qa for linac kv imaging systems -4442-40814...kv p accuracy & stability kvp fluctuations 100 105...

QA for Linac KV Imaging QA for Linac KV Imaging SystemsSystems

J-P Bissonnette, D Moseley,T Purdie, E White, D Jaffray

Learning ObjectivesLearning Objectives

• Understand the reasoning behind QA– Define quality metrics– Analyse metrics for quality improvement

• Apply to a device: cone-beam CT

• Apply to an existing RT process

What is Quality?What is Quality?

• Several dimensions of quality– Technical specs– Staff/client education & credentials– Consistency of performance– Efficiency of service delivery– Continuity and timeliness– Safety– Human factors engineering

Applicable to devices and processes both

Quality MetricsQuality Metrics

• Analyse a process or device in each of its dimensions

• For each dimension:– What is it that can go wrong?– How does this risk impact quality?

• Define an output that can be controlled, measured, or quantified

Quality MetricsQuality Metrics

• Quantifiable measures of success– Recommended maintenance performed– Error rate less than 0.5%– 1 mm objects can be seen

• Can be statistically analysed

• Can be tracked and monitored

Risk AnalysisRisk Analysis

• Faulty mechanical design causing harm to patient?– FDA, EC, Health Canada– Preventative maintenance & daily visual checks

• System unreliable?– Buggy software, unstable components– R&D identifies these issues & report to users

Risk AnalysisRisk Analysis

• Fault resulting in poor image quality?– Portal imaging as a back-up system

• Excessive dose to the patient?– Validation of imaging techniques– Monitor tube output periodically

• Reports incorrect geometrical info?– Subtle, not immediately obvious– May impact success or failure of RT

ConeCone--beam CT: QA of a Devicebeam CT: QA of a Device

• Safety

• Geometric

• System stability

• Image quality

• System infrastructure

• Dose

Safety TestsSafety Tests

• Collision detectors on flat panels– Motion inhibits when activated

• X-ray tube deployment inhibit

• X-ray area monitors

• Grounding tests• Visual inspection for electrical hazards


• Safety

• Geometric


• Image quality


• Dose

GeometryGeometry

• Pixel size calibration

• MV and kV source alignment

• Flexmaps– Refresh periodically– Repeatability

• kV source positioning• Flat panel positioning• Each imaging position

Pixel Size at IsocentrePixel Size at Isocentre

0.52 mm/pixel

MV GeometryMV Geometry

• Imaging and treatment beams coincide

MV/kV CoincidenceMV/kV Coincidence

• Treatment is orthogonal to imaging

kV/MV Calibration ConceptkV/MV Calibration Concept

kV

BB (Reconstruction Iso-centre)

MV Mechanical isocentre

x

y

z

MV Radiation isocentre

Calibrated isocentre

MV/kV Calibration Procedure MV/kV Calibration Procedure

u

v

Results for Six UnitsResults for Six Units

-5-4.5

-4-3.5

-3-2.5

-2-1.5

-1-0.5

00.5

11.5

22.5

33.5

44.5

5

-180 -135 -90 -45 0 45 90 135 180

Gantry angle (degrees)

Abs

olut

e U

dis

plac

emen

t (m

m)

Unit 7Unit 8Unit 9Unit 10Unit 14Unit 17

-5-4.5

-4-3.5

-3-2.5

-2-1.5

-1-0.5

00.5

11.5

22.5

33.5

44.5

5

-180 -135 -90 -45 0 45 90 135 180


Abso

lute

V d

ispl

acem

ent (

mm

)

Unit 7Unit 8Unit 9Unit 10Unit 14Unit 17

LongLong--term Stability:term Stability: FlexmapFlexmap

12 calibrations over 28 months

u

v

-1.5

-1.25

-1

-0.75

-0.5

-0.25

0

0.25

0.5

0.75

1

1.25

-180 -135 -90 -45 0 45 90 135 180


Resi

dual

dis

plac

emen

t (m

m)

95% confidence interval = 0.25 mm

Targeting StudyTargeting Study• 21 sessions:

– Over a 3 month period – Three different operators– Single calibration map

• Geometric calibration maps monitored weekly in parallel study.

MV/kV DifferenceMV/kV Difference

Daily Geometry QADaily Geometry QA

• Align phantom with lasers

• Acquire portal images (AP & Lat) & assess central axis

• Acquire CBCT• Difference between

predicted couch displacements (MV & kV) should be < 2 mm

-2.00

-1.50

-1.00

-0.50

0.00

0.50

1.00

1.50

2.00

01-Jan-06 20-Feb-06 11-Apr-06 31-May-06 20-Jul-06

Dev

iatio

n fr

om is

ocen

tre,

X d

irect

ion

(cm

)

ES07

ES08

NS09

NS10

WS17

Effect of Incorrect CalibrationEffect of Incorrect Calibration


• Safety

• Geometric


• Image quality


• Dose

System StabilitySystem Stability

• Dark image calibration– Acquired prior to each scan

• Gain stability & defect maps– Refresh floods every month

Dark image performanceDark image performanceDark fields

0

1

2

3

4

5

6

7

8

14-J

an-0

4

23-A

pr-0

4

1-Au

g-04

9-N

ov-0

4

17-F

eb-0

5

28-M

ay-0

5

5-Se

p-05

14-D

ec-0

5

24-M

ar-0

6

2-Ju

l-06

10-O

ct-0

6

Date

Stan

dard

dev

iatio

n in

pix

el v

alue

CenteredFull offsetMid offset

Flood Fields: image SNRFlood Fields: image SNR

0

20

40

60

80

100

120

14-J

an-0

4

23-A

pr-0

4

1-Au

g-04

9-N

ov-0

4

17-F

eb-0

5

28-M

ay-0

5

5-Se

p-05

14-D

ec-0

5

24-M

ar-0

6

2-Ju

l-06

10-O

ct-0

6

Date

Sta

ndar

d de

viat

ion

in p

ixel

val

ue

X ray tube changes

X ray Generator StabilityX ray Generator Stability

• Reproducibility and Accuracy– kVp, mA, ms

XX--ray Generator: ms Accuracyray Generator: ms Accuracy

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90

Programmed time (ms)

Mea

sure

d tim

e (m

s)

Second experimentLinear fitFirst experiment

Linear fit: Y = 1.002 x + 0.789

mAs linearity: fixed currentmAs linearity: fixed currentmAs linearity

0

50

100

150

200

250

0 5 10 15 20 25 30

programmed mAs

Mea

sure

d ex

posu

re (m

R)

40 mA80 mA32 mA320 mA160 mA

kVkVpp accuracy & stabilityaccuracy & stability

kVp fluctuations

100

105

110

115

120

125

1 4 7 10 13 16 19 22 25 28 31 34 37 40 43 46 49 52 55 58 61 64 67 70 73 76

shot

Mea

sure

d kV

p

fixed mA

40 mA 80 mA 160 mA 320 mA 32 mA

117.6 ± 4.6 kVp


• Safety

• Geometric


• Image quality


• Dose

Image QualityImage Quality

CatPhan 500 phantom

ScaleScale

• Geometric calibration to tie isocentre to centre of volumetric reconstruction

• Scale to relate all pixels to isocentre

5 cm

ScaleScale

• Geometric calibration to tie isocentre to centre of volumetric reconstruction

• Scale to relate all pixels to isocentre

UniformityUniformity

• Standard CT tests– Cupping, capping

• Baseline non-uniformity index:

minmax

minmax

CTCTCTCT

+−

Linearity of CT NumbersLinearity of CT Numbers

Linearity of CT numbersLinearity of CT numbers

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 200 400 600 800 1000 1200 1400 1600 1800 2000

Theoretical Housfield unit

Mea

sure

d Ho

unsf

ield

uni

t Unit 7Unit 8Unit 9Unit 10Unit 12Unit 16Unit 16 with annulusUnit 17

Add ScatterAdd Scatter

IEC standard 61675-1

Linearity of CT numbersLinearity of CT numbers

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 200 400 600 800 1000 1200 1400 1600 1800 2000

Theoretical Housfield unit

Mea

sure

d Ho

unsf

ield

uni

t Unit 7Unit 8Unit 9Unit 10Unit 12Unit 16Unit 16 with annulusUnit 17

Linearity of CT NumbersLinearity of CT Numbers

• Fairly linear (χ2 > 0.99) for all systems

• Beam hardening

• Scatter conditions

• Non-standard metric; use only as a baseline

Spatial ResolutionSpatial Resolution

GE CT scanner2.5 mm slices 0.5 mm pixels

120 kVp, 100 mA

Low Contrast ResolutionLow Contrast Resolution

1 % contrastvisible

0.5 % contrastvisible

1 % contrastvisible

GE CT scanner2.5 mm slices 1 mm pixels

120 kVp, 100 mA


• Safety

• Geometric


• Image quality

• System Infrastructure

• Dose

System InfrastructureSystem Infrastructure

• Data safety

• Storage space available

• Review of QA data

Daily CBCT QA ProgramDaily CBCT QA ProgramDimension Procedure Tolerance

Detector stability Dark image calibrationGeometry Localising lasers < 1 mm

MV/kV/laser alignment < 1 mmAccuracy of shifts < 1 mm

Safety Interlocks: interrupts or prevents irradiation

Functional

Warning lights FunctionalWarm-up Generator operation Functional

Detector operation FunctionalDetector signal Within expected rangeCollimator operational Functional

Clinical process issues Database integrityStorage space availability

Monthly CBCT QA ProgramMonthly CBCT QA ProgramDimension Procedure Tolerance

Imaging system Gain stability Replace or refreshperformance Defect maps Replace or refreshImage quality Scale and distances ± 0.5 mm

CT number linearity & stability

Baseline

Image uniformity BaselineHigh contrast spatial resolution

Baseline

Artefacts AbsenceGeometric Geometric calibration Replace / refresh

Accuracy of couch shifts

< 1 mm

Clinical process issues Review of daily test results

Annual CBCT QA Program (service)Annual CBCT QA Program (service)Dimension Procedure Tolerance

X-ray generator kVp accuracy Baselinestability mAs linearity Baseline

Radiation quality (HVL) BaselineAccuracy of mA & mAs Baseline

Geometry Couch scales 1 mmCouch motion accuracy (manual or remote)

1 mm

Detector tilt BaselineDetector skew BaselineDetector scale Baseline

Annual CBCT QA Program Annual CBCT QA Program (upgrades)(upgrades)

Dimension Procedure ToleranceData transfer Link to treatment

planningFunctional and

accurateLong term and short term storage

Functional

Dosimetry Axial and skin doses BaselineClinical process issues Database integrity and

maintenance Baseline

Documentation of imaging procedure

Up-to-date

Review of daily and monthly test results

Completeness

Radiation Therapy ProcessRadiation Therapy Process

Imaging Planning

TreatmentdeliveryVerify

ImageImage--Guided RT: ProcessGuided RT: Process

Verify

Adjust

Imag

e tra

nsfer

Image

Target

Adjust

Verify

BeginTx

InitialSetup

Within tolerance

With

in tol

eran

ceRe-try

Shift

Verify: Quality MetricsVerify: Quality Metrics

• Patient ID

• Prescription

• Location

• Patient set-up

Illustrative Example: SBRT LungIllustrative Example: SBRT Lung

• Small, inoperable lung lesions

• Radio ablative doses (60 Gy in 3 fx)

• High level of confidence– Targeting– Planning– Treatment

Method: Lung SRT ProcessMethod: Lung SRT Process

Patient consentand education

Immobilisation

Steretactic framefor abdominalcompression

Helical and 4DCTimaging: derive

GTV

Planning

Image guidedpositioning at the

treatment unitCBCT

± 3 mm

Treat

Image Guidance: CBCTImage Guidance: CBCT

Planning CTPlanning CT

CBCT Fx #1CBCT Fx #1



GTVGTVPTVPTV

CBCT Image Guidance AccuracyCBCT Image Guidance AccuracyDisplacements (mm)

Medio-lateral Antero-posterior Cranio-caudal

4.7 ± 5.3 7.3 ± 9.4 5.6 ± 4.8

2.1 ± 2.32.7 ± 2.22.0 ± 1.8

Cranio-caudalAntero-posteriorMedio-lateral

Residual Error (mm)

10 patients

ObservationsObservations

•IGRT clearly reduces both the systematic and random errors.

•Residual errors reflect our current tolerance (± 3 mm).

Radiation TherapyRadiation Therapy

• Who?– Record & verify / electronic file

• What?– Planning CT, diagnostic work-up

• When?– Record & verify / electronic file

Radiation TherapyRadiation Therapy

• How?– Treatment plan

• Where?– Daily CBCT

IGRT is a QA tool for externalbeam radiation therapy

Summary and ConclusionsSummary and Conclusions

• Introduced framework for a QA program

– Goal: reduce errors and uncertainties• Devices • Processes


• Quality has several dimensions

– Dimensions can be formulated as metrics• Quantifiable• Traceable

– Use quality metrics• Assess performance• Improve quality with time


• Have applied QA framework to a device– Cone-beam CT

• Have applied QA framework to a process– SBRT lung

ConclusionsConclusions

• Demonstrated that IGRT is routinely used to reduce systematic and random errors & uncertainties

• In this sense, IGRT is a QA tool

AcknowledgementsAcknowledgements

• Doug Moseley • Tom Purdie• Elizabeth White• David Jaffray

qa for linac kv imaging systems -4442-40814...kv p accuracy & stability kvp fluctuations 100 105...

Documents