QUALITY ASSURANCE FOR IMAGE-GUIDED RADIOTHERAPYSource and detector position. MARINELLO, AAPM, Prague 2008 MV-aSiPIDE (AS 500) kV-aSi detector (Pax Scan 4030 CB) kV X-ray source VARIAN-

MARINELLO, AAPM, Prague 2008

QUALITY ASSURANCE FOR

IMAGE-GUIDED RADIOTHERAPY

Ginette MARINELLO, Ph D

Unité de Radiophysique et Radioprotection du patient

Centre hospitalo-universitaire Henri Mondor51 avenue de Lattre de Tassigny

94010 CréteilFRANCE


• A comprehensive QA program ismandatory at the timeof commissionningandto maintain the requirements ofthe system

QUALITY ASSURANCE PROGRAM

• Quality control (QC) tests should :

- be reasonable, simple and fast to perform

- include safety and functionalities, precision ofthe OBI and treatment couch movements, image quality and for CBCT or MV-CT, HU accuracy

- involve algorithms allowing mechanical couchcorrection or image reconstruction and matching, etc

- include patient dose evaluation


IMAGE GUIDED RADIOTHERAPYInvolves multiple imaging procedures:

- during planning process:CT images sometimesfused with PET, SPECT, ultrasound or MR imaging

- to optimise patient set-up: film or electronicimaging using a kV simulator, or use of calculated digitallyreconstructed radiographs (DRRs) from the planning CT used as the simulation images

- to check patient set-up: use of EPID , diagnostic X-ray or kV or MV cone beam CT imaging

- to characterize organ or tumor motion due to therespiration or other influences: breathing correlated CT (referred as 4D CT) or linac, etc


OVERVIEW OF EXISTING COMMERCIALLY AVAILABLE TECHNOLOGIES IN IMAGE GUIDANCE

• Planar imaging, EPID

• kV-CBCT , only availableon Elekta and Varian


MV-CBCT , onlyavailable on Siemens

Paco hernandez 2007

MV-CT , only avalaibleon helical Tomotherapy

• Stereoscopic X-ray imaging, ultrasound imagingindependent of linac manufacturer...


COMMISSIONNING AND QUALITY ASSURANCE

Irrespective of the imaging equipment used, the followingfeatures must be verified:

• mechanical and electrical safety

• geometrical performance and reproducibility

• calibration

• image quality

• sofware performance, etc

The controls must be undertaken only if mechanicalperformances and radiation stability of the linac arecorrect


ANTI-COLLISION AND MECHANICAL VERIFICATIONS

After M. Djordjevic, 2007


must be made and stored ...Example:

AAPM report 75 (TG58),2001


CHECK OF AGREEMENT BETWEEN VISUAL AND DISPLAYED SCALES

Source and detector position


MV -aSiEPID(AS 500)

kV-aSi detector(Pax Scan 4030 CB)

kV X-ray source

VARIAN- Clinac 2100 C


EPID GENERALLY BASED ON:• matrix ion chambers• combination of camera and phosphor screen

• active matrix flat panel (photodiode array made of amorphoussilicium, or photoconductor array made of selenium)...


EPID DETECTION PRINCIPLE

• Compton electrons produced in copper plate : energy released in the scintillator

=> fluorescence photons detected by photodiodes

VARIAN

aS 500 or

aS 1000

cover

copper plate(1 mm)

glass (0.1 mm)

a-Si photodiodes

Beam

cover

scintillator


• copper platereplaced by aluminium plate

• anti-scatter grid addedto avoid scattered radiation frompatient and table

kV-X-RAY DETECTORS

cover

aluminium plate (2mm)

thin lead plate

a-Si photodiodes

Beam

cover

scintillator

antiscatter gridVARIANPax Scan

4030CB

RECEPTOR


Photodiodes signal read out line by line and sent to a converter=> digitalized images through pulses continously processed

and displayed

Courtesy of VARIAN

IMAGE CONSTRUCTION


QUALITY CONTROLS (QC)

1. ELECTRONIC PORTAL IMAGING DEVICE

(EPID)


CALIBRATION OF EPID

• Method:

Should be carried out for all energies, dose-rates and geometrical conditions used for patients,following the recommendations madeby the vendor, and using the tools provided withthe system

• Purpose:

Check thatdark current, noiseandhomogeneityare acceptable


HOMOGENEITY CHECK

NOISESD

A

B

B

A

RX 6MV20 x 26 cm2

maximal field size


CALIBRATION OF EPID

Pixel defect can be detected and repaired through software interpolation


IMAGE QUALITY : CONTRAST

whereΦP1, ΦP2, andΦS are theprimary and scatter photon fluences

( )/22ΦΦΦ

ΦΦ

signalmeansignal

CSP1P2

P1P2

++−==

( )SF1

2SFe1

e-12 C

∆-

-∆

−++

=

withwaterµboneµXL∆ −=

( )PSS ΦΦΦSF +=

AAPM, Report 58, 2001

MARINELLO, AAPM, Prague 2008EPID QC Phantom - PTW

minimum distancebetween 2 adjacent features, or

minimum size of a featurethatcan be detected by EPID.

IMAGE QUALITY : SPATIAL RESOLUTION


Tolerances for EPID

• acceptable contrast :1%,

• spatial resolution 2-3 mm

IMAGE QUALITY

AAPM TG 58, Report N° 75

Med. Phys. 28, 712-737, 2001

⇓⇓⇓⇓


Spatial resolution given by thevisible columnsandcontrast

deduced from thevisible lines

ALUMINIUM LAS VEGAS PHANTOM

6 MV

side to place opposite to X-ray source


EPID: on line or off line 2 D tissue and bone imaging => patient set-up and treatment verification by comparison with

DRR or reference EPID image

DRR

EPID image matched with DRR

= > softwares check mandatory


Alderson phantom

EXAMPLE OF SOFWARE CHECKS

CT Scan of the phantom

in "treatment" position

⇓

DDR PRODUCING

using TPS

⇓

Transfert to linac

through network

STEP 1


EPID image

taken without

phantom

⇓

check of the field shape

using

sofware

⇓

check of geometric

reproduction of fieldshape

STEP 2


Anterior field

EPID Image DRR

REFERENCE

VISUAL COMPARISON

STEP 3


Lateral field

EPID Image DRR

ANATOMY

CHECK

WITH

CORRELATION

POINTS

REFERENCE

STEP 4


2. MV-kV & kV-kV

QUALITY CONTROLS

MARINELLO, AAPM, Prague 2008After Bissonnette, 2007


• safety and anticollision checks

• mechanical checks such as coincidence between rotation and beam axis,etc

note: no light field => need to use special toolsadapted to X-ray energyandmodeused.

kV-kV2 portal images

taken at0° and 90°either in

radiographic or fluoroscopic modes


CHECK OF CENTRAL AXIS

1. Check lasers2. Align cube phantom with light and lasers3. Acquire AP and lateral images4. Check rotation axis

lead ball(ΦΦΦΦ = 2mm)

at the center ofthe cube


CHECK OF IMAGE QUALITY

http://www.leedstestobjects.com

Same method as for EPID but with tools adapted to diagnostic energy*...

* AAPM Report N°74QA in Diagnostic Radiology,2002


50kV – small focus spot

It is important to evaluatecontrastandspatial resolutionwith the sameparameters as those used for patients

MARINELLO, AAPM, Prague 2008CHECK OF IMAGE GEOMETRY ......

kV-kV


Printed Board : Set-up

Source surface distance : 1m

Printed Board : Acquisition

Source surface distance : 1m

Various surface detector distance :

20cm to 50cm


angles

44.9°

45°

distances 20.1 cm

20 cm

PRINTED BOARD

QA OF IMAGE GEOMETRY

To checked for bothperpendicular directions


CHECK OF TABLE DISPLACEMENT

Example of tool used to checkautomatic correction of table displacement

by software

⇓

It can be used for:• kV-kV• kV- MV• CB- CT• MV- CT


CHECK OF TABLE DISPLACEMENT

2. Check lasers3. Setup the phantom with light and lasers according to white

crosses4. Move the couch to set up the phantom according to the red or

green crosses5. Check the deviation founded by the software6. Repeat points 3 to 4 ; then check and apply automatic correction

of table displacement in order to check it

1. Acquire CT scan with phantom. Treatment isocenter aligned according to white crosses

accuracy and repeatability⇓⇓⇓⇓


3. CONE-BEAM CT(CBCT)

QUALITY CONTROLS


CONE BEAM CT: 3 D on linebone and soft tissue imaging=> patient set-up corrections and evaluation of organ movements

through comparison withCT used for planification

available on ELEKTA and

VARIAN linacs...


CHARACTERISTICS OF VARIAN – CBCT(model VZW2940FH2-02)

• a kV x-ray sourceandan a-Si flat panel detectormountedorthogonally to the EPID (aSi500 PortalVision) on linacgantry via 2 robotically controlled arms

• source- axis distance 100 cm and axis - detector distance 50 cm

• detector area 40 x 30 cm2 => maximum beam size at isocentre:20 cm (longitudinal axis) and 26 cm (transversal axis)

• CBCT acquired by a rotation of the linac gantry over 370° (10° overlap on left side) and using x rays in radiographic or pulsed-fluoro mode (focal spots 0.4mm or 0.8 mm)

• 2 acquisition modes depending on the size of anatomical site ...


CBCT ACQUISITION MODES

FULL FAN MODEused to scan site where the

size is less than 24 cm(head andneck), with detector centered

HALF FAN MODEused for sites larger than 24 cm(mostlyfor body scans), with detector shifted to

1 side to cover more than the halfpatient volume

SA

D =

100

cm

AD

D =

50

cm

KVS

KVD

KVS

KVD

Half Bow-Tie

BladesBlades

Full Bow-Tie

axis


Dark grey = scanner, Cleargrey = CBCT

TOOLS TO CHECK THE QUALITY OF THE CORRECTION


image avec couleurs

TOOLS TO CHECK THE QUALITY OF THE CORRECTION

Reference : CT imagesCB CT images

CB CT imagesCB CT images

CT window

CT window CT window


lead ball(ΦΦΦΦ = 2mm)

at cube center

Coincidence between the image of lead ball and axis < 2mm (After Bissonnette, 2007)

CHECK OF ROTATION AXIS


EXAMPLES OF ARTEFACTS

A B

A : Ring artefactB: Streaking

Truncation artefactAfter ZHANG, IJROBP,2005


FLEXMAP

• ELEKTA: Flexes are corrected for in reconstruction algorithm

• VARIANFlexes are compensated for by roboticarm motion

If artefacts remain once the calibration of thedevice made again => defective componentsor sofware problems


QC OF THE CBCT IMAGE

CATPHAN

phantoms 500 or 600

(or equivalent) allow

most of the checks at

the same timeusing

the same protocols as

for patientsphantom made ofdifferent modules



QUALITY CONTROL OF CBCT IMAGE

CBCT Varian (half-fan- max field-125 kV- mA- 25 s)

Uniformity ResolutionDetectability &slice thickness

Results obtained highly dependent of the equipment,sofware versions... and parameters of irradiation(beam hardening, scatter, energy, dose-rate...)


IMAGES OF THE HOMOGENEOUS MODULE

Full fan mode Half-fan mode

VARIAN- CBCT

M. DJORDJEVICMaster of sciences

thesis, 2007

MARINELLO, AAPM, Prague 2008 After Bissonnette, 2007


20 cm* (longitudinal axis) x 26 cm *maximal size

5 cm (longitudinal axis) x 26 cm at the axis

INFLUENCE OF THE SCATTERING VOLUME ONCONTRAST

MARINELLO, AAPM, Prague 2008After Bissonnette, 2007


CBCT Varian

M. DJORDJEVICMaster of sciences

thesis, 2007


IMAGES CAN BE VISUALLY

OR AUTOMATICALLY ANALYSED USING

SPECIAL SOFTWARES


The The The The ARTiScanARTiScanARTiScanARTiScan* Solution* Solution* Solution* Solution****http:// http:// http:// http:// www.aquilab.comwww.aquilab.comwww.aquilab.comwww.aquilab.com

Integrated solution for Quality Controlin Diagnostic or Radiotherapy Imagingincluding:

• Equipmentindependent PC connected to the PACSor network

• Software ARTiScanDICOM, Image gestion

• Modules for automaticallyanalysing the images

Specific module for each test object and imaging device



Results can automatically becompared to :

- reference data,either published in national or international recommendations (IEC...) or establishedby users (if possible acceptance testsof equipment) when no recommendations are existing

reference

measuredvalues

- previous checksin order to point out possible changes andundertake preventive actions

INTEREST :


scannerreferencecontour

CBCT

CBCT CBCT

shift

shift

shift

USE OF CBCT TO CHECK PATIENT SET-UP


Automatic (version 2.0) ormanual correction

of couch position and angle

END OF CORRECTION


EVALUATION OF PATIENT DOSE


Howeverquestions are raised

• What is the dose delivered by IGRT ?

• Does daily use of IGRT for patient setup significantlyincreases the already high level of therapeutic radiation and the doses to the normal tissues of the patient ?

• How taking it into account?

Benefits of IGRT are obvious => possibility to seethe target and organ at risk (rectum) before the treatment

PATIENT DOSE


IMAGING DOSE DURING IGRT

Although different in modality and method, allradiographic techniques have in common the fact thatthey can delivera significant radiation dose to the patient(especially when they are used for accurate day-to-dayset-up)

⇓⇓⇓⇓

REPORT OF THE AAPM TG 75

The management of imaging dose duringimage-guided radiotherapy

Medical Physics, Vol. 34, N°10,4041-4063


EVALUATION OF IMAGING DOSE

Fundamental distinction between:

• dose: dose deposited locally per unit of mass by ionizing radiation (J/kg or Gy, mGy...).It depends on thefluence(rate and duration)

• integral dose: total dose integrated over the exposedmass (volume) of the anatomy (J).It depends on fluence and irradiated volume

⇓⇓⇓⇓

Absorbed dose is the meaningful quantity for estimating the biological effect of irradiationand integral dose to quantify the risk ofsecond cancers in future


• Planar imaging(portal imaging, kV-kV, MV-kV) :doseto the patient /image greateast at the skin surfacenearest to the source and falls off progressively as the radiation transits the body to the image detector.

• Axial imaging (CT, CBCT, MV-CT):dose distributed nearly uniformly throughout the imagevolume to produce 3D image of uniform cross-sectionalquality

GENERALITIES


DOSE DELIVERED BY EPID

• The image being created with the megavoltage beamwhich is used to treat the patient

• Generally source/detector distances are variable from120 to 150 cm => cumulative doseto acquire single image using aSi panels ranges from 1 to 5 MUfor most users(AAPM, 2007)

• For scenarios that involve daily or numerous portal images, it is recommended to substract the numberof MU used for imaging from the number of MUnecessary to treat the patient (possibility to do itautomatically with some sofware).


DOSE DELIVERED BY kV- kV

When used in radiographic mode to acquireAP and lateral alignment radiographs, thedose to the patient is approximately1-3 mGyper image,depending on the technique (Perkins et al. 2006)

=> in most cases, it can be neglected.


DOSE DELIVERED BY CBCT

kV CBCT

2.3 cGy

~ 5 cGy

2.3 cGy

Dose

(lateral)

68 cGy1.7 cGySiemens

(Prototype)3

120 cGy~3 cGyOBI (120 kV)2

64 cGy1.6 cGySynergy (120kV)1

80Gy

Daily image

Dose

(center)

Phantom

(Φ= 30 cm)

MV CBCTProstate (center) 6.4 cGy 256 cGy

1 M.K. Islam et al, Med Phys 33(6) 1573-1582, 2 N. Wen et al Phys Med Biol 2007 52 2267-2276, 3 U. Oelfke Comm. DKFZ Jan 2007

L.C. Peng et al, J. Appl Clin Med Phy 2007 8(1) 10-20


The dose delivered by kV-CBCT can be optimalizedby adapting the collimation aperture to the size of thevolume to be explored:• by sofware (Varian linacs)• by inserting a special accessory(Elekta)


• Irradiation conditions

identical to those used for

patient CBCT (prostate):

- 125 kV

- Half bow-tie

- 80 mA

- 25 ms/pulse

PREPARATION AND CHECK OF THE IVD METHOD

• Use of TLD 700H for point dose measurements…

• Phantom Rando Alderson


... anduse of EBT GAFCHROMIC FILMfor evaluating the dose distribution

• A-P thickness 23 cm• Table height 11.5 cm• 15 CBCT =>

100% ~ 0.7 GyDpost

110 %

100 %

120 %130 %

90 %

xDleft

Dant

Dright

140 %

Dpost

Rectum

axis100% =

(Dant+Dpost) / 2


• For geometrical and antomical reasons , maximal skin dosesare measured along theantero-posterior axisof the phantom (smallestthickness), and theirmean is close or higherto the dose at mid-thickness of the phantom or at therectum position (organ at risk)

=> mean = REFERENCE FOR PATIENT DOSE CORRECTION

DOSES (cGy)

6.25.44.7

--

4.9

--

4.9

6.66.86

86.55.5

1412.311.5*

Dmid-thick nessDrightDleftDpostDant

TH (cm)

RESULTS - 1 CBCT (Alderson phantom)

* mid-thickness

• Dose heterogeneous within the irradiated volumeand verydependent on the patient position/axis (TH)


Posterior dosimeter under the tablesheet (centered with light beam)=> measured correction factor1.24


/ 26 CBCT


CONCLUSION

• In vivo evaluation of maximal dose susceptible to bedelivered to organ at risk (rectum dose for prostate irradiation) possible with TLD

• Important number of CBCT => contribution of dose to betaken into account... but how to do it and what EBR for low X-rays and high MV X-ray beams)*

*Wambersie Ph D recommends to use a correction factor of 1.13 for X-rays of 200 kV compare to 60Co but for higher doses

• Waiting for a general consensus, we have decided to subtract thephysical dosedelivered by CBCT from theprescribed dose (without correction for biological effects)

Documents

QUALITY ASSURANCE FOR IMAGE-GUIDED RADIOTHERAPYSource and detector position. MARINELLO, AAPM, Prague 2008 MV-aSiPIDE (AS 500) kV-aSi detector (Pax Scan 4030 CB) kV X-ray source VARIAN-