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Tools for IMRT QAN. Dogan, Ph.D

Department of Radiation OncologyVirginia Commonwealth University

Medical College of Virginia HospitalsRichmond, VA, USA

N. Dogan /July 2005 N. Dogan

Objectives• To identify the QA tasks involving

IMRT• To describe the QA tools for all

aspects of IMRT process• To explain the limitations of the current

IMRT QA tools• To compare the IMRT QA tools and

techniques


QA tasks for IMRT • Machine QA- Acceptance and routine QA of

the MLC for IMRT delivery - dosimetric and geometric characteristics

• Algorithm QA for IMRT - QA of planning system and data consistency with machine

• Patient Specific QA – prove plan works1D and 2D dosimetry of treatment components such IM beams and segments3D dosimetry of entire treatment delivery

• Post Treatment QA• Log-file analysis


IMRT QA Tools

• Detectors• Phantoms• Scanners• Dosimetric Analysis Tools


Detector Requirements for IMRT QA• Geometric and dosimetric accuracy• Volumetric simultaneously integrating dosimeter

to faithfully quantify the dose delivered over the total time of treatment

• Good spatial resolution, tissue equivalent response

• Ability to provide 3-D information• Portability to multiple phantoms• Ease of use• Sufficiently large dynamic range and be

insensitive to photon energy spectrum and dose rate response which is independent of the energy spectrum


IMRT QA ToolsDetectors• Many of them available for IMRT

measurements• Necessary to characterize the detector

response for both static and dynamic fields for linearity

• Need to be calibrated for absolute measurements

• Need to determine stem and cable effects


IMRT QA ToolsDetectors• Need to determine energy dependence

and angular response• Small field detectors required for small

field characterizationSensitive to positionDetector should be smaller than homogeneous region of dose to be measured

• Assess electrometer response


IMRT QA ToolsDetectors, cont.• Need to determine necessary

resolutiondepends on the resolution of the beamlet grid that is used for planning and sequencing fields for delivery Chambers with the smaller volumes are more sensitive to position and will have a higher response when positioned at an opposing leaf pair junction and between adjacent leaves

Dose (cGy)

70605040302010

Poor detectorposition

More stablemeasurementpoint

Courtesy of Jean Moran, UofM


IMRT QA Tools

1-D and 2-D Detectors• Ion chamber (1-D)• TLDs and MOSFETs (1-D)• Detector arrays (2-D)• Film (2-D)

RadiographicRadiochromic

• Gels (3-D)


Small 1-D Detectors

0.0019

NA

NA

0.3

0.015

0.009

Volume(cm3)

0.45

0.4

0.73

NA

0.2

0.6

Diameter(cm)

< resolution than diodes, dose rate dependence, expensive

Diamond

Non-linear dose response for <30 cGy

MOSFET

Stereotactic diode

p-type Si diode

Over-respond to low energy photons

Martens et al. 2000

Pinpoint chamber

Poorer resolution than diodesMicro-chamber

DisadvantagesDetector

IMRT QA Tools


IMRT QA ToolsIon Chamber• Advantages

Available in different shape and sizesDosimetric response is well understood.Absolute dose measurements – theory is well establish, they can be used as a benchmark standardEasy to calibrate


IMRT QA ToolsIon Chamber, cont.• Disadvantages

Only one measurement point for each irradiation – does not yield sufficient information to evaluate the dose throughout the target and/or critical structuresVolume averaging – the measurements are to be considered as an average throughout the chamber’s active volume - does not yield significant errors if the ion chamber is placed in a low dose-gradient region even for relatively large chambers


IMRT QA ToolsIon Chamber volume averaging, cont.

D.A. Low et al. “Ionization chamber volume averaging effects in dynamic intensity modulated radiation therapy beams, Med. Phys.30(7): 1706-1711 (2003

Micro cham: 0.009cc

PTW: 0.125cc

Farmer:0.65cc


IMRT QA ToolsTLDs• Advantages

Multiple measurement points in a single irradiationReusableEasy to use in multiple phantomsSmall size and versatility in placementReadily available readout equipmentAchievable accuracy: 2-3%


IMRT QA ToolsTLDs• Disadvantages

Requires calibration to determine calibration factor for each TLD chipRequires calibration of subset of TLD chips for each measurementTLD reader response and oven temperature should me routinely monitored to maintain consistent TLD responseAutomatic reader recommended for IMRT field verification due to large number of TLDsrequired for verification in a plane (60 or more) – inefficient for routine IMRT QA


IMRT QA Tools

D.A. Low et al. “Phantoms for IMRT Dose Distribution Measurementand Treatment Verification, Int J Radiat Oncol Biol Phys 40: 1231-1235 (1998).

TLDs, cont.


IMRT QA ToolsMOSFET systems• Advantages

Excellent spatial resolution – small size (~0.04mm2)Multiple detectors can be irradiated simultaneouslyAutomatic and immediate readoutCan be re-used immediatelyLinear dose response > 30 cGy Response independent of depthCommercially available phantoms to accommodate the small detectors


IMRT QA Tools

MOSFET systems• Disadvantages

Decrease linearity for < 30 cGy – limited to high dose applicationsOver-response for the phantom scatter factor for small fields Specific application and measurement conditions should be carefully assessed and the detector should be used in the appropriate dose range


IMRT QA ToolsMOSFET systems

Reader

Bias Box

MOSFET

TNRD50 system

Courtesy of Cynthia Chuang, UCSFAn axial image of MOSFET

phantom


0

200

400

600

800

1000

1200

1400

0 100 200 300 400

MOSFET Linearity

MOSFET1MOSFET2MOSFET3MOSFET4

Radiation (cGy)

-4.0

-3.0

-2.0

-1.0

0.0

1.0

2.0

3.0

4.0

0 2 4 6 8 10 12 14 16 18 20Number of Measurements

MOSFET1 MOSFET2 MOSFET3

Mosfet Consistency

IMRT QA Tools

Courtesy of Cynthia Chuang, UCSF

MOSFET systems


0

0.2

0.4

0.6

0.8

1

1.2

0 5 10 15 20 25 30 35

Percent Depth Dose Comparison

Ion ChamberMOSFET

Per

cent

age

(%)

Depth (cm)

Angular Dependence

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

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1.0

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0 20 40 60 80 100 120 140 160 180

Degrees

IMRT QA Tools


MOSFET systems


Cal. 1.64 GyMeas. 1.72 GyDiff 4.6 %

Cal. 0.70 GyMeas. 0.68 GyDiff - 2.8 %

IMRT QA Tools

Calc. 2.18 GyMeas. 2.09 GyDiff –4.35%



Courtesy of

Cynthia Chuang, UCSF


Current IMRT QA Tools

2-D Detectors• Film

RadiographicRadiochromic

• Beam imaging system, CCD, SLIC, AMFPI• 2-D Detector arrays

Diode array (Mapcheck) Ion chamber

• Active matrix flat panel detector (AMFPD)


IMRT QA ToolsRadiographic Film• Advantages

Readily available (XV, EDR2, …)Can be cut into any desired shapeExcellent spatial resolution (<1mm)Less expensive than other 2-D systems


IMRT QA ToolsRadiographic Film, cont.• Disadvantages

Over-response to low energy x-rays – high atomic number of the active material – not good for absolute dosimetryDependent on QA of film batchDependent on processor and digitizerSensitive to storage conditionsNeed to measure the response to dose for each experiment – H&D curve each timeProper normalization is critical


Current IMRT QA ToolsRadiographic Film, cont.• Other issues

Store in a cool and dry placeMake sure that the temperature for the film processor is stableFilm digitizer pixel spacing, integrity of OD, beware of artifactsVerify spatial and optical density accuracy


Rapid Film Calibration120MU

240MU

90MU

180MU

150MU

210MU

60MU

30MU

Childress et al Med Phys 29(10), 2002.

IMRT QA Tools• Multiple dose levels per

film-3x3 cm2 fields of different dose levels

• Step-and-shoot or SMLC delivery

• Different dose values required for XV and EDR2 film (15 -120MU for XV and 30-240MU for EDR2)

• Saves both time and film


XV vs. EDR Film

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0.0 100.0 200.0 300.0 400.0 500.0 600.0

XV2, 6 MV

XV2, 15 MV

EDR2, 6 MV

EDR2, 15 MVNet

Opt

ical

Den

sity

Dose (cGy)Chetty and Charland 2002

PMB 47: 3629-3641Dogan et al. 2002 PMB 47: 4121-4130

EDR

XV

IMRT QA Tools

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 50 100 150 200 250 300 350

Dose (cGy)O

ptic

al D

ensi

ty

Co60-EDR2 6MV-EDR2

10MV-EDR2 18MV-EDR2

Depth-corrected H&D


Dogan et al. 2002 PMB 47: 4121-4130

EDR

XV

IMRT QA Tools

Depth-corrected H&D curves

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 50 100 150 200 250 300 350Dose (cGy)

Opt

ical

Den

sity

6MV-EDR2-Depth corrected6MV-EDR2-Regular

0

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0 50 100 150 200 250 300 350

Dose (cGy)

Opt

ical

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sity

18MV-EDR2-Depth corrected18MV-EDR2-Regular


Ion Chamber

Film- depth corrected H&D

Film regular H&D

(a) (b)Ion chamber and EDR2 film depth-dose curves for a) 6 x 6 cm2, b) 14 x 14 cm2 films for 10 MV beam. Films were positioned parallel to the beam and OD to dose conversion was done using regular and depth-corrected H&D curves.

IMRT QA Tools

Dogan et al. 2002 PMB 47: 4121-4130


IMRT QA Tools

Childress et al. Med. Phys. 32(2) 2005


IMRT QA ToolsAs compared to XV film, EDR2 film• has less dependence on the

processor, field size• less response to low energy photons• have better reproducibility and

agreement with ion chamber measurements

• can be used to measure a complete fraction of an IMRT treatment


Radiographic Film: 2-D Dosimetric Measurements

Intensity mapfrom Opt System

Calculated

LeafSequencer

Measured

Calc-Meas

Courtesy of Jean Moran, UM

IMRT QA Tools


DMLC field 14x14 cm2

at SSD =100 cm, 2 cm separated strips

• Using radiographic filmsIntensity-modulated pattern fieldCheck leaf position, acceleration, motion stability Check for hot and cold density Visual check

IMRT QA ToolsRadiographic Film: Routine DMLC QA


IMRT QA ToolsFilm – Processor issues• Should do routine maintenance and quality

assurance – verify spatial intensity, characteristic response, noise due to large changes in optical density ( Dempsey et al, Med Phys, 26; 1721-1731, 1999).

• Should be warmed up prior to use• Should have appropriate amount of chemicals

- Several films should be run in advance• Should have stable temperature• Should have a consistent rate of feeding into

the processor


IMRT QA ToolsFilm – Other Issues• Accurate positioning of the film in the

phantom – for the registration with treatment planning system

• Minimized errors by using a solid-water slab designed for film

• Have pins between slabs that puncture the film


IMRT QA ToolsRadiochromic Film (RCF)• Advantages

No significant energy dependence –decreased sensitivity to low-energy photonsInsensitivity to visible lightVery high spatial resolution - well-suited for measurements in high-dose gradient fieldsSelf-developing – no developer or fixer is requiredEasy to handleTissue equivalent


IMRT QA ToolsRadiochromic Film, cont.• Disadvantages

Takes a couple of hours for the color change to stabilize, and it may be necessary to wait up to two days before evaluating the filmSensitive to the air temperature and humidityUltraviolet light may cause a color change without exposure to ionizing radiationSize, availability, and costNon-uniform response to radiation – double exposure technique minimizes this effectIssues with thermal history, wavelength dependence, and local sensitivity of the film


IMRT QA ToolsRCF (Gafchromic HS and MD55-2) vs radiographic films (XV and EDR2)

O. Zeidan et al., Med. Phys., 31 (10):2730-2737 (2004)


IMRT QA ToolsRCF profiles vs. Ion chamber

J. Dempsey et al., Med. Phys., 27 (10):2462-2475 (2000)


IMRT QA ToolsRCF – digitizer issues• Response of the digitizer

• Light source characteristics• DesignGluckman et al, Med Phys, 29(8); 1839-1846, 2002.


Other 2-D systems• Beam imaging system, CCD, SLIC,

Amorphous silicon flat panel detector (AMFPD)

EPID systems attached to gantryInvestigated more for pre-treatment QA currently

• 2-D Detector arraysDiode array (e.g; MapCheck) Ion chamber (e.g; LA48 linear array)

IMRT QA Tools


EPID Systems• Charged coupled device (CCD) camera systems• Scanning liquid ion chambers (SLICs)• Amorphous silicon flat panel detector (AMFPD)• Active matrix flat panel imagers (AMFPIs)

Transit Dosimetry

Patient orPhantom

Pre-Tx 2-D Measurements

FilmReplacement

IMRT QA Tools


EPID Systems• aS500 EPID

1 mm copper platePhosphor scintillating layer (Kodak Lanex Fast B –Gd2O2S:Tb, 70 mg/cm3)Array of photodiodesAmorphous Silicon panel each pixel consists of:

Light sensitive photodiode Thin film transistor

16-bit ADC

Munro et. al, Med. Phys. 25, 1998

IMRT QA Tools


EPIDsAdvantages• Many centers have installed EPIDs and being

primarily used for patient-specific pretreatment field verification and MLC QA

Logical extension to investigate dosimetric applications• Mounted to linear accelerator - known

geometry with respect to the beamDetector sag must be accounted for at different gantry anglesPositioning reproducibility important

• Real time digital evaluationNo processor, data acquisition takes less time

IMRT QA Tools


• EPIDs were primarily designed for patient localization

High resolution, good contrast imagesAdditional dose to the patient should be minimized

• The conversion of imager response to dose is complex

Imaging system dependent

• Other problemsGhostingLag

IMRT QA ToolsEPIDs - Challenges


• Imager response must be calibrated to a standard

• Absolute calibration to ion chamber at a point over a ROI

E.g. ion chamber in a mini-phantom or slab at same SDD as EPID

• 2-D calibration to actual beam distribution at the imager plane

Can be measured with film or a diode array

IMRT QA ToolsEPIDs – Dose determination


Factors for EPID Response• Water-equivalent depth of the

detector• Field size dependence and scatter

properties within the imager• Short- and long-term reproducibility• Dose rate• Energy dependence• Spatial integrity

IMRT QA Tools


EPID: DMLC measurements

Pasma Med Phys 26: 2373-2378 (2376) 1999

PredictedEPIDIon Chamber+

Discrepancies in the penumbra region (up to 10%)

Overall: Good agreement

10 MV 25 MV

IMRT QA Tools


Withoutshort rangepenumbra correction

Linear Diode Array in water vs. CCD


IMRT QA Tools


Dose Determination using EPID (SLIC)

Chang et al., Int J Radiat Oncol Phys 47: 231-240 (p. 233)

IMRT QA Tools


Agreement : Within +/- 2 cGy

Calculated (Calculated – Measured)


IMRT QA ToolsCalculation vs. measured using AMFPD for DMLC


• EPIDs can provide a much-needed replacement for pre-tx QA film dosimetry

Only if proper QA of the EPID is establishedNeed better understanding of regions where EPIDsare inadequate for dosimetrySystems must be verified at more centers against accepted QA methods such as film and ion chamberAdditional software is required before more facilities can do proper validation of the methods (Software must be commissioned)Can be part of a comprehensive QA program in conjunction with other methods such as computational checks (monitor programs, log file analysis, etc.)

IMRT QA Tools


IMRT QA ToolsGel Dosimeters

• Advantages3-D information in one irradiationEnergy and dose-rate independentHigh sensitivity and linear responseCumulativeGel density can be changed - Ideal for anthropomorphic phantomsNear tissue equivalentMultiple readout techniques (MR, optical-CT)New gel formulations and readers commercially available



• DisadvantagesSensitive to time, preparation, temperatureCylindrical container required for optical readers - less accurate readout at gel/container interfaceMR time is often limited and expensive - long scan times for accurate readout, e.g. 5% accuracy over 10 hr scan time (Gum et al. 2002)Relative dosimeter -require cross-calibration technique – batch to batch they are differentCost


• In-house optical CT scanner – cost is less

• Oldham and Kim, Med. Phys. 31 (5), 1093-1104.

• Upgraded motors, motion control, and user interface. (Pacific Scientific: step motors. National Instruments: motion control and Labview.)

IMRT QA ToolsGel dosimetry

8cm


Gels: Optical Density to Dose Calibration

• 6 Beam calibration irradiation• BANG gel phantom diameter 17.4cm

IMRT QA Tools

Courtesy of Mark Oldham, Duke University


• Five Field Prostate IMRT

• Re-computed for a 3 L BANG gel dosimeter.

•Dmax scaled to 1.8 Gy to fit dynamic range of optical scanner

• BANGkitTM from MGS Research. Optical-CT @ 1x1x3mm, 5hours

Courtesy of Mark Oldham, Duke University



Isodose comparison: Pinnacle (red), Gel-dosimetry (black)Courtesy of Mark Oldham, Duke University




Gum, et al. “Preliminary study on the use of an inhomogeneous anthropomorphic Fricke gel phantom and 3D magnetic resonance dosimetry for verification of IMRT plans ,” Phys Med Biol 47; N67-77 2002.


Phantoms for IMRT Measurements

• multiple phantoms for commissioning • Fiducials for reproducible setup of

phantom and detectors• User-customized for different detectors –

allow special holders• Simple vs. anthropomorphic• Homogeneous or heterogeneous

IMRT QA Tools


• Water tankAccommodate different ion chambersUse for measurements of depth dose and profilesOutput, flatness, symmetry, and linearity assessment

• Cylindrical mini-phantomUse with ion chamber to assess dependence of output on gantry angle

• Water-equivalent plastics: slab w/ custom chamber inserts

1-D and 2-D measurementsDetector position can be varied with depth

• Cylindrical phantoms (plastic or water filled)Straightforward geometryIon chamber at single positionPlastic phantoms may hold films

Current IMRT QA ToolsSimple Geometric Phantoms


Water-equivalent square IMRT Verification Phantom

D.A. Low et al. “Phantoms for IMRT Dose Distribution Measurement and Treatment Verification, Int J Radiat Oncol Biol Phys 40: 1231-1235 (1998).

IMRT QA Tools


A Cylindrical Phantom containing movable ion chamber

Current IMRT QA Tools

L. Xing et al. “Dosimetric verification of a commercial inverse treatment planning system, Phys. Med. Biol. 44: 463-478 (1998).


A cylindrical Plastic PhantomDetector

IMRT QA Tools





Plastic Cylindrical Phantom with MOSFETs

IMRT QA Tools


Spiral Phantom

Paliwal et al “A spiral phantom for IMRT and tomotherapy treatment delivery verification” Med Phys (2000).

IMRT QA Tools


Anthropomorphic: RPC Head Phantom

Target Volumes

Critical Structure

TLDs in Target VolumesRadiochromic film through multiple plansDelivery is required by RTOG for participation in IMRT trials

Removable DryInsert

WaterWater

IMRT QA Tools



Dosimetric Analysis Tools

• Provide a comprehensive and quantitative comparison between two dose distributions

• Different ones available• Important to know the limitations

IMRT QA Tools


Dosimetric Analysis Tools

• Overlay of isodoses• 2-D dose difference displays with

colorwash• Dose difference histograms• Distance-to-agreement (DTA)• Gamma evaluation• Normalized agreement test (NAT)

IMRT QA Tools


Isodose lines and Dose Difference Display

IMRT QA Tools

+/- 10%CalcsFilm

70 cGy60 cGy50 cGy20 cGy10 cGy



Dose difference display

• Useful in shallow dose gradients• Overly sensitive in steep dose gradients

– e.g.; a small spatial shift (due to experimental measurement errors) between two dose distributions yield large dose differences

IMRT QA Tools


IMRT QA ToolsDose difference histogram and profiles


Distance to Agreement (DTA)• Is the distance between a reference point

and the nearest point in the compared dose distribution that exhibits the same dose

• Is not overly sensitive in steep dose gradients

• In shallow dose gradients, a large DTA value may be computed even for relatively small dose differences

• May be hard to interpret

IMRT QA Tools


Combination of dose difference and DTA• Identify regions where the dose difference

and DTA are simultaneously by greater than a pre-selected criteria – points that fail both criteria are identified on a composite distribution

• The display of the dose difference may emphasize the impression of failure in high dose gradient region

• Provides no information on the magnitude of the failure

IMRT QA Tools


IMRT QA ToolsGamma Analysis- Generalization of composite distribution

• Measures the closest distance between each reference point and evaluated dose distribution after scaling by ∆D and ∆d

spatial distance between evaluated and reference dosepoints

∆D : Dose difference criteria∆d : DTA

• The point with the smallest deviation from reference point is a quantitative measure of the accuracy of the correspondence -> the quality index, γ (rr) of the reference pointγ (rr) ≤ : 1 ->correspondence is within the specified acceptance criteria

{ } { }2 2

r2 2

( , ) ( , )( , ) ( r ) m in ( , )e r e re r e r e

r r r r rr r r r rd D

δ γΓ = + = Γ ∀∆ ∆

( , ):e rr r r

Low et al, Med Phys 30(9) 2455-64 (2003).


IMRT QA ToolsDose Difference and DTA

Dose Difference and DTA Analysis Summary

Dose Diff and DTA criteria : 2% ofDmax and 2mm

Points Checked = 5348Points Passed DTA = 5312Points Passed DD = 4363Points Passed Either = 5343Points Passed Both = 4332

99.3269 % of the points passed DTA

81.5819 % of the points passed DoseDiff

99.9065 % of the points passed eitherEither

81.0022 % of the points passed Both

Dose Difference Statistics Summary

Mean Dose Diff = 0.488805 0.877915

DTA SummaryMean DTA = 0.0477486 0.0747123


IMRT QA ToolsGamma Analysis

Gamma Analysis Summary

Dose Diff and DTA criteria : 2% ofDmax and 2mmPoints Checked = 5348 Points Passed = 5348 100 % of the points passed Gamma

Gamma Statistics SummaryGammaBar = 0.0406743 0.0620086

Dose Diff and DTA criteria : 3% ofDmax and 3mmPoints Checked = 5348 Points Passed = 5348 100 % of the points passed Gamma

Gamma Statistics SummaryGammaBar = 0.0271162 0.0413391


Normalized Agreement Test (NAT)IMRT QA Tools

( )1( )( )

sca le

indexsca le

N AT DAve NATNATAve D

δ= × −

=

• Is based on a 2D array of calculated image of NAT values derived from comparisons of measured and computed doses.

• Assumes that two dose distribution images are registered each other and NAT is calculated using

δ : lesser of Abs(∆D/ ∆Dm) or∆d/ ∆dm

Dscale: Di /Dmax

• NATindex represents the average deviation from the ∆Dm and ∆dm criteria for every dose pixel, ignoring the ones less than the set criteria

N. Childress et al, “The design and testing of noval clinical parameters for dose comparison,” Int J. rad. Oncol Biol Phys 56(5) 1464-1479 (2003).


IMRT QA ToolsNAT Index

N. Childress et al, “The design and testing of noval clinical parameters for dose comparison,” Int J. rad. Oncol Biol Phys 56(5) 1464-1479 (2003).


Other Analysis Tools• MU check software

In-house dose calcCommercial packages (e.g; Radcalc)Monte Carlo (e.g; Peregrine, EGS4, …) –Patient QA

• Software for Post-treatment QA Analysis of IMRT delivery log files (e.g; in-house analysis software, Argus IMRT QA package)

IMRT QA Tools


∆=10%Superposition Monte Carlo

IMRT QA ToolsMC verification


Summary• Multiple detectors and phantoms are

typically required for IMRT QA• Quantitative dose analysis tools are

necessary for proper evaluation of delivery - identify the cause of discrepancies between delivery and measurements

• Treatment planning vendors are starting to provide dosimetric evaluation tools

• Aware of the limitations of each tool


Summary

• Verify that all equipment is functioning properly

Film processor, digitizerDetectors, cables, electrometers (automatic leakage correction)TLD reader, ovens

• Input/output to treatment planning system• Standardize measurement setup when

possible• Monitor software and hardware changes

and QA


Summary• Measurements may show dosimetric

differences that planning systems may not model at this time – curved leaf ends

• Need to know the limits of the mechanical systems and interactions with controller and accelerator software for delivery

• Continued need for improvements to software for delivery system, measurement devices, phantoms, and dose analysis tools


Acknowledgements

Jean Moran – U of MichiganCynthia Chuang – UCSFMark Oldham – Duke University

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