imrt quality assurance · 1 imrt qa ii chester ramsey, ph.d. stephen mahan, ph.d. thompson cancer...

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IMRT QA II

Chester Ramsey, Ph.D.Stephen Mahan, Ph.D.

Thompson Cancer CenterKnoxville, Tennessee, U.S.A

IMRT Quality Assurancen Why do we need IMRT QA?

n Do I really need to do QA foreach IMRT patient?

n If I use an independent MonitorUnit calculation program do Istill need QA for each Patient?

n Will I still need to IMRT QAafter we’ve treated 500patients?

n If I expand my monthlymachine QA can I eliminateIMRT QA for each patient?

Medicare Billing Guidelinesn The accuracy of dose delivery must be documented for each course of

treatment by irradiating a phantom that contains either calibrated film tosample the dose distribution or an equivalent measurement system to verifythat the dose delivered is the dose planned

n The dosimetry should be verified using an ionization chamber

Good Practice Guidelinesn Absolute dose must be verified by measurement prior to the delivery of the

first IMRT Fractionn Relative (or absolute) dosimetry must be analyzed prior to the delivery of

the third fraction using calibrated film or an equivalent measurementsystem

IMRT Quality AssuranceWhat’s the Worst that Could Happen?

n Patient Deathn Severe Complicationn Misadministrationn Major Treatment Deviationn Minor Treatment Deviationn Litigationn Lost Revenue

IMRT Quality Assurance

Least

Worst

FDA Adverse Event Report(06/16/2004)n Patient Overdosed by 13.8%n Patient subsequently died as a

result of complications related tothe mistreatment

FDA Adverse Event Report(04/07/2005)n Medical center reported that

between 2004 and 2005 77 ptsreceived radiation approx 52%in excess of their prescribeddose

n The excess radiation was a resultof a calculation error by themedical center physicist duringcalibration

n This incident has beenrecognized/identified as "humanerror"

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What’s the Worst that Could Happen?n Patient Deathn Severe Complicationn Misadministrationn Major Treatment Deviationn Minor Treatment Deviationn Litigationn Lost Revenue


Least

WorstFDA Adverse Event Report(04/09/2005)n A patient was reported to have

potentially received 13 Gy perfraction for 3 fractions (39 Gytotal)

n It was reported that thetreatments apparently were givenas open field treatments when theMLC was intended to be active

n Control points did not save due toan "abnormal" programtermination during the saveprocess

FDA Adverse Event Report(04/22/2005)n Prostate IMRT patient treated to

a higher dose than prescribedn Reported as Medical Physics user

error

What’s the Worst that Could Happen?n Patient Deathn Severe Complicationn Misadministrationn Major Treatment Deviationn Minor Treatment Deviationn Litigationn Lost Revenue


Least

WorstMedical device Correction(05/1/2005)n Intermittently for IMRT treatment

type, MLC leaves may remain openafter therapists have opened leavesfor the field light, then hit “Go”

n The frequency in which theanomaly occurs is much greaterwith real-time virus scanningenabled

n Manufacturer states that “Thisanomaly is unrelated tomisadministration's that have beenpublicized in recent newspaperaccounts…”

FDA Adverse Event Report(04/29/2005)n Hospital reported that couch,

collimator, and gantry anglebecame reversed on one fieldout of seven

Multileaf Collimator QA

Light Field Edge

The Light-Field Edge is Defined by theTangent Point on the MLC Leaf



3

ü

0-cm

Carriage Skew

Carriage Skew is the Alignment between the MLCleaves and the Primary Jaw or Backup Diaphragm

Î

0-cm

Carriage Skew

Carriage Skew is the Alignment between the MLCleaves and the Primary Jaw or Backup Diaphragm

Local ComputerLocal Computer Control ComputerControl Computer

Carriage Skew

The MLC Field Alignment Tool can be used toAdjust Skew on Varian MLCs

Minimum Leaf Gap

Minimum Leaf Gap is Needed to Prevent MLC Collision



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Minimum Leaf Gapn A directly independent

measurement of the mechanicalgap can be made with a feelergauge

n The opposed pair of MLC leavesare set to a 1-mm field atisocenter

n The gauge should read thedemagnified value at the plane ofthe MLC

n Be aware that vendors havefrequently changed thespecification for this offset

Local ComputerLocal Computer Control ComputerControl Computer

Minimum Leaf Gap

The MLC Field Alignment Tool can be used tothe Leaf Gap Offset on Varian MLCs

X-Ray Field Edge

The Rounded leaves allow SignificantTransmission in the First Millimeter Boyer et al, Med. Phys. 24: 757

Light-FieldEdge

X-RayField Edge

X-Ray FieldEdge = 1 HVL

X-Ray Field Edge



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X-Ray Field Edge

The Effective Leaf Shift is the DistanceBetween the Light and X-Ray Field Edges

X-Ray Field Edge

The Effective Leaf Shift is the DistanceBetween the Light and X-Ray Field Edges

Effective Leaf Shift

AAPM IMRT subcommittee Med. Phys. 30 (8)

n The Effective Leaf Shift ismeasured using Multiple StaticMLC fields

n The MLC leaves on one side movebeyond the desired position by 1/2the estimated gap width

n The opposing MLC leaves thenmove beyond the desired positionby 1/2 the estimated gap width

n A double exposed film is used tomeasure the junction between thetwo fields



n The Effective Leaf Shift ismeasured using Multiple StaticMLC fields

n The MLC leaves on one side movebeyond the desired position by 1/2the estimated gap width

n The opposing MLC leaves thenmove beyond the desired positionby 1/2 the estimated gap width

n A double exposed film is used tomeasure the junction between thetwo fields



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0.4-mm

0.6-mm

1.2-mm1.4-m

m0.8-m

m1.0-m

m

Varian 6MV



1.4-mm

1.6-mm 2.2-m

m2.4-m

m1.8-m

m2.0-m

m

Varian 18MV

Image-Guidance Commissioning

Imaging System Commissioning

n MVCT images can beacquired with slice thicknessof 6-mm (coarse), 4-mm(normal), and 2.5-mm (fine)

n The dose for each slicethickness is measured with aspecial ion chamber

n 2 cm3 collecting volume witha collecting length of +4-cm

n 0.5 (coarse) to 4.0 cGy (fine)is delivered to the imagingvolume for each MVCT



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n Imaging system accuracy is measured by placing an object ina reference position and then CT imaging that object

n An anthropomorphic head phantom is imaged 30 times


n The calculated shifts should be 0.0-mm in all directions

n Any non-zero shifts that are calculated represent the totalimaging and fusion accuracy

n The measured (and now published) accuracy is 0.6-mm

Radiographic Film for IMRT QA

Film Calibration

n EDR2 Film (Kodak, Rochester,NY) is a Ready-Pack form ofthe original EC film used forportal localization

n EDR2 is a very slow speed andfine grain film with a usefulrange up to 600 cGy

n Double emulsion layers arecoated on a 0.18-mm Estarbase, which allowsconventional processing

Zhu et al. Med Phys 29: 16870

1

2

3

4

5

0 500 1000 1500cGy

OD

TLXV2EDR



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n A 13 step calibration film can be generatedusing a dMLC step-and-shoot file

n Each stripe delivers a progressively lowerdose

n Easier than using square films

96.1 cGy

65.1 cGy57.3 cGy49.6 cGy

41.8 cGy34.0 cGy

26.1 cGy

18.2 cGy10.4 cGy

87.1 cGy79.8 cGy72.4 cGy

93.4 cGy

Film Calibration Film Calibration

n The dose must be measured ineach of the 13 stripes using anionization chamber

n Place a water equivalent phantomat isocenter

n Align the chamber in the center ofthe first 2-cm wide stripe

n Move the couch longitudinally 2-cm to move the chamber to thenext stripe

n Scan the film into a dosimetrypackage and create a dose-to-density curve

Film Calibration

n Calibration variations havebeen studied over an 18 monthtime period (547 individualcalibration films)

n Large standard deviations havebeen observed in filmcalibration

n These observed differences arelikely due to the variation overtime in the film processorproperties (temperature,chemical age, etc…)

Childress et al. Med Phys 32: 539

Film Calibration

n The amount of time betweenexposure and processing altersKodak EDR2 film’s absolutedose response

n If calibration films are exposedfollowing patient QA, and allfilms are processed together, thetime delay for the calibrationwould be shorter

n The film was within 1% of itsstable values after 1 hour, andwithin 2% after 30 min

Childress et al. Med Phys 31:2284



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IMRT Patient QA Techniques

Planar Dosimetry

Planar Dosimetry is when measurements are taken withbuild-up material one field at a time

Planar Dosimetry

Planar Dosimetry is when measurements are taken withbuild-up material of one field at a time

Composite Dosimetry

Composite Dosimetry is when measurements are taken withbuild-up material for all fields from the treatment gantry angles



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Composite Dosimetry

Composite Dosimetry is when measurements are taken withbuild-up material for all fields from the treatment gantry angles

Patient Specific Quality Assurance

n The phantom is placedon the treatment andaligned with the REDlasers

n Ion Chamber(s) areused to measureabsolute dose prior tothe first treatment

n Film (or equivalent) isused to measure theshape of the dosedistribution

Sold Water Phantom with Farmer ChamberSold Water Phantom with Farmer Chamber


Cylindrical Phantom with Small Volume ChamberCylindrical Phantom with Small Volume Chamber





MedMed--Tec Phantom with Small Volume ChamberTec Phantom with Small Volume Chamber






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The Film is marked at knowThe Film is marked at knowlocations by metal pins that arelocations by metal pins that are

built into the phantombuilt into the phantom

Metal PinsMetal Pins

(0, 11)

(0, -11) (11, -10)

(13.5, -5)

Film Registration

MVCT kVCTkVCT

CT Images are acquired of the phantom and the metal pinsCT Images are acquired of the phantom and the metal pinsare aligned with the planning CT imageare aligned with the planning CT image

Film Registration

Film Registrationn Do not need a phantom

with built-in pins

n The film dosimetrysoftware must have a toolfor registering the film tothe calculated doses

n The Pin Pricks on thefilm are matched toknown points on theplanned doses

n The use of film pricksallows a spatialcalibration of the locationof the doses

Inverse Planning SystemCommissioning



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Planning System Commissioningn The first step in planning system

commissioning is to developsimple geometric test cases

n In the treatment planning system,contour 5 rectangular structures ona single slice that corresponds to 5-cm depth in phantom

n Contour 2x2, 4x4, 6x6, 8x8, and10x10 squares

n Add a single anterior field

n Using the inverse planningsoftware to created a dose pyramid

200180160140120

Film Plane

Planning System Commissioningn For the next commissioning test,

reverse the dose sequence to createa dose well

n Again, use the inverse planningsystem to create a plan with asingle anterior beam

n Experiment with different leafsequencing options to find a goodbalance between the calculateddose and the number of segments

n Verify these test plans using filmdosimetry on the accelerator

Film Plane

200180160140120

Planning System Commissioning

Gamma (3%/3-mm)Green < 1Red >1

ThresholdGreen < 3%Red >3%

Pyramid




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Well



XR: 5-mm

25-mm

PTV

Cord

n The next commissioning stepinvolves creating complexmulti-field IMRT treatments

n Create a PTV that is 25-mmthick and Spinal Cord had adiameter of 10-mm

n Create five test cases with theseparation between the PTVand the Spinal Cord set to 2-mm, 4-mm, 6-mm, 8-mm, and10-mm


XR: 5-mm

25-mm

PTV

Cord

n Prescribe 30 Gy to 90% of thePTV

n The minimum dose to the PTVis 27.9 Gy

n The maximum dose to the PTVis 33Gy

n During optimization minimizethe dose to the spinal cord

n Verify these test plans usingfilm and ion chamber dosimetryon the accelerator



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Film and Ionization Chamber measurements were taken inthe cylindrical phantom for each test case


Dose Gradient• 10% per mm• R2 = 0.988

PTV Uniformity• PTVmax / PTV95%• 4 to 11%

Absolute doses (as measured with film andchamber) agree within ±2% of the prescribed dose

Diode Arrays for Patient SpecificIMRT QA

Sun Nuclear MapCheck

n The MapCHECK consistsof 445 N-Type diodes thatare in a 22 x 22 cm2 array

n Each detector has an activearea of 0.8 x 0.8-mm

n The Outer Band Array as2.0-cm horizontal andvertical spacing

n The Inner 10 x 10-cm Arrayhas 1.0-cm horizontal andvertical spacingSun Nuclear Corporation

MapCHECKJursinic et al. Med Phys 30:870



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n The N-Diodes are aproprietary design thatmakes them very resistantto radiation damage(compared to other N-TypeDiodes)

n Two acrylic plates that haveconductive surfacessurround the diodes, whichare mounted on a multilayercircuit board

Sun Nuclear CorporationMapCHECK

Jursinic et al. Med Phys 30:870


0%

20%

40%

60%

80%

100%

0 50 100 150 200 250 300 350

Dose (cGy)Si

gnal

n The response of the systemis linear with dose

n However, the N-Typediodes do have atemperature coefficient of0.54% / 0C

n As such it is recommendedthat the MapCHECK bestored at a temperture closeto that of the treatmentroom



n The system has a radiologicalbuildup of 2 g/cm2 to thedetector junctions

n The physical thickness of thebuild-up is 1.35-cm

n The system also has 2.7g/cm2 of Backscatter

n As such, the system can beused with or without additionwater-equivalent build-upmaterial

95.65-cm98.65-cm

3.0-cm


MapCheck

Build-Up (3-cm)

100-

cm

95.6

5-cm

Build-Up

100-

cm 95-c

m


Actual Measurement Setup Treatment Planning Setup



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n Each diode reading isconverted to doseusing individualcalibration factors

n Calculated Doses areImported from theTreatment PlanningSystem

n MapCHECK’scontroller software isthen used to analyzethe results


Film for Patient Specific IMRT QA

IMRT Dosimetric Verification



5 FieldAxial Composite



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IMRT Dosimetric Verification IMRT Dosimetric Verification



AP Field

>10%

IMRT Dosimetric VerificationLarge Region of Interest

Small Region of Interest

90% pixelswithin ±3%

76% pixelswithin ±3%

Statistics must be calculatedwith a consistent ROI


Green < 1Red >1


Coronal Composite

5 Field (Planned)



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Green < 1Red >1


Coronal CompositeDelivery Had a Reversed

Gantry Angle


Axial Composite



Delivery Had a ReversedGantry Angle

SpinalCord


Pick a Film Orientation that provides you with informationPick a Film Orientation that provides you with informationabout doses to theabout doses to the PTVPTV and Critical Structureand Critical Structure

PTV

Parotids





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PTV

SpinalCord



Absolute ion chamber readings should be withinAbsolute ion chamber readings should be within ±2%,and must be withinmust be within ±4% of the prescribed doseof the prescribed dose


EPID Dosimetry

n Varian Linear AcceleratorsEquipped with AmorphousSilicon EPIDs can be used tofor patient-specific IMRT doseverification

n Planar dosimetry can bemeasured for individual IMRTfields and analyzed using eitherthe Varian Portal VisionDosimetry Software or theRIT113 Dosimetry Software

EPID Dosimetry



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EPID Dosimetryn Users who have the Varian EPID, Eclipse (version 7.1.13 or

higher), and the Eclipse Portal Vision Dosimetry Upgrade cancalculate a dose map of the true patient plan at the 105cm planein the amorphous silicon panel

n Users who have the Varian EPID and a Third Party PlanningSystem must calculate a homogeneous phantom plan with thedose map calculated at the 105 cm distance

n Images for Third Party Planning Systems are acquired at 0degrees (IEC Scale) and 100 cm SSD with 2cm of solid watersitting directly on the EPID

n NOTE: There is an anti-collision interlock for the EPID arm sothe solid water must be placed on the EPID very gently in orderto not activate this interlock

EPID Dosimetry

Adaptive Therapy



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n Adaptive therapy is the nextstep in the evolution of IGRTand IMRT

n In Adaptive Therapy, newIMRT treatment plans arecreated every 1 to 5 fraction

n It will not be possible toperform the required patientspecific IMRT QA for adaptivetherapy using currenttechniques

Adaptive Therapy

S.V.

Rectum

Prostate

n In order for adaptive therapy tobe used clinically, techniquesmust be developed forautomated IMRT QA

n Ideally, this will be performedusing Exit Dosimetry duringthe actual treatment delivery

n Calculated and measured MLCpositions and doses would becompared in real-time by thedelivery system

Adaptive Therapy

(A)(B)

(C)

(D)

(E)

There are three aspects of IMRT QA:n QA for IMRT Treatment Planning

n IMRT Delivery System QA

n Patient Specific QA

Summary

An quality assurance program must be created by themedical physicist that periodically evaluates each ofthese areas



imrt quality assurance · 1 imrt qa ii chester ramsey, ph.d. stephen mahan, ph.d. thompson cancer...

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