Patient Specific Quality Assurance

for IMRT and VMATbased on Radiochromic Film Dosimetry

with EBT2/EBT3 FilmSou-Tung Chiu-Tsao, PhD

Quality MediPhys LLC, Denville, NJ

Presentation at the New Jersey Medical Physics SocietyJun. 12, 2012

Disclosure Statement

• Consultant to Ashland Inc. (Formerly ISP)

Background

• Safety and quality of radiation therapy• Machine equipment QA• Complex treatment modalities• Imaging devices• Immobilization/support devices• Skin dose issue

Complex Treatment Modalities

• SRS (Stereotactic Radiation Surgery, single fx)

• SRT (Stereotactic Radiation Therapy, multiple fx)

• SBRT (Stereotactic Body Radiation Therapy)

• Helical Tomotherapy• IMRT (Intensity Modulated Radiation Treatment)

• VMAT (Volumetric Modulated Arc Therapy)

IMRT

• Fixed gantry angle• Constant output rate• Multileaf collimator (MLC)

leaves move during the treatment

• Non-uniform beam intensity• Technique

– Sliding window– Step and shoot

VMAT

• Arc delivery of IMRT

• MLC leaf motion

• Gantry rotation

• Output rate variation

Webb & McQuaid, PMB 2009; 54: 4345-4360.

VMAT/IMRT Prostate

• Evolving rapidly• MU ~ 300• Less treatment time• Output rate varies with angle• Rotating gantry

– Single arc, Multiple arc– Partial arc, Split arc

• Established • MU ~ 600• More treatment time• Output rate remains fixed• Fixed gantry angle for each field

IMRTVMAT

30 50 70 90 100 105 %

Patient Specific QA• Consult, diagnostic data gathering• Decision making on

– Prescription dose to target– Tolerance doses to organs at risk (OAR)

• Treatment planning CT scan simulation• Delineation of contours of target and OARs• Treatment planning calculation

• Pre-treatment QA• On-treatment, in vivo monitoring• Post-treatment analysis• Skin dose

Pre-Treatment QA

• 2D dosimetry– Ion chamber array– Diode array: MapCHECK– Radiochromic film: EBT2, EBT3 film– EPID

• 3D, 4D dosimetry– Gel– Delta4 (2 perpendicular diode arrays )– ArcCHECK– 4D Monte Carlo simulation

Planar Dose: Gamma Index

Solid: TPSDotted: EBT2 Film

Low, Med Phys 2003; 30: 2455-2464.

Gamma index:

Distance difference:

Dose difference:

DTA criterion: ∆d

Dose diff criterion: ∆D

Pre-Treatment QA Procedure: EBT2/3 Film

Export viaR&V System

Import Plan viaDICOM

Results

ExposePhantom

FlatbedScanner

cGy

Film

Film Dosimetry

• Radiographic film, Kodak– XTL, XV2, EDR2

• Radiochromic film, Ashland (Formerly ISP)– XR-QA, XR-RV2, EBT, EBT2, EBT3, MDV3, HDV2

• Fine spatial resolution• Darker shade with higher dose

EBT2 and EBT3 Films

EBT2

Contains yellow marker dyeColor: yellow to greenLess sensitive to ambient light

Asymmetric Symmetric

Clear Polyester - 50 mAdhesive Layer - 25 m

Active Layer - 28 m

Clear Polyester - 175 m

Matte Polyester - 120 m Active Layer -28 m

Matte Polyester - 120 m

EBT3

Matte Polyester in EBT3 Film

Common Setup for IMRT QA

Solid Water

Measurementprobe for absolutepoint dose

EDR2 Film EBT2/3 Film

Film for 2Ddose distribution

EBT2/3 Film in Phantom on CouchEBT2/3 Film in Phantom on Couch

EBT2/3 Film in Coronal Plane

IGRT couch

EBT2/3 Film Dosimetry

• Technical Considerations– Note model and lot #– Note film orientation and alignment– Control time between irradiation and scanning– Environmental factors: humidity, temperature– Store in dark envelopes

• Scanner influence, uniformity, and software

Tips for Scanning EBT2/EBT3 Film for IMRT and VMAT QA

• Scan all films in the same orientation Landscape or portrait orientation

• Place the film at the center of the scanner bed• Practical spatial resolution for IMRT/VMAT QA

– 72 dpi, i.e. 0.35 mm per pixel• Use 48 bit color mode, red, green, blue channels• Use transmission mode• Disable color correction feature

Epson 10000XL Flatbed ScannerLandscape orientation of film on the scanner bed

10000XL10000XL

Coating direction

Scanning direction

Transparencyadapter

Scanner bed

Epson 10000XL Flatbed ScannerPortrait orientation of film on the scanner bed

Scanning direction

Coating direction

10000XL

EBT2/3 Film Image w/48 Bit Color Mode

EBT2/3 film RGB imageconsists of 3 components

Separated into 3 images in red, green and blue channels

Absorption Spectra of EBT2/3 Film

Courtesy: Dr. David Lewis

0.0

0.5

1.0

1.5

2.0

2.5

350 400 450 500 550 600 650 700 750 800

Abso

rban

ce

Wavelength, nm

Before exposure

After exposure of 2 Gy

636 nm

585 nm

420 nm

Absorption Spectra of EBT2/3 Components

0.00.20.40.60.81.01.2

400 500 600 700Abs

orba

nce

Wavelength, nm

Visible Spectrum of Active Component after Exposure

• Active component– Signals in red

and green channels

• Marker dye– Signal in blue

channel

Courtesy: Dr. David Lewis

0

1

2

3

350 400 450 500 550 600 650 700

Ab

sorb

ance

Wavelength, nm

Visible Spectrum of Marker Dye

Before exposure

After exposure to 50Gy

Data Analysis Method

• Single channel method– Red channel only– Green channel only– Blue channel only

• Triple channel method– Red, green and blue channels– Removes film and scanner artifacts– Allows compensation for film thickness variation– Significantly improves the accuracy of dose map

Micke, Lewis and Yu, “Multi-channel film dosimetry with non-uniformity correction”, Med. Physics 37, 2523-2534 (2011).

IMRT/SBRT QA, EBT2 Film

Single field

SBRT patient QAwith EBT2 film(Red Channel data)

Lung CA: 2000 cGy x3

Solid line: FilmDotted line: TPS

D(cGy)

98%3 %3 mm

2 cm

IMRT, H&N Field, EBT3 Film

98%2 %2 mm

Thcik line: TPSThin line: EBT3

SBRT Lung Treatment

• EBT2 film used for patient QA• Small field size• Require fine spatial resolution, which is

NOT achievable with diode or ion chamber array

cGy

OAR

D(cGy)Composite of 3 fields

SBRT patient QAwith EBT2 film(Red channel data)

Lung CA: 2000 cGy x3

Solid line: FilmDotted line: TPS

95%3%3mm

2 cm

Brain SRS QA, EBT2 Film

2 cm

Chan, International Journal of Medical Physics, Clinical Engineering and Radiation Oncology, 2012, 1, 1-7.

Solid line: iPlan, Dotted line: EBT2

97%2 %2 mm

96%2 %2 mm

2 cm

VMAT QA, EBT2 Film

D(cGy)

Solid line: Film. Dotted line: TPSRed channel data

96%3 %3 mm

2 cm

Prostate VMAT QA, EBT2 Film

Ozawa, ESTRO 2009 and 9th AOCMP 2009

VMAT, Double Arc, EBT3 Film Scanned Image, Portrait Orientation

0 Gy1.7 GyApplication FilmReference Films

Scanning direction

Coating direction

VMAT, Double Arc, EBT3 FilmIsodose Curve comparison

Thick line: TPS, Thin line: EBT3 Film

96.4%2 %2 mm

IMRT QA (Single Field)EBT2 vs. TPS MapCHECK vs. TPS

cGycGy

Solid line: Measurement, Dotted line: TPS

DiscrepancyAgreement

Radiochromic EBT/EBT2/EBT3 Film

• Fine spatial resolution (e.g. 0.35 mm)• No angular dependence of film response

van Battum, Med. Phys. 2008; 35: 704-716. Lin, Master’s thesis, Taichung, Taiwan, 2006.

• Suitable for IMRT and VMAT QA• Immediate visualization of color change

• Drawback:Scan film after radiation exposure and then perform quantitative data analysis

THANK YOU.

We have come a long way in radiation therapy.To assure the safety and quality of treatment,More work to be done,Further progress to be made,Future refinement to be achieved.