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The Department of Radiation OncologyUniversity of MichiganThe Department of Radiation OncologyUniversity of Michigan

Dosimetry Metrology for IMRT

Jean M. Moran, Ph.D.Jean M. Moran, Ph.D.

AAPM Summer SchoolAAPM Summer SchoolJune 2003June 2003

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Outline

• Acceptance testing• Detectors for commissioning• Phantoms• Dosimetry analysis tools• Commissioning tests

– Varying complexity and geometry• Potential Pitfalls• Summary

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Acceptance

• Prior to purchase and installation, review manufacturer’s acceptance tests

• If needed, adapt tests and tolerances with manufacturer in purchase order

• Test basic functionality of equipment• Tests may be dependent on the MLC

design

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Collimator Designs

Elekta

Lower Jaws

Exit Window

Isocenter

Diaphragm

MLCUpper Jaws

Exit Window

Isocenter

Siemens

MLCLower Jaws

Upper Jaws

Exit Window

Isocenter

Varian

MLC

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Physical MLC Characteristics

No

31 cm

1 and 6.5 cm

Divergent

40x40 cm2

(40x27 cm2)

Siemens

No

=1 cm gap

32.5 cm

1 cm

Rounded

40x40 cm2

Elekta

YesInter -

digitation

Length

Leaf width

Leaf ends

Field

Width

16 cm (14.5 cm Carriage)

0.5 and 1 cm

Rounded

40x40 cm2

Varian

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Effect of Rounded Leaf Ends & Linear Motion

Uncorrected Corrected

Graves 2001

Cold

Hot

UniformCorrection

Xmlc

Xlight

Linear leaf motion

Leaf position offset = Xrad-Xlight

Xrad

2

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Static MLC Testing

• Carriage skew– Orientation of MLC carriages in the accelerator

head – Determined by measurement with feeler

gauges for very small field size– Important to prevent collisions especially for

systems with interdigitation of leaves

– Verify with film

• Alignment of each leaf bank to central axis• Leaf position reproducibility and accuracy

– Verify with graph paper

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MLC Transmission

• Dependent on system design

• Values range from 1.5 to 3% for midleafand interleaf transmission

Film exposed under closed leavesEvaluate compared to open field

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Tongue and Groove Effect

Caused by overlap of leaves and leaf design

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Uniform Square Field Illustration of TG Effect

Area in Red Never Directly Exposed

+

?

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Cold Spots from T&G Effect

~25% cold spot where leaves overlap

0

1 0

2 0

3 0

4 0

5 0

6 0

7 0

-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5

Position (cm)

Dos

e (c

Gy)

square fieldsmall leaves

large leaves

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DMLC and/or SMLC Mode Tests

• Leaf speed• Dose rate evaluation• Leaf position tolerance and

reproducibility• Leaf acceleration and deceleration• Rounded leaf tip transmission• Beam stability (output, flatness,

symmetry, linearity)• Interrupted treatments

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Example Test: FenceTest

• All leaves move across field and deliver dose to small field gap (0.1 or 0.2 cm)

• Sensitive to errors of 1 mm in leaf position

Multiple incorrect leaf positions

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Equipment and Software for IMRT Commissioning

• Ideal measurement system:– Excellent spatial resolution, accuracy, tissue

equivalent response, provide 3-D data, portable to multiple phantoms, and easy to use

• Multiple detectors required– Verifying IMRT fields is more complex than

static fields– Profiles along major axes in a water phantom

are insufficient for characterizing systems– Profiles are also time-consuming to obtain in

water phantom for IMRT delivery

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Equipment

• Depth dose and profiles in water– Ion chamber – required for absolute dose

measurements– Ion chamber or linear diode array– Note: Depth dose curve measurements must

be made with a repeat delivery for each datapoint on the curve (delivery stability)

• Detector arrays are useful for profile measurements in water but 2-D detectors are still required

• 2-D measurement methods– Film– Detector arrays

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Detector Characterization

• Linearity

• Energy dependence• Stem and cable effects

• Angular response• Calibrated if for absolute

measurements• Small field detectors

required for small field characterization– Sensitive to position– Detector should be smaller

than homogeneous region of dose to be measured

Dose (cGy)

70605040302010

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1-D Detector Characteristics

0.10.0260.730.056/0.0730.0019Diamond

NA0.1NA0.04NAMOSFET

0.06NA0.20.0100.015Pinpoint chamber

0.070.0060.450.011NAStereotactic diode

0.60.060.40.490.3p-type Si diode

0.2NA0.60.240.009Micro -chamber

Effective Point ofMeasurement (cm)

Thickness(cm)

Diameter(cm)

SensitiveArea(cm2 )

MeasurementVolume (cm 3)

Detector

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1-D Detectors

< resolution than diodes, expensive, Rustgi et al, Laub et al Diamond

Non-linear dose response for <30 cGyChuang et al 2002MOSFET

Pinpoint chamber

Stereotactic diode

Over-respond to low energy photonsMartens et al. 2000

p-type Si diode

Poorer resolution than diodesMicro -chamber

DISADVANTAGESDETECTOR

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Reader

Bias Box

MOSFET

C. Chuang, UCSF

MOSFET System

• Excellent spatial resolution

• Automatic and immediate readout

• Can be re-used immediately

• Linear dose response

• Response independent of depth

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-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 20N u m b e r o f M e a s u r e m e n t s

MOSFET1 MOSFET2 MOSFET3

MOSFET Consistency

Cynthia Chuang, UCSF

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TLDs

• TLDs must be characterized– Time consuming

• Reusable• Achievable accuracy: 2-3%• Automatic reader required for multiple

TLD measurements

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TLD Holder for Phantom Measurements

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

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2-D Detectors

• Film– Radiographic: XV and EDR– Radiochromic

• Beam imaging system, CCD, SLIC, AMFPI– EPID systems attached to gantry– Investigated more for pre-treatment QA

currently

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Radiographic Film

• Advantages– Excellent spatial resolution

– Readily available– Less expensive than other 2-D systems

• Disadvantages– Over-response to low energies– Dependent on QA of film batch

– Dependent on processor and digitizer QA– Sensitive to storage conditions

– Need to measure the response to dose for each experiment

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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 “Investigation of Kodak EDR film for megavoltage photon beam dosimetry” PMB 47: 3629-3641 (2002). JMM03 26

Depth Dependence of EDR Film

0

200

400

600

800

1000

1200

1400

0 50 100 150 200 250 300 350

Dose (cGy)

Opt

ical

Den

sity

6MV-EDR2-Depth corrected6MV-EDR2-Regular

Dogan et al “Comparative evaluation of Kodak EDR2 and XV2 films for verification of intensity modulated radiation therapy,” PMB 47: 4121-4130 (2002).

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Radiochromic Film: Advantages

• Decreased sensitivity to low energy photons compared to radiographic film

• No processing– Film changes color with irradiation

• Insensitivity to visible light• High spatial resolution

Niroomand-Rad et al. “Radiochromic film dosimetry: recommendations ofAAPM Radiation Therapy Committee Task Group 55,” Med Phys 25: 2093-2115.

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Radiochromic Film: Disadvantages

• Non-uniform film response– Can be minimized by using double-exposure

technique

• Response dependent on time and temperature

• More expensive than radiographic film• Digitizer response is dependent on the

light source of the digitizer and may need to be modified

Niroomand-Rad et al. “Radiochromic film dosimetry: recommendations ofAAPM Radiation Therapy Committee Task Group 55,” Med Phys 25: 2093-2115.

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Active Matrix Flat Panel Dosimeter(AMFPD)

512 x 512 pixels

0.508 mm pitch

26.0 x 26.0 cm2

El-Mohri, et al. “Relative dosimetry using active matrix flat-panel imager (AMFPI) technology,” Medical Physics 26, 1999.

Preamplifiers AD converters

a-Si array

Direct Detection-No fluorescent screen

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Example: SMLC Delivery

AMFPD Film

184 mu600 mu/mind=10 cm

Non-commercial systemAMPFD at 10 cm depth in solid water

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Example: SMLC Delivery

B4

-4

-2

0

2

4

Pos

ition

(cm

)

-4 -2 0 2 4

Position (cm)

150

130 120

80 70 100 50 30 20

16012080400

184 mu600 mu/mind=10 cm

FilmAMFPD

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Gel Dosimetry: Advantages

• Obtain 3-D information in one irradiation

• Gels can be prepared with different density therefore ideal for heterogeneous measurements

• Gel can be used in containers of different shapes

• Ideal for anthropomorphic phantoms

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Gel Dosimetry: Disadvantages

• Sensitive to many factors such as time, preparation, temperature

• Optical reader requires cylindrical container for gel

• MR time is often limited and expensive (unless dedicated scanner)– Long scan times are required to increase

accuracy of readout– E.g. 5% accuracy over 10 hr scan time (Gum et al

2002)

• Interface of gel and container results in less accurate readout at edges of the gel

• Not ready for routine use in the communityJMM03 34

Gel Dosimetry

Gum et al. “Preliminary study on the use of an inhomogeneous anthropomorphic Fricke gel phantom and 3D magnetic resonance dosimetry for verification of IMRT treatment plans” PMB 47: N67-N77 (2002).

Slice 1

Slice 2

Slice 3

MR Dose Maps IMRT Plan MR Dose Maps- IMRT Plan

Dose % Dose Deviation %0 20 40 60 80 100 120 -60 -40 -20 0 30 40 60

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Considerations for Phantoms

• Fiducials for reproducible setup of phantom and detectors

• Flexibility to accommodate different detectors

• Simple vs. anthropomorphic• Homogeneous or heterogeneous

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Water Tank

• Restricted to gantry at 0 degrees– Unless mylar window for 90 degrees

• Flexibility in chamber position• Important for basic depth dose and

profile measurements• Output, flatness, symmetry, and

linearity assessment

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Water-equivalent Plastics

• Flat phantoms with custom chamber inserts– Accommodate film at multiple depths– Detector position only varies with depth

• Cylindrical phantoms (plastic or water filled)– Ion chamber at single position– Possibly hold films

• Can be customized to hold various detectors and have multiple positions

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Simple Geometric Phantom

20 cm

Van Esch et al. “Acceptance tests and QC procedures for the clinical implementation of IMRT using inverse planning and the sliding window technique: experience from five radiotherapy departments,” Radiother Oncol 65: 53- 70 (2002).

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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).

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Calc. 1.37 GyMeas. 1.42 GyDiff –3.52%

Calc. 0.81 GyMeas. 0.78 GyDiff –3.45%

C. Chuang, UCSF

Cylindrical Phantom:Ion Chamber and MOSFETs

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Spiral Phantom for Dosimetric Verification

Paliwal et al “A spiral phantom for IMRT and tomotherapy treatment delivery verification” Medical Physics pp. 2503-2507 (2000).

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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

Water

Water

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Anthropomorphic Phantom for Gel Dosimetry

Gum et al. “Preliminary study on the use of an inhomogeneous anthropomorphic Fricke gel phantom and 3D magnetic resonance dosimetry for verification of IMRT treatment plans” PMB 47: N67-N77 (2002).

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Dosimetry Analysis Software

• Transfer patient fluence maps and beam geometry to phantom geometry

• 2-D dose difference displays with colorwash

• DVHs• Highlight of differences• Gamma analysis

– % agreement and distance-to-agreement

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Phantom Measurements

Film at depth in water equivalent plastic

• Calculate IMRT plan for each field in measurement geometry

• Film measurement at depth for individual field at gantry=0

CalcsFilm

70 cGy60 cGy50 cGy20 cGy10 cGy

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Dosimetric Analysis: Overlay of Isodose Lines

Intensity Map

70 cGy60 cGy50 cGy20 cGy10 cGy

CalcsFilm

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Dosimetric Analysis : Dose Color Wash

CalculationIndividual Field

Film DataIndividual Field

Dose (cGy)

70605040302010

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Dosimetric Analysis: Dose Difference Display

Intensity Map +/- 10%

Differences due to tongue-and-groove

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Dosimetric Analysis: Dose Difference and DTA

D. A. Low – Washington University

ReferenceEvaluated

3% DoseDifference

3 mmDTA

Dose

Spatial

BothUnmod

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Dosimetric Analysis: Gamma Evaluation

3%/3 mm 5%/5 mm

D. A. Low – Washington University

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Simple Geometry Tests

• Leaf position reproducibility in dynamic or step-and-shoot mode

• Effect of gravity on leaf position accuracy and reproducibility

• Sweeping gap test• Fence test

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Simple Geometry Tests

• Leaf speed stability• Leaf acceleration/deceleration• Output checks for small to large fields

– Including smallest field size – 1 x 1 cm 2

• MLC limits (field size restrictions)• Depth dose curve measurements

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Simple Geometric Tests

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Geometric Tests

L. Xing et al. “Dosimetric verification of a commercial inverse treatment planning system” PMB 44: 463-478 (1999).

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Geometric Tests

Acceptance tests and quality control (QC) procedures for the clinical implementation of intensity modulated radiotherapy (IMRT) using inverse planning and the sliding window technique: experience from five radiotherapy departments Van Esch et al Radiotherapy Oncology 65: 53-70 (2002). JMM03 56

Complex Cases – Simple Geometry

Evaluate dose across field as a function of regional beamlet intensity

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Complex Geometry: Anthropomorphic phantom

M. A. MacKenzie et al. “Dosimetric verification of inverse planned step andshoot MLC fields from a commercial planning system,” J Appl Clin Med Phys 3: 97-109 (2002)

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Potential Pitfalls

• Limit number of monitor units per segment to verified Linac behavior

• Limit field width for IMRT• Overshoot-undershoot phenomenon• Incorrect tolerance value • Integration with record-and-verify

system

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Potential Pitfalls: Prior to Measurements

• Verify all equipment is functioning properly– Film processor, digitizer– Detectors, 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

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Detector Issues

Dose (cGy)

Vol

ume

11

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Detector Position

• Small ion chambers are very sensitive to position

• Position should be considered with respect to MLC design– Example: CAX of Varian MLC is a junction

of four 0.5 cm wide leaves

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Ratio Calc/Ion Chamber - CAX

0.90

0.95

1.00

1.05

1.10

1.15

1.20

regular spike=cax spike=high spike=low

Spikiness values on part of field

Rat

io

(Cal

cula

tion/

Mea

sure

men

t)

Beam 1 Ratio 9/21/02

Beam 1 Ratio - 9/25/02 -20 segBeam 1Ratio 9/25/02 -100 segBeam 2 Ratio 9/21/02

Beam 2 Ratio 9/25/02 -20 segBeam 2 Ratio 9/25/02-100 seg

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Ratio Calc/Ion Chamber --0.5, -0.5 cm from CAX

SMLC Single Field 30 Fractions

0.85

0.90

0.95

1.00

1.05

1.10

1.15

1.20

regular spike=cax spike=high spike=lowSpikiness values on part of field

Rat

io (C

alcu

lati

on

/Mea

sure

men

t)

Beam 1 Ratio - Off-axis - 20 segBeam 1 Ratio off-axis - 100 segBeam 2 Ratio Off-axis - 20 segBeam 2 Ratio Off-axis-100 seg

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Effect of Leaf Position Offset on IMRT

Cadman et al “Dosimetric considerations for validation of a sequential IMRTProcess with a commercial treatment planning system” PMB: 3001-3010 (2002).

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Cadman et al “Dosimetric considerations for validation of a sequential IMRTProcess with a commercial treatment planning system” PMB: 3001-3010 (2002).

Effect of Leaf Position Offset on IMRT

No leaf offset correction-3-12% errors

With leaf offset corrected+/- 5%

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Overshoot Phenomenon:Example Varian System SMLC Delivery

Planned

Given

Planned

Given

Planned

Given

Ezzell and Chungbin, “The overshoot phenomenon in step -and-shoot IMRTDelivery,” J Appl Clin Med Phys 2: 138-148 (2001).

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System Communication Delay

AcceleratorControl MLC

Controller

AcceleratorControl Console

MLCWorkstation

~ 0.055 secondJMM03 68

MLC Dynamic Log File (Dynalog)

• Varian 21 EX• Expected and actual leaf positions• Beam hold-off events• Position tolerance setting • Recorded approximately every 55 ms

• Information can be compared to imported 2-D information

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Deviation in Leaf Trajectory

-5.0

-4.5

-4.0

-3.5

-3.0

Po

siti

on

(cm

)

1.41.21.00.80.60.40.20.0

Time (sec)

1.0

0.0Hold

Expected Trajectory Actual Trajectory MLC Beam Hold-Off

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DMLC: Effect of Incorporating Machine Limitations

44 mu, tolerance 0.1 mmno gap: 137 beam hold-offs

BEFORE AFTER

54 mu, tolerance 0.25 mmgap 1.1 mm: No beam hold-offs

Case1, B1

B1

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Summary

• Basic MLC characteristics should be tested in static mode prior to IMRT testing

• IMRT tests should be specific to delivery mode and device

• Be aware of potential issues with delivery systems that may need further investigation

• Multiple detectors and phantoms are typically required for IMRT commissioning

• Quantitative dose analysis tools are required for proper evaluation of delivery

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Summary

• Measurements may show dosimetric –mechanical differences that planning systems may not model at this time

• Need to know the limits of the mechanical systems and combination of software + delivery

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

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Acknowledgements

University of MichiganDale LitzenbergJeff RadawskiTim NurushevBenedick Fraass

Dan Low – Washington UniversityCynthia Chuang - UCSF