Radiochromic Film

Radiochromic Film

David F. Lewis, Ph.D.

Senior Science Fellow

Advanced Materials Group

International Specialty Products

[email protected]

October 20, 2010

High Resolution Imaging

• 1984: Could we make a processless film for recording very high resolution images in an electron-beam recorder? – Tri-phenyl methane dyes in PVC

• Photolysis produces HCl and activates a pH-sensitive leuco-dye

• No amplification - slow

– Diacetylenes • Solid-state polymerization

• >>100x amplification - faster

Performance of Photosensitive Systems

Silver halide – fast : 5x10-10J/cm2

Silver halide – slow: 10-8 J/cm2

Xerography: 10-6 J/cm2

Photopolymerization: 10-3 J/cm2

Photochromism: 1 J/cm2

Imaging Systems, Jacobson and Jacobson, Focal Press 1976

Amplification

• One photochemical event effecting >1 molecule

– Silver halide – a whole crystal, 1010 silver atoms, can become

developable after as few as 4 photochemical events

– Photopolymerization – up to thousands of monomer molecules

react to form a polymer

Processless Electron Recording Media 1988

1988 - Monochrome color 1996 – Monochrome B&W

1998 – Polychrome in one active layer

Core Radiochromic Film Technology

Original active component : Pentacosa-10,12-diynoic acid (PCDA)

Current active component is lithium pentacosa-10,12-diynoate (LiPCDA)

The diacetylene monomer must be in an ordered form to be active

Radiation Dosimetry Applications –

GAFCHROMIC® Films

Five products introduced between 1990 and 2002

Primarily for MeV radiation

• HD-810 transparent, single layer, 5 – 400Gy

• MD-55 transparent, laminated double layer, 1 – 80Gy

• HS transparent, laminated, single layer, 0.5 – 40Gy

Primarily for keV radiation

• XR-R opaque, laminated, single layer, 0.1 – 15Gy

• XR-T transparent, laminated, double layer, 0.02Gy – 10Gy

Features of radiochromic film include…

• No film processing

• No darkroom

• No chemicals

• Low energy dependence

• Fractionation independence

• Dose-rate independence

Quantitative film dosimetry also requires ….

•Film Digitizer

•Software

However, some issues prevented wide

clinical use…

• Dose range >10 Gy

• Non-uniformity

• Time/temperature effects

• Small film size

• Complicated protocols for use

– 4 days for measurement

– TG-55

• High price

Development of High Sensitivity Film

• Development began in December 2002

• GAFCHROMIC® RTQA – 2-500cGy, radiotherapy QA where use is qualitative

– Introduced July 2004

• GAFCHROMIC ® EBT – 1-800cGy, for quantitative film dosimetry, e.g. IMRT – Currently in final clinical evaluations

– Introduction late September 2004

• GAFCHROMIC ® XRQA – keV photons 0.1-20cGy – Currently in field evaluations

– Introduction about December 2004

Developing a 3D Radiochromic Gel Courtesy of James F. Dempsey, Univ. Of Florida

The novel film active layer was “discovered” while investigating a novel 3D radiochromic gel see Med. Phys. 30(6) (2003) 1425

Active Component is Hair-Like

A New Film is Developed Courtesy of James F. Dempsey, Univ. Of Florida

EBT is still a RCF, but it has new properties, the most promising of which, was it’s greatly increased radiosensitivity

LiPCDA – Active Component in EBT and EBT2

GAFCHROMIC® Dosimetry Films

Radiology/diagnostic Energy Range Dose Range

GAFCHROMIC® XR- R kV 0.1Gy to 15 Gy

GAFCHROMIC® XR-CT kV 0.1cGy to 20 cGy

GAFCHROMIC® XR-M kV 0.1cGy to 20 cGy

GAFCHROMIC® XR-QA kV 0.1cGy to 20 cGy

Radiotherapy Energy Range Dose Range

GAFCHROMIC® HD-810 MV 5Gy to 500Gy

GAFCHROMIC® MD-V2-55 MV 1Gy to 100Gy

GAFCHROMIC® RTQA MV 0.02Gy to 8Gy

GAFCHROMIC® EBT kV-MV up to 50Gy

GAFCHROMIC® EBT Coating

CLEAR POLYESTER ~97 microns

SURFACE LAYER ~3 microns

ACTIVE LAYER ~17 microns

EBT Film Structure and Composition

CLEAR POLYESTER - 97 microns

CLEAR POLYESTER - 97 microns

ACTIVE LAYER - 17 microns

ACTIVE LAYER - 17 micronsSURFACE LAYER - 6 microns

COMPOSITION (ATOM%)

C H O N Li Cl Zeff

25.0% 54.2% 9.6% 10.4% 0.7% 0.1% 7.14

22.5% 53.3% 11.1% 12.7% 0.2% 0.2% 6.84

45.5% 36.4% 18.2% 0.0% 0.0% 0.0% 6.64Polyester

Layer

Active layer (contains about 7.5% water)

Surface layer (contains about 15% water)

GAFCHROMIC® EBT: rgb Color Scanning

The End of EBT Film

• Fortune meets reality

• Contract coating facility (Polaroid Corp.) closed in

2008

• Forced changes in coating technique

• Active component remained the same at in EBT

• Binder material changed from natural to synthetic

polymer

• Active layers reduced from two to one

• Voluntary change

• Incorporate a marker dye in the active layer

GAFCHROMIC EBT2 dosimetry film

Polyester Laminate - 50 microns

Adhesive Layer - 25 microns

Active Layer - 28±3 microns

Polyester Base - 175 microns

Why is EBT2 film yellow?

• Contains a yellow dye – a marker dye

• But why?

– Allows for correction of film non-uniformities

• U.S. Patent 6,285,031 September, 2001

– Original intent to use red and blue color channels

• Red – signal dominated by dose information

• Blue – signal dominated by uniformity information

Spectra of EBT2 components

• Active component

– Signal in red channel

• Marker dye

– Signal in blue channel

0.0

0.2

0.4

0.6

0.8

1.0

1.2

400 450 500 550 600 650 700

Ab

so

rba

nc

e

Wavelength, nm

Visible Spectrum of Active Component after Exposure

0

1

2

3

350 400 450 500 550 600 650 700

Ab

so

rba

nc

e

Wavelength

Visible Spectrum of Marker Dye

Before exposure

After exposure to 50Gy

Spectra of EBT2 Before and after exposure

0.0

0.5

1.0

1.5

2.0

2.5

350 400 450 500 550 600 650 700 750 800

Ab

sorb

ance

Wavelength, nm

Handcoating with Yellow Marker-dye: rgb pre-exposure

Handcoating with Yellow Marker-dye: Red channel pre-exposure

Handcoating with Yellow Marker-dye: Blue channel pre-exposure

Dose/profile Before Correction

Dose/profile After Correction

Protocol for using the marker dye in EBT2

Step 1: Calibration

ODR = -log10(PVR/65535)

ODB = -log10(PVB/65535)

Dual channel dosimetry

Red channel

Composite channels

Blue channel

There‟s a better way, a much better way!

• Use all the color channels – “Multi-channel film dosimetry with non-uniformity correction”,

A. Micke, et al. submitted to Med. Physics August, 2010.

Anisotropy

Coating direction (downweb) Cross web direction

•Active material crystals are:

•Rod shaped approx. 2 μm x 15μm

•Preferentially aligned in downweb direction

(parallel to coating direction)

•Light polarizers (after exposure)

Scan Orientation and Response of EBT2

0

100

200

300

400

500

600

-0.10 0.00 0.10 0.20 0.30 0.40 0.50 0.60

Do

se

, c

Gy

Red density

Dose Response EBT2, lot# 061109

Landscapeorientation

Lateral response dependence

Menegotti et al 2008 Med Phys 35 3078-84

Lateral Response Dependence

• Why? – Light scattering? No

– Path length of rays transmitted through the active

layer? No

– Path length of rays transmitted through the CCD

filters? Yes, but minor

– Polarization of light transmitted through the film

Polarization and Radiochromic Film

• Diacetylene polymers are highly anisotropic

• Light transmitted by the polymer is polarized

– Vibrational component perpendicular to the backbone is absorbed

• If the particles in radiochromic film were randomly oriented

the transmitted light would be:

– Lower intensity (equivalent to crossed polarizers)

– Unpolarized

• However particle orientation in is induced by fluid flow in the

coating process

– Therefore the transmitted light is polarized

Note: The particles in the original radiochromic film were square in cross-section. Therefore

the particles were randomly oriented and the transmitted light was unpolarized

Crossed Polarizers

R: 0.551

G: 0.483

B: 0.702

R: 0.640

G: 0.535

B: 0.739

Optical Elements in a Flatbed Scanner

Single channel dosimetry

in extreme lateral position

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

2.00

0 50 100 150 200 250 300 350

Do

se,

Gy

Position, x 0.353 mm

Side scan - red channel dosimetry, nolateral correction

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

2.00

0 50 100 150 200 250 300 350

Do

se,

Gy

Position, x 0.353 mm

red channel dosimetry, no lateral correction

red channel dosimetry, with lateral correction

Lateral response scanner calibration

Triple channel dosimetry

in extreme lateral position

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

2.00

0 50 100 150 200 250 300 350

Do

se,

Gy

Position, x 0.353 mm

Side scan - red channel dosimetry, nolateral correction

side scan - 3-channel dosimetry, no lateralcorrection

Triple channel film dosimetry:

Features and advantages

Separates Dose and Dose-independent effects

– Allows compensation for film thickness variation

– Allows „smart‟ noise reduction (not yet implemented in

FilmQA Pro software)

Enable the use of full film dose sensitivity of all channels

RGB without “transition error”

Significant improvement of dose map accuracy

Allows to „sense‟ calibration errors

Attenuates the lateral response artifact

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

2.00

0 50 100 150 200 250 300 350

Do

se,

Gy

Position, x 0.353 mm

3-channel dosimetry with lateral correction

red channel dosimetry, no lateral correction

3-channel dosimetry, no lateral correction

red channel dosimetry, with lateral correction

Lateral response scanner calibration

Polymerization and Post-Exposure Growth

Gap = D + N.ΔD

Intermolecular distance = (D - ΔD)

Diacetylene polymer - N monomer unitsDiacetylene monomer

Intermolecular distance = D

hν

Polymerization and Post-exposure Changes

• Polymerization starts within 100 μsec of exposure

• At low exposures (<100 Gy) polymerization proceeds as

a first order reaction for about 30 msec

• The polymer shrinks relative to the monomer

– The distance between the end of a growing chain and the next

monomer increases as polymerization increases

• Within a second the initial fast phase converts to a slow

phase where changes in absorption are proportional to

log(time)

• Protocols for using radiochromic film must

accommodate the post-exposure changes

Spectral Changes Post-exposure

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

610 620 630 640 650 660

Ab

so

rba

nc

e

Wavelength, nm

GAFCHROMIC EBT: Post-Exposure Growth

90 minutes

45 minutes

10 minutes

7 hours

Post-exposure Change

y = 0.0076x + 0.2991 R² = 0.9956

y = 0.0044x + 0.2446 R² = 0.9952

y = 0.0022x + 0.5024 R² = 0.9795

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

Den

sity

Log10(time after exposure, min)

Post-exposure change

Red

Green

Blue

Energy Independent Dose Response

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0 50 100 150 200 250 300 350

Ne

t V

isu

al D

en

sit

y

Dose (cGy)

30kVp, 2mm Al

100kVp, 2mm Al

150kVp, 2mm Al

Cobalt 60

Energy Dependence of EBT2 Film

Alexandra Rink and Pamela Lindsey, AAPM Annual

Meeting, 2009

* Proportions of Cl and Br in the entire film

EBT2 Lot # Ratio OD105kVp/6MV Cl, ppm* Br, ppm*020609 1.18±0.04 1520±118 738±60

031109 0.77±0.022 1390±90 43±1

050709 1.02±0.03 1330±41 532±20

Elemental Composition and Zeff of EBT2

H Li C N O Na S Cl Br

Polyester film base* 50 1.35 36.4% 0.0% 45.5% 0.0% 18.2% 0.0% 0.0% 0.0% 0.0%

Adhesive* 25 1.2 57.1% 0.0% 33.3% 0.0% 9.5% 0.0% 0.0% 0.0% 0.0%

Active layer (assumes 7.5% moisture)** 30 1.2 58.3% 0.8% 29.6% 0.1% 10.8% 0.1% 0.0% 0.2% 0.1%

Polyester film base* 175 1.35 36.4% 0.0% 45.5% 0.0% 18.2% 0.0% 0.0% 0.0% 0.0%

Overall Composition 40.57% 0.09% 42.68% 0.01% 16.62% 0.01% 0.00% 0.02% 0.01%

* The composition of these layers is a good faith estimate based on the manufacturer's identification of the constituents. The composition should not be used as a specification.

** The composition of these layers is a good faith estimate based on the proportion of the chemical constituents. The composition should not be used as a specification.

*** The thicknesses are approximate and are not specifications.

6.64

6.26

8.27

6.64

6.78

LayerThickness***

microns

Approximate

density g/cm2

COMPOSITION (ATOM%)Zeff = [∑ αi (Zi)

a]1/a

EBT2 is Usable in Water Phantoms

0

1

2

3

4

5

6

7

8

0 4 8 12 16 20 24

Pe

ne

tra

tio

n, m

m

Time, hours

Penetration of Water into Radiochromic Film

GAFCHROMIC EBT-2

GAFCHROMIC EBT

Water front at

position 365

Edge of film at

position 388

Profile

Magnification

What can go wrong?

• Changes to the signal from the marker dye

•

• The active component changes sensitivity

• The alignment of the active component differs

Performance Issues and Corrective Actions

• Coating uniformity

– Installed precision coating dies (ex-Polaroid) (3/09)

– Installed direct-drive motors for web transport (3/09)

– Optimized fluid formulations and machine parameters

• Uniformity of particle alignment

– Restricted coating width to <18” (from 21½”) (6/10)

• Changes in absorbance of marker dye induced by

moisture

– Eliminated K⁺ ion (5/09)

– Adjusted marker dye and Na⁺ ion concentrations (5/10)

Uniformity of EBT Film

0.95

0.96

0.97

0.98

0.99

1

1.01

1.02

1.03

1.04

1.05

1 6 11 16 21 26 31 36 41

Rela

tive r

esp

on

se

Position

Typical Cross-Web Uniformity: EBT Film Lot# 37249-5H

2σ/mean: 0.8%

Measurement error: ±0.45%

Uniformity of EBT2 Film

0.95

0.96

0.97

0.98

0.99

1

1.01

1.02

1.03

1.04

1.05

0 25 50 75 100 125 150 175 200 225 250 275 300

Rela

tiv

e r

esp

on

se,

blu

e c

han

nel

Distance, mm

Crossweb Uniformity: EBT 2 Coating 090110-1

2σ/mean: 0.5%

Measurement error: ±0.25%

Crossweb Profiles Particle Alignment is not Uniform

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

1.05

1.1

0 2 4 6 8 10 12 14 16 18 20 22

Den

sit

y

Crossweb position, inches

Red Blue

Is there an EBT3?

EBT2 Improvement Projects

• Change to a pigment dye

– Greater chemical stability, will not diffuse

• Water lamination and symmetric structure

– Re-create the structure used in the original EBT film

• Using a polyester substrate with microscopic silica on

the surface

– Eliminates Newton‟s Rings

• Cross lamination

• Energy independence

– Making the film energy independent from 10 kV into the MV

range

• Enhancing scanner response with selected optical filters

Hypothesis

• We could approximate the random orientation

condition by crossing two pieces of film

• This approaches the state of crossed polarizers

– Perfect polarizer transmits 50% of incident

unpolarized light

– Two perfect polarizers aligned transmit 50% of

incident unpolarized light

– Two perfect polarizers crossed transmit no light

Polarization in Radiochromic Film

• Crystal alignment is the source of polarization

effects in EBT2

• Particle alignment is very difficult to control

– In EBT2 there is only a partial (preferential) alignment

• Would be easier to deal with if: – Alignment was 100% perfect

– Alignment was 100% “imperfect”

• # crystals/unit volume large and crystals randomly oriented

In-line lamination

Cross lamination

Crossweb Profiles

0.75

0.80

0.85

0.90

0.95

1.00

1.05

1.10

1.15

1.20

1.25

0 2 4 6 8 10 12 14 16 18 20 22

Den

sit

y

Crossweb distance, inches

Cross lamination

Red

Blue0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

1.05

1.1

0 2 4 6 8 10 12 14 16 18 20 22

Den

sit

y

Crossweb position, inches

In-line lamination

Red

Blue

Advantages of Cross-lamination

• Improves sensitivity/contrast at doses >50 cGy

• Assists the proper function of the marker dye

• Eliminates orientation effects on the scanner

• Reduces noise level

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 100 200 300 400 500 600 700 800

Den

sit

y

Dose, cGy

In-line, red

In-line, green

In-line, blue

Cross-laminate, red

Cross laminate, green

Cross-laminate, blue

Cross-lamination: No orientation dependence

@ 0 degrees @ 90 degrees

12000

17000

22000

27000

32000

37000

42000

47000

52000

0 100 200 300 400 500 600 700 800

Cross-laminated EBT2 : Red channel response

Original orientation

Flipped

Turned 90 degrees

Turned 45 degrees

Cross-lamination: No orientation dependence

Noise Reduction

In-line laminate Cross-laminate

[email protected]

Thank you