cm step, respectively

Maximum source speed: 0.83 cm/sFor the 7 cm balloon the transit time is 8.4 s (0.5cm step size) This relatively large transit time should be investigated and accounted for in the treatment planning processFor a 4cm diameter balloon there is a calculated 0.5% +/-1.4% dose uncertainty due to transit time

Catheter length: 25 cmCatheter length: 25 cmSource travel distance: 7.5 cmSource travel distance: 7.5 cm–– HDR sources:HDR sources:

NucletronNucletron--12 cm for 0.25 cm step size; 24 cm for 0.5 cm step size12 cm for 0.25 cm step size; 24 cm for 0.5 cm step sizeVarisourceVarisource--5 cm and 10 cm at 0.25 and 0.5 cm step, respectively5 cm and 10 cm at 0.25 and 0.5 cm step, respectively

Maximum source speed: 0.83 cm/sMaximum source speed: 0.83 cm/sFor the 7 cm balloon the transit time is 8.4 s (0.5cm step For the 7 cm balloon the transit time is 8.4 s (0.5cm step size) size) This relatively large transit time should be investigated and This relatively large transit time should be investigated and accounted for in the treatment planning processaccounted for in the treatment planning processFor a 4cm diameter balloon there is a calculated 0.5% +/For a 4cm diameter balloon there is a calculated 0.5% +/--1.4% dose uncertainty due to transit time 1.4% dose uncertainty due to transit time

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Source Positional AccuracySource Positional AccuracySource Positional Accuracy

Determined using RC film – Precision better than 1 mm over span of source travel

(7.5 cm)

Step size accuracy (pull back mechanism)– External markers

– ≤0.5mm

All sources manufactured to identical lengths

A better source positioning QA device will be useful

Determined using RC film Determined using RC film –– Precision better than 1 mm over span of source travel Precision better than 1 mm over span of source travel

(7.5 cm)(7.5 cm)

Step size accuracy (pull back mechanism)Step size accuracy (pull back mechanism)–– External markersExternal markers

–– ≤≤0.5mm 0.5mm

All sources manufactured to identical lengthsAll sources manufactured to identical lengths

A better source positioning QA device will be A better source positioning QA device will be useful useful

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“Flexibility” of the Source Catheter““FlexibilityFlexibility”” of the Source Catheterof the Source Catheter

Maximum bend in the catheter– 15 deg flex

– Source out of the nest

– Fault

Comparison– HDR source wires can negotiate a curvature with a minimum 2 cm

radius

Clinical Implications– Proper balloon placement

– Ensure straight approach to balloon’s X-ray source lumen

Maximum bend in the catheterMaximum bend in the catheter–– 15 deg flex15 deg flex

–– Source out of the nestSource out of the nest

–– FaultFault

ComparisonComparison–– HDR source wires can negotiate a curvature with a minimum 2 cm HDR source wires can negotiate a curvature with a minimum 2 cm

radiusradius

Clinical ImplicationsClinical Implications–– Proper balloon placement Proper balloon placement

–– Ensure straight approach to balloonEnsure straight approach to balloon’’s Xs X--ray source lumenray source lumen

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Treatment PlanningTreatment PlanningTreatment Planning

Source parameters specified using the TG43 framework and modeled using Plato TPS

Xoft versus Ir-192 dose distributions

Xoft surface to target ratios (STR) 2.1 to 3.5 versus 1.8 to 2.8 for Ir-192

Source parameters specified using the TG43 Source parameters specified using the TG43 framework and modeled using Plato TPS framework and modeled using Plato TPS

XoftXoft versus Irversus Ir--192 dose distributions 192 dose distributions

XoftXoft surface to target ratios (STR) 2.1 to 3.5 surface to target ratios (STR) 2.1 to 3.5 versus 1.8 to 2.8 for Irversus 1.8 to 2.8 for Ir--192192

IrIr--192 18 192 18 posnposnXoftXoft 1 1 posnposn XoftXoft 5 5 posnposn

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Safety Characteristics Safety Characteristics Safety Characteristics

Treatment pause button:

– worked

– treatment resumption possible

Treatment interruption

(displace source from nest):

– treatment resumption possible

Radiation Safety

– FlexiShield adequate

– Distortion of the balloon by the shield was not investigated

Treatment pause button:Treatment pause button:

–– workedworked

–– treatment resumption treatment resumption possiblepossible

Treatment interruption Treatment interruption

(displace source from nest):(displace source from nest):

–– treatment resumption treatment resumption possiblepossible

Radiation SafetyRadiation Safety

–– FlexiShieldFlexiShield adequateadequate

–– Distortion of the balloon by Distortion of the balloon by the shield was not the shield was not investigated investigated

Emergency stop button:

– worked

– treatment log file recovered

Sudden power failure (unplugged unit):

– treatment log file recovered

Treatment stop button:

– worked

– treatment log file recovered

Emergency stop button:Emergency stop button:

–– workedworked

–– treatment log file recoveredtreatment log file recovered

Sudden power failure Sudden power failure (unplugged unit):(unplugged unit):

–– treatment log file recoveredtreatment log file recovered

Treatment stop button:Treatment stop button:

–– workedworked

–– treatment log file recoveredtreatment log file recovered

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Possible Design DeficienciesPossible Design DeficienciesPossible Design Deficiencies

Tube source is easily displaced from the nest on positioning arm

Source attachment clip is fragile

Source travel might be too limited (7.5 cm)

Physics mode will be helpful

Pressure and temperature gauge as part of the system

Interlock failure: X-ray generation with no water flow in cooling tube

– not reproducible

Tube source is easily displaced from the nest on Tube source is easily displaced from the nest on positioning armpositioning arm

Source attachment clip is fragileSource attachment clip is fragile

Source travel might be too limited (7.5 cm)Source travel might be too limited (7.5 cm)

Physics mode will be helpfulPhysics mode will be helpful

Pressure and temperature gauge as part of the systemPressure and temperature gauge as part of the system

Interlock failure: XInterlock failure: X--ray generation with no water flow in ray generation with no water flow in cooling tubecooling tube

–– not reproduciblenot reproducible

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ConclusionsConclusionsConclusions

Very stable outputSource position accuracy < 1.0 mmTimer accuracy and linearity adequate“Turn on” dose needs to be quantified and documentedSource transit effect on delivered dose is ≤1.0% STR’s range is “similar” to Ir-192 Safety interlocks functionalPatient exposure history was always recoverableSource movement should be extendedSource attachment to nest could be redesignedThe accuracy of the dose distribution was not investigated

Very stable outputVery stable outputSource position accuracy < 1.0 mmSource position accuracy < 1.0 mmTimer accuracy and linearity adequateTimer accuracy and linearity adequate““Turn onTurn on”” dose needs to be quantified and documenteddose needs to be quantified and documentedSource transit effect on delivered dose is Source transit effect on delivered dose is ≤≤1.0% 1.0% STRSTR’’ss range is range is ““similarsimilar”” to Irto Ir--192 192 Safety interlocks functionalSafety interlocks functionalPatient exposure history was always recoverablePatient exposure history was always recoverableSource movement should be extendedSource movement should be extendedSource attachment to nest could be redesignedSource attachment to nest could be redesignedThe accuracy of the dose distribution was not investigatedThe accuracy of the dose distribution was not investigated