Shani G. Radiation Dosimetry: Instrumentation and Methods

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320 Radiation Dosimetry: Instrumentation and Methods

was established by Chiu-Tsao et al. [13] The net optical

density was found to be a power function of dose with

exponents of 0.858 and 0.997 for the Macbeth and LKB

densitometers, respectively. Film sensitivity with the LKB

densitometer was about double that with the Macbeth

densitometer.

The dose-response curves of the high-sensitivity

GafChromic ﬁlms were plotted for

125

137

Cs, and

and shown in Figures 5.31a, b, and c, respectively, for

doses up to 200 Gy. The curves for the same radionuclide

obtained with the LKB and Macbeth densitometers were

compared to the ﬁgure. The net OD increases with

increasing dose. The values with the LKB were about

twice those with the Macbeth, depending on the dose.

This means that the ﬁlm sensitivity measured with red

light from the He-Ne laser is higher than that obtained

with the broadbeam spectrum. The data obtained with the

Macbeth densitometer were ﬁtted to a power function of

dose, with exponents of about 0.858, 0.839, and 0.849

for

125

137

Cs, and

Co, respectively. As for the LKB

densitometer, the curves were linear for

137

Cs and

Co.

The curve for

125

I, however, was better-ﬁtted to a power

function of dose with a power of 0.997, i.e., essentially

linear.

FIGURE 5.28 Transmission spectra for the 660 nm ﬁlter (dashed line) and 621 nm ﬁlter (solid line) measured using the Du 640B

spectrometer. (From Reference [12]. With permission.)

FIGURE 5.29 Absorbed spectra of GafChromic ﬁlm (MD-55-2) for the dose levels 0, 10, 20, 30, 50, and 100 Gy. The spectra were

measured on a Beckman DU 640B spectrometer. (From Reference [12]. With permission.)

Ch-05.fm Page 320 Friday, November 10, 2000 12:01 PM

Film Dosimetry 321

FIGURE 5.30 (a) Predicted dose response curves when mea-

sured at ﬁxed wavelengths 400 nm, 510 nm, 632.8 nm, 650 nm,

and 671 nm for GafChromic ﬁlm (MD-55-2), determined from

its absorption spectra using a Beckman DU 640B spectropho-

tometer at 31.9 days post-irradiation (solid curves). Measured

data (

) using the He-Ne laser is also shown for comparison

with the predicted dose response curve at ﬁxed wavelength of

632.8 nm. Three samples were given the same total dose of 30

Gy and used to check for measurement reproducibility. (b) Pre-

dicted dose response curves (solid curves) for a tunable light

source to the minor peak (A), major peak (B), and for valley

between peaks (C), and for complex light sources: an LED light

source coupled to a 660-nm band-pass ﬁlter (D), an LED light

source coupled to a 671-nm ﬁlter (E), and a broadband light

source (F). Measured dose response curves are also shown for

comparison, namely a 660-nm ﬁltered densitometer (

), 671-nm

ﬁltered densitometer (O) and a broadband densitometer (x). Three

samples were given the same total dose of 30 Gy and were used

to check for measurement reproducibility. (From Reference [12].

With permission.)

FIGURE 5.31 The dose-response curves of the high-sensitivity

GafChromic ﬁlm for three radionuclides,

125

137

Cs, and

Co, in

a, b, and c, respectively. The square and circle symbols are for

the reading using Macbeth and LKB densitometers, respectively.

The solid curves represent the corresponding function ﬁt. (From

Reference [13]. With permission.)

Ch-05.fm Page 321 Friday, November 10, 2000 12:01 PM

322 Radiation Dosimetry: Instrumentation and Methods

The dose response of the radiochromic ﬁlm for 4-MV

x-rays is shown in Figure 5.32. [14] The x-rays were

produced by a Clinac 4 linear accelerator, and the ﬁlm

was irradiated at a depth of 1.25 cm in an acrylic phan-

tom set up at an SSD of 41 cm. The dose rate was about

900 cGy/min. The lower curve shows the results from

optical density measurements using broadband absorp-

tion, and the upper curve shows the results of measure-

ments using 632.8-nm light.

A family of dose-response curves for radiochromic ﬁlm

for different photon energies is shown in Figure 5.33. These

curves are in the low-dose range for this ﬁlm, although the

doses are about 250 times the doses used for the silver

halide ﬁlm curves (Figure 5.34) and about 600 times the

doses used for the LiF chip curves (Figure 5.35). The curves

show linearity of dose response, and the similarity of slopes

shows that the dependence on photon energy is slight. The

optical densities are for 632.8-nm light.

An “improved ﬁlm” was developed with higher sensi-

tivity and better uniformity than the model MD-55 ﬁlms by

Meigooni et al. [15] The improved ﬁlm is a multilayered

structure composed of a nominal 30-



m thickness of the

FIGURE 5.32 Dose response curves for radiochromic ﬁlm.

(From Reference [14]. With permission.)

FIGURE 5.33 Family of dose response curves for radiochromic

ﬁlm. (From Reference [14]. With permission.)

FIGURE 5.34 Family of dose-response curves for silver halide

ﬁlm. Each curve is for one x-ray beam (one equivalent photon

energy). (From Reference [14]. With permission.)

FIGURE 5.35 Family of dose-response curves for LiF TLD

chips. Each curve is for one type of x-ray beam (one equivalent

photon energy). (From Reference [14]. With permission.)

Ch-05.fm Page 322 Friday, November 10, 2000 12:01 PM

Film Dosimetry 323

sensitive material, sandwiched between two pieces of 75-



thick polyester base material and two pieces of 13



m lam-

inated material, yielding a total thickness of 206



m. In

contrast, MD-55 ﬁlm is composed of a layer (23-



m-thick)

of sensitive material on one side of a 100-micron polyester

base. These ﬁlms are colorless and transparent before being

exposed to ionizing radiation. As they are irradiated, their

color changes to blue, due to the polymerization of radio-

chromic dye. The darkness of the blue color depends on the

absorbed dose and can be measured with a laser densitom-

eter with a wavelength ranging from 610–675 nm.

Figure 5.36 shows the measured sensitometric curves

of the improved radiochromic ﬁlm and MD-55 ﬁlm as a

function of absorbed dose. This ﬁgure indicates that the

improved ﬁlm requires about 40% less dose than MD-55

ﬁlm to yield an optical density of 0.5. This difference is

reduced to about 30% for an optical density of about 1.5.

The combination of all sources of uncertainties in these

measurements is estimated to be less than 5%. This

uncertainty includes the reproducibility of the laser scan-

ning system, uncertainty of determination of absorbed

dose given to the ﬁlm, and standard deviation of the optical

density measured along a path of about 5 mm in a uni-

formly exposed ﬁlm.

The uniformity of the improved radiochromic ﬁlm and

MD-55 ﬁlm along two orthogonal directions was deter-

mined by measuring the optical density of a ﬁlm irradiated

with a uniform radiation ﬁeld. Figure 5.37a shows very

good uniformity (within 4%) along one direction of the

improved radio-chromic ﬁlm. A similar result was

obtained at a distance of 5 cm from the beam axis, along

the same direction of the ﬁlm. However, along the per-

pendicular direction (Figure 5.37b), a non-uniformity of

up to 15% was observed. This non-uniformity is not con-

sistent from one sample of ﬁlm to another.

Figure 5.38 shows the variation of relative optical

density of the improved radiochromic ﬁlm and MD-55

ﬁlm as a function of time after irradiation for absorbed

dose as shown in the ﬁgures.

Evaluation of the inﬂuence of irradiation temperature

differences on the response of three widely used radiochro-

mic ﬁlm dosimeters was made by McLaughlin et al. [16]

Freestanding nylon base ﬁlm (mean thickness 48



FWT-60-00, thin-coated (~7



m) sensor on 100-



m poly-

ester base GafChromic DM-1260 dosimetry media, and

freestanding polyvinylbutyral base ﬁlm (mean thickness



m) Risø B3 were investigated.

The irradiations were made with gamma-rays from two

Co sources, one at an absorbed dose rate of 12.2 kGy/h

and the other at 30.0 kGy/h. During irradiation the ﬁlms

(three ﬁlms for each irradiation) were held between two

electron equilibrium layers of 3-mm-thick polystyrene and

sealed at 50% relative humidity in a triple layer pouch of

FIGURE 5.36 Sensitometric curves of the improved radiochro-

mic ﬁlm (solid symbol) compared with that of MD-55 ﬁlm (open

symbol). These results were obtained by irradiating the ﬁlms to

doses ranging from 1–72 Gy of gamma rays from a

Co tele-

therapy unit. (From Reference [15]. With permission.)

FIGURE 5.37 Relative optical density of a uniformly irradiated

improved radiochromic ﬁlm (a) measured in one direction along

the ﬁlm and (b) measured along the orthogonal direction. (From

Reference [15]. With permission.)

Ch-05.fm Page 323 Friday, November 10, 2000 12:01 PM

324 Radiation Dosimetry: Instrumentation and Methods

30-



m-thick polyethylene–aluminum foil laminate. Each

packet was conditioned for 30 min at each irradiation

temperature before irradiation, then irradiated, and then

stored at room temperature immediately after irradiation.

Figures 5.39, 5.40, and 5.41 show, respectively, the

temperature dependence of the gamma-ray responses of

the three most recent batches of radiochromic ﬁlms, FWT-

60-00 (batches 8W9-1989 and 1D0-1990), GafChromic

1260 (batch 09031501-1989), and Risø B3 (batch 85-128-

1989). In all three cases, it is seen that a different trend

occurs at different absorbed doses, particularly at the higher

temperatures.

A technique of readout was developed by Kellermann

et al. [17] to measure the optical density distributions of

the ﬁlm in purely directed light. This technique imple-

ments radiochromic ﬁlm dosimetry near the ﬁlm’s

absorption maximum by using a single-mode top-surface

emitting laser diode (675.2 nm). The effective sensitivity

of the ﬁlm, compared with a helium-neon laser densito-

meter (632.8 nm), is increased approximately threefold.

Figure 5.42 shows the optical density of the radiochro-

mic ﬁlm depending on the wavelength of the analyzing

beam after irradiation with four different doses. [18] In

the helium-neon laser densitometer, the ﬁlm is scanned

with an analyzing beam and the transmission is measured

by a photomultiplier. The emission wavelength (632.8 nm)

of the analyzing beam lies in the valley of the ﬁlm’s

absorption spectrum. Since the absorption spectrum of the

radiochromic ﬁlm contains two peaks centered at approx-

imately 615 nm and 675 nm, it has been suggested [17]

that signiﬁcant improvement in response might be

achieved by tailoring the wavelength of the incident den-

sitometer light to one of these peaks. Densitometers with

a light-emitting diode (LED) (660 nm, spectral line width

30 nm) and with an LED coupled to a band-pass ﬁlter

(670 nm, bandwidth 11 nm, GatChromic densitometer)

have been presented, but their spectral line width is too

broad to match the absorption maximum exactly, and the

improvement in response is reduced.

The radiochromic ﬁlm’s sensitive layer consists of

microcrystals of radiation-sensitive monomer uniformly

dispersed in a gelatin binder. When the microcrystals are

exposed to ionizing radiation, polymerization occurs and

the polymers alter the crystal color to various shades

of blue. The optical density increases continuously with

the absorbed dose. The optical density depends neither

FIGURE 5.38 Variation of the optical density with time after

irradiation for the improved radiochromic ﬁlm (solid symbol)

compared to MD-55 ﬁlm (open symbol). These results were

obtained by simultaneous irradiation of pieces of each ﬁlm-type

absorbed doses of 5, 16.92, and 33.85 Gy using a

Co telether-

apy unit. (From Reference [15]. With permission.)

FIGURE 5.39 Temperature dependence, relative to that at

20°C, of the gamma-ray response of two batches of radiochromic

ﬁlm FWT-60-00. Batch 8W9 was irradiated at three absorbed

doses (6, 15, and 30 kGy) and batch 1D0 was irradiated at 15

kGy. The absorbances per unit thickness (

A mm

1

) were mea-

sured at 605-nm wavelength. (From Reference [16]. With per-

mission.)

Ch-05.fm Page 324 Friday, November 10, 2000 12:01 PM

Film Dosimetry 325

on photon energy in the range 0.1 to 20 MeV nor on

the energy of secondary electrons in the range 0.01 to

20 MeV.

GafChromic MD-55™ irradiated in a uniform radia-

tion ﬁeld showed a non-uniformity in optical density of

up to 15% in one direction and within 4% in the direction

orthogonal to it. This would result in a dose error of

or %.

A laser diode with an emission wavelength of 675.2 nm

was chosen by Kellermann et al. as a light source to tran-

silluminate the radiochromic ﬁlm. At 675.2 nm, near the

ﬁlm’s absorption maximum, a dose gradient in the radiation

FIGURE 5.40 Temperature dependence, relative to that at 20°C, of the gamma-ray response of radiochromic ﬁlm GafChromic

Dosimetry Media, DM-1260, batch 09031501, irradiated at two absorbed doses. The wavelengths of absorbance measurements:

•

650 nm; 600 nm; X  510 nm; ◊ 400 nm. (From Reference [16]. With permission.)

FIGURE 5.41 Temperature dependence of the gamma-ray response of radiochromic ﬁlm Risø B3, batch 85-128, irradiated at two

absorbed doses. The two right ordinates show the response relative to that at 20°C. The absorbances per unit thickness (

A mm

1

)

were measured at 554-nm wavelength. (From Reference [16]. With permission.)

10% 2.5

Ch-05.fm Page 325 Friday, November 10, 2000 12:01 PM

326 Radiation Dosimetry: Instrumentation and Methods

ﬁeld leads to a change in optical density that is up to three

times larger than the change that is caused by the same

dose gradient in the valley (632.8 nm) of the ﬁlm’s absorp-

tion spectrum. The laser light propagating in the active

region excites a transverse electrically polarized (TE

) sur-

face mode in the waveguide structure on the top of the

laser diode through grating coupling. The surface mode

couples both into the vacuum light cone, resulting in top-

surface emission with narrow beam divergence, and back

to the active region, leading to a gain mechanism and thus

to single-mode emission.

The ﬁlm is fastened (with double-sided adhesive tape)

to a stage that allows precise movements along the scan-

ning direction (and orthogonal to it) in 50-



m steps along

a distance of 75 mm. The schematic of Figure 5.43 dis-

plays the measurement setup.

After irradiation, the optical density of radiochromic

ﬁlm increases logarithmically. Therefore, the optical den-

sity measurements with the densitometry system are done

four days after the end of irradiation, by which time the

changes in optical density are small.

The calibration curve—change in optical density at

675.2 nm vs. radiation dose—is shown in Figure 5.44. The

curve rises rapidly, but the upward gradient becomes smaller

with higher doses. In comparison, the optical density vs.

radiation dose at 632.8 nm is ﬂat but approximately linear.

[19] At 675.2 nm, a dose gradient from 0 Gy to 20 Gy in

the radiation ﬁeld leads to a change in optical density that is

3.1 times larger than at a wavelength of 632.8 nm. This ratio

decreases to 1.9 with a dose gradient from 20 Gy to 40 Gy.

Film digitizers are, compared with mechanical scanners,

much faster, while their spatial resolution is even higher.

FIGURE 5.42 Optical density of the radiochromic ﬁlm (Gafchromic MD-55

, Model No. 37-041), depending on the wavelength

of the analyzing beam after irradiation with four different doses. [18] The wavelength of the helium-neon, laser (632.8 nm) lies in

the valley of the ﬁlm’s absorption spectrum, while the wavelength of the laser diode (675.2 nm) used by Kellermann et al. is near

the ﬁlm’s absorption maximum. (From Reference [17]. With permission.)

FIGURE 5.43 The schematic measurement setup is sketched. The laser light transmits the ﬁlm and is sent via an optical ﬁber to a

solar cell, where it is transformed to an electrical signal. This is measured with a lock-in ampliﬁer. The light of the edge emission

is used as a reference signal. (From Reference [17]. With permission.)

Ch-05.fm Page 326 Friday, November 10, 2000 12:01 PM

Film Dosimetry 327

Most commercially available digitizers are based on either

a scanning He-Ne laser beam or a one-dimensional charge-

coupled device (CCD) combined with a standard ﬂuores-

cent lamp. Mersseman and De Wagter [20] investigated a

CCD-based digitizer.

The principles of the digitizer are as follows. A broad

light beam from the ﬂuorescent lamp (Philips F17T8/

TL841) crosses the light-diffusion plate and travels

through the scrolling ﬁlm to be digitized. Two ﬁxed ver-

tical and two movable horizontal shutters subsequently

collimate this light beam to a narrow rectangular ﬁeld that

covers the ﬁlm lengthwise. The transmitted light is sub-

sequently reﬂected by a stationary mirror, focused by the

40-mm lens and ﬁnally detected by the linear CCD ele-

ment. The CCD detector reads line per line at 300 dots

per inch (dpi) over its full length of 35.6 cm (14 in). In

order to achieve a 150, 75, and 60 dpi resolution, the ﬁlm

digitizer combines data from nearby pixels. A 12-bit

analog-to-digital converter digitizes the signal. Conver-

sion tables (built-in or loaded from the host computer) are

allowed to further translate the data. The resulting data

matrix is transferred to the host computer (Pentium PC)

using an SCSI interface.

Nuclear Associates’ GafChromic Dosimetry Media

is a radiation-sensitive imaging ﬁlm designed for use in

medical and industrial quality control measurement

applications. The media is colorless and grainless and

offers a very high spatial resolution (1200 LP/mm), mak-

ing it invaluable for measuring dose distribution around

small brachytherapy sources and stereotactic radiosur-

gery ﬁelds. [21] GafChromic Dosimetry Media MD-55

(Model 37-041) comprises clear ﬁlm sheets that

are suitable for medium-dose measurement applications

covering the 1.5 to 50-Gy dose range, when used with the

radiochromic densitometer. Higher doses (100 Gy and

greater) can be measured using other commercially avail-

able densitometers. MD-55 has a higher sensitivity than

HD-810 and was developed for use in external-beam radio-

therapy and brachytherapy applications. MD-55 is a lam-

inated ﬁlm composed of two pieces of 2.65-mil polyester

base, each with a nominal 15-micron-thick coating.

HD-810 (Model 37-040) ﬁlms are low-sensitivity

clear ﬁlm sheets suitable for high-dose beam pro-

ﬁling and dose-mapping applications in the 10 to 300-Gy

dose range, when used with the radiochromic densitome-

ter. Higher doses (up to 2500 Gy) can be measured using

other commercially available densitometers. This media

has a nominal 7-micron-thick radiation-sensitive layer on

a 3.9 mil polyester base.

GafChromic Dosimetry Media is composed of materials

with low atomic numbers, so the ﬁlm can be considered

tissue-equivalent for most radiation therapy applications.

Additionally, this material has low sensitivity to ambient

room light, which simpliﬁes handling procedures and

enhances image stability. Its response is dose rate indepen-

dent; it is linear dose response and energy independence

above 100 keV. [21]

TECHNICAL DATA

MD-55: Polyester Base: 67-



m; Sensitive Layer: 15-



Pressure-Sensitive Adhesive Layer: 44.5



m; Polyester

Base: 25



m; Pressure-Sensitive Adhesive Layer: 44.5



Sensitive Layer: 15



m; Polyester Base: 67



HD-810: Sensitive Layer: 7



m; Adhesive Layer: 1.5



Conductive Layer: 0.05



m; Polyester base: 99



FIGURE 5.44 Calibration curve—change in optical density at 675.2 nm vs. radiation. The curve rises rapidly, but the upward gradient

becomes smaller with higher doses. In comparison, optical density vs. radiation dose at 632.8 nm is ﬂat but approximately linear.

(From Reference [17]. With permission.)

5 5

8 10

Ch-05.fm Page 327 Friday, November 10, 2000 12:01 PM

328 Radiation Dosimetry: Instrumentation and Methods

37-041 MD-55 GAFCHROMIC Dosimetry Media;

Package of Five Sheets, Weight: .10 lb (.04 kg).

37-040 HD-810 GAFCHROMIC Dosimetry Media:

Package of Five Sheets, Weight: .50 lb (.24 kg)

The GAFCHROMIC sensitivity to energy compared

to that of silver based ﬁlm is shown in Figure 5.45.

Another promising thin ﬁlm polymer dosimeter for high

doses consists of a ﬁlm or gel cast from an alcohol solu-

tion of blue tetrazolium or triphenyl-tetrazolium chloride

combined with a polymer matrix material (e.g. polyvinyl

alcohol, polyvinyl pyrrolidone), McLaughlin [22]. Figure

5.46 shows the radiation response properties of one exam-

ple of this ﬁlm system, in this case with a thickness of



FIGURE 5.45 GafChromic sensitivity to energy composed to that of silver based ﬁlm on LiF TLD. (From Reference [21]. With

permission.)

FIGURE 5.46 Response of radiochromic ﬁlm of tetrazolium chloride in polyvinyl pyrrolidone (in terms of increase in absorbance

measured at 522 nm wavelength) as a Junction of the gamma ray absorbed dose. The inset shows the increase in the absorption

spectral amplitudes with dose. (From Reference [22]. With permission.)

5 5

8 10

Ch-05.fm Page 328 Friday, November 10, 2000 12:01 PM

Film Dosimetry 329

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Ertl, A. et al., Phys. Med. Biol., 43, 1567, 1998.

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Mersseman, B. and De Wagter, C., Phys. Med. Biol.,

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McLaughlin, W. L., in Proc. High Dose Dosimetry for

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