MP 6757

Radiochromic Film for Medical Dosimetry

STEVEN HUPCHER

4/26/2013

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I. Introduction

Coloration detectors, or materials that turn color upon being irradiated, have had a long

history in radiation detection and dosimetry. Barium platinocyanide screens were instrumental to

Wilhelm Röntgen’s discovery of the x-ray. Silver halide films, used in most photographic film and

paper, were also used by Röntgen to demonstrate the medical applications of x-rays. Silver

halide film have the drawback of requiring processing after exposure1. One of the earlier units of

radiation dosimetry, known as the “erythema dose”, was defined as the amount of ionizing

radiation required to produce visible reddening of the skin. Unlike halide film, this method did not

require any type of processing; however it included the obvious drawback of needing to irradiate

a person. Fortunately, this unit was quickly replaced by other quantification methods that were

more reproducible and, importantly, did not require irradiation of the skin2.

There are a large number of measurement tools to provide two-dimensional radiation

exposure information besides film. Diodes and ionization chambers, for instance provide point-

by-point measurements which can be scanned in two-dimensions to provide spatial information

relating to dose. These methods are slow because of the scanning requirement, and are also

expensive due to the necessity of precise movement2. Thermoluminescent detectors (TLDs) are

generally cumbersome and time consuming when it comes to two-dimensional dose distribution

measurements. In addition, dosimetric data cannot be easily stored long-term3.

Although film dosimetry offers exceptional spatial resolution and flexibility, not all film is

created equally. Silver halide films enjoys several advantages over other dosimetric tools

including being relatively inexpensive, providing a permanent archival image, and being well-

characterized in terms of the relationship between film darkening and dose. There are several

disadvantages as well: a chemical development process whose variability can affect darkening 2

and energy-dependent response3 in the energy spectrum from 10-200 keV, and energy

absorption and transfer properties that do not match those of tissue3. Radiochromic film (RCF)

offers high spatial resolution, relatively little energy-dependent response, and near tissue

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equivalence. In addition, it is relatively insensitive to visible light and does not require chemical

processing4. Here, the current status of RCF for dosimetry will be discussed along with the

remaining limitations in terms of dosimetric accuracy.

II. RCF Mechanism of Action

Modern radiochromic films have been used for high-dose industrial and medical

applications for over 40 years. These early films are made from a colorless transparent material

consisting of hydrophobic-substituted triphenylmethane leunides5. Upon irradiation, heterolytic

scission breaks the nitrile bonds, causing the formation of a colored polymer. The films required

a high radiation dose to create measurable coloration, on the order of 104-106 Gy. More recent

RCF is based on polydiactylene, which is much more sensitive to radiation. As a result, these

films are sensitive in the range from 1-2500 Gy, depending on the film type, with the correct film

selected for the medical application5. These films have an effective Z number ranging from 6-

6.5. While insensitive to visible light, they have some sensitivity to ultraviolet light and changes

in temperature6.

The resulting color change upon irradiation typically stabilizes within 24 hours6 and is

used to quantify dose received. This quantification is in terms of optical density (OD) given by

𝑂𝐷 = log10

𝐼0𝐼,

(1)

where I0 is the light intensity with no film present and I is the light intensity after passing through

the film. These measurements can be made with a densitometer where a light source, often a

HeNe laser, is scanned point-by-point across the film and the intensity of light emerging from

the film is measured by a detector. The detector of choice is typically a photomultiplier tube or

an array of charge-couple devices2.

There has also been interest in measuring the OD of RCF using a standard flatbed

scanner7-9. Flatbed scanners have relatively low cost and have been shown to have accuracies

to within 1%10. In addition, these scanners have pixel sizes of less than 85 µm and can reduce

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inaccuracies that may be generated by point-by-point mechanical scanning. Generally,

scanners of 12 bits (4096 levels of contrast) or greater are recommended for dosimetry2.

In order for absolute dose measurements using RCF, a calibration curve must be

measured to convert OD to absolute dose. A typical calibration curve is shown in figure 1 with

an absolute dose measured by a calibrated detector. Absolute dose, D, can be derived by fitting

the calibration curve with a single power function11 given by

𝐷 = 𝑏 ⋅ 𝑛𝑒𝑡𝑂𝐷 + 𝑐 ⋅ 𝑛𝑒𝑡𝑂𝐷𝑛, (2)

where b and c are fitting parameters. The term netOD refers to the measured film OD

subtracted by the background OD with no film present. The power term, n, has been evaluated

in the range12 from 0.5 to 5 and in one study.

Several characteristics of the film as well as the imaging system and environment must

be taken into account when using RCF for absolute dosimetry. These are described by

Dempsey et al.13. Major sources of uncertainty in dose measurement will be further discussed in

section IV, however a few will be mentioned briefly. If the active layer on the RCF is dispersed

nonuniformly, the point-by-point measurement of a uniform dose can vary on the film by as

much as 15%. This can be corrected by pre-exposing the film to a known dose for calibration

purposes and then exposing the film a second time to the unknown dose, using the calibration

to correct for spatial dependent sensitivity. It has also been determined that the measured OD

can be affected by the storage temperature of the film during the time between irradiation and

scanning. In addition, the temperature of the RCF during reading can also affect the measured

OD. As such, it is recommended that the film be stored and read at relatively constant

temperature throughout use. It has also been shown that the crystalline active layer of RCF can

act as a polarization filter such that when using a linearly polarized He-Ne laser to measure OD,

a 4-15% variation occurred with changes in RCF orientation. With these sensitivities, it is

important to remember the importance of careful handling, storage, and use of radiochromic

film.

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III. Gafchromic Film for Medical Dosimetry

Gafchromic film is one of the most widely used film products for medical dosimetry5. The

earliest Gafchromic film was used for high dose radiation monitoring in industry14. Later, model

MD-55 film was introduced which was more sensitive, with an effective dose range of 10-100

Gy. Early studies with this model demonstrated the film to be as accurate as TLDs with the

added benefit of 2D mapping surface dose with a linear response in the clinical range15.

The MD-55-2 model (now known as MD-V2-55) was introduced by combining two MD-

55 layers together providing an effective thickness of 32 µm4. This new film was sensitive in a

dose range from 1-250 Gy and was better suited for clinical use than the MD-55 film. One study

using the MD-55-2 film as a dosimeter for 192Ir brachytherapy of cervical cancer demonstrated

dose measurement accuracy within 10% and reproducibility of measurement within 5%16.

Although Ashland, the manufacturer of Gafchromic film, currently boasts several film

models for dosimetry17, the most popular for radiotherapy is the EBT (External Beam Therapy)

line. The latest models are the EBT2 and EBT3 films. The EBT2 film claims a dose range from 1

cGy to 40 Gy, with uniformity better than 3%17. The more recent model, EBT3 claims a dose

range from 1 cGy to greater than 40 Gy, also with uniformity better than 3%17. The changes in

absorbance of the EBT 2 model as measured by a flatbed scanner are provided by the

manufacturer and shown in figure 2. One study of the EBT2 system measured an effective dose

range of 0.2-100 Gy with a precision and accuracy below 2%18, in agreement with manufacturer

specifications. The EBT3 film system, made commercially available in September 2011, has

similar dosimetric properties to the EBT2 system. Unlike the EBT2 system, it is symmetric so it

can be scanned on either side and it also has silica particles embedded in the surface layer to

prevent some imaging artifacts that may appear in the EBT2 film when scanned using a flatbed

scanner. In addition, the EBT3 demonstrates less energy-dependent sensitivity in the

kilovoltage x-ray range19.

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IV. Sources of Uncertainty in RCF Measurements

Like all dosimetry systems, radiochromic film has sources of uncertainty that can result

in inaccurate dose measurement. Devic et al.4 groups these sources into two categories:

uncertainties during the calibration process and uncertainties during dose measurement.

Several groups3, 5, 20, 21 have studied these uncertainties to under different conditions. The

testing conditions and the resulting uncertainties will be discussed, as well as current and/or

proposed solutions.

Spatial Uncertainty

Butson et al.5 described two levels of spatial uniformity of concern in film dosimetry:

“microscopic” scales and “macroscopic” scales. Microscopic uniformity refers to fluctuations in

OD at a point of interest compared to the average OD of the film. Macroscopic uniformity has

been studied by researchers and is likely more of a concern when using RCF for measuring

dose deposition in radiotherapy. The results of two studies are discussed below.

van Battum et al.21 examined the dosimetric qualities of RCF irradiated with a 6 MV x-ray

beam. In order to remove errors associated with plastic phantoms and air gaps, the film was

immersed in a water tank. After irradiation, the optical density of the film was measured using a

flatbed scanner. They first examined scanner accuracy, which can influence both the calibration

process as well as dose measurement. The scanner showed no warmup effects and no

changes in measured optical density when the film was flipped and scanned from the other side.

However, the measured OD did vary with lateral scan position on the scanner as well as optical

density itself. Lateral scan accuracies were determined by repetitively scanning a film with

known dimensions and was found to be 0.35 mm. These uncertainties could be corrected by

plotting the changes in OD as function of off-axis position laterally.

Another study22 found a sizeable dependence of direction of irradiation on the measured

OD. These measurements were obtained by orienting the film and irradiating with respect to 6-

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and 18-MV x-ray beams. The dose along the length of the film irradiated in one direction varied

by less than 4%, however, when the film was irradiated after rotating it 90°, the dose uniformity

along the length of the film varied by as much as 15%. This nonuniformity is not consistent from

film to film and is due to variability in the manufacturing process of the film. The dose response

is much more uniform when it is irradiated parallel to the direction the sensitive layer is applied5.

Currently, manufacturers mark the film so that users can identify the direction to orient the film

for minimum variation.

Although users can orient film along the direction of deposition of the sensitive layer to

minimize uncertainty, error in any one direction can still be as high as 6%. Yet, for most

radiotherapy applications, acceptable uncertainty is on the order of 3-5%. Zhu et al.23 have

devised a double exposure technique, mentioned earlier, in order to normalize macroscopic

response over the whole film. This technique begins with irradiating the film with a uniform dose

of 20 or 40 Gy and marking two corners. After a 2-4 day time interval, the film is scanned and

the optical density is recorded on a pixel-by-pixel basis. Since the film received a uniform dose,

ideally the OD of each pixel should be identical. In reality, the OD is not uniform along the

entirety of the film, so a sensitivity correction matrix, f(i,j), is created

𝑓(𝑖, 𝑗) =

𝑂𝐷1(𝑖, 𝑗)

⟨𝑂𝐷1(𝑖, 𝑗)⟩,

(3)

where i,j are the X and Y position indices, OD1(i,j) is the optical density at pixel (i,j), and

⟨𝑂𝐷1(𝑖, 𝑗)⟩, is the mean optical density over the entire film. This pre-exposed film is then

exposed to an unknown dose distribution to be measured. After 2-4 days, the film is scanned

again, leading to a cumulative optical density distribution, OD2(i,j). The measured OD1 and OD2

are compared on a pixel-by-pixel basis by lining up the fiducial markers on the two scanned

images. The net optical density, ΔOD(i,j), is the corrected response of the film to the unknown

radiation is given by

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∆𝑂𝐷(𝑖, 𝑗) =

𝑂𝐷2(𝑖, 𝑗) − 𝑂𝐷1(𝑖, 𝑗)

𝑓(𝑖, 𝑗).

(4)

This correction technique yielded a reproducibility in measurement of 30 Gy to within 2%, which

was an improvement of the ~6% uncertainty reported by manufacturers. The film also yielded

measurements with spatial resolution of 0.25 mm. The success of this technique highlights

another advantage of RCF, namely the ability to deliver fractionated doses without any change

in response. The measured netODs from identical doses delivered in a single fraction or in

multiple fractions have been found to match within uncertainty24.

Energy Dependence

Although the response of radiochromic film is less energy dependent that silver halide

film, due to the lower atomic number of most RCF, there still exists energy dependent behavior5.

Specifically, the films tend to under-respond to low-energy X-rays. This defect is important for

dosimetry during orthovoltage skin treatments where peak photon energies are less than 1 MeV

as well measuring low percentage depth dose (PDD) regions of megavoltage beams. Several

Gafchromic models (MD-55-1, MD-55-2, and HS) were tested and were found to under-respond

to photons of energies below roughly 100 keV5. It is interesting to note that this response is

opposite to many other radiation dosimeters such as TLDs, silicon diodes, and radiographic

films which usually over-respond to low-energy X-rays. Similar results are found in the American

Association of Physicists in Medicine (AAPM) Report No. 63 from Task Group 55 (TG-55) 3. The

response of MD-55-2 film has been shown to be approximately 40% lower for photons in the 20-

40 keV range than for 137Cs or 60Co gamma radiation. For megavoltage photon beams, it has

been shown that MD-55-2 is 40% more sensitive than MD-55-1. It has been suggested that the

reduced response of MD-55-2 at energies less than 100 keV is likely due to the larger carbon

content of the film relative to tissue, which results in a lower mass-energy absorption coefficient

relative to water at these energies25.

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Scanning Errors

Although the sources of dosimetric error described above apply to uncertainties in the

film itself, radiochromic film must also be scanned or imaged in order for its OD to be converted

to an absolute dose. These systems can introduce their own sources of error to the dose

conversion process. The two general scanning procedures are to scan the light source over the

film or to translate the film itself3. In both procedures, the film is scanned point-by-point with light

transmission averaged over the aperture of the light source. In the case of polychromatic light

used in typical flatbed scanners, the transmission is also a function of the light absorbance

spectrum of the film and the relative output spectrum of the light source. The efficiency of the

detector is also a factor. The resolution for scanning systems is determined by the size of the

light source and the distance between point measurements3. One source of error in the

scanners themselves is dead pixels. Devic et al.26 recommended eliminating this problem by

scanning with no film present to identify the pixels and replacing their value with an average of

the surrounding pixels.

Most flatbed scanners acquire up to 48-bit images in red-green-blue (RGB) mode, with

16-bits per color26. Since RCF absorbance reaches a maximum in the red region as seen in

figure 2, sensitivity can be improved by measuring the red channel only. According to Beer’s

law, light transmission is a function of both extinction coefficient and path length. As such,

apparent OD can be affected by non-uniformity in film thickness. The dependence on path

length can be removed by taking the ratio of two color channels of the apparent OD. More

accurate dose measurements have been achieved by taking ratios of the red to green

channels27 or the ratios of red to blue channels28 compared to examining the red channel alone.

The latter procedure is also recommended by the manufacturer28.

The positional accuracy of the measured OD along the film is highly dependent on the

accuracy and reproducibility of the scanning process. Hysteresis of the scanning mechanism,

either translating the film or the light source, depending on the scanning system, can cause an

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over- or under-estimate of the OD3. This is especially noticeable in scanners that operate bi-

directionally, meaning that they alternate scan directions line-by-line as opposed to starting from

the same side. This can usually be detected and corrected. In order to assure reproducibility in

measurements, especially when scanning multiple films that have been irradiated

simultaneously, it is also important to have a system for securing the film in a repeatable way.

This is because the scanning mechanism designates its origin based on the scanning bed, not

the film. Most scanners do not come with a positioning system, so another method needs to be

devised to correctly place film. In addition, fiducial markers on the film itself can be used to

register multiple scanned images that need to be aligned with one another.

As an alternative to a light scanning system, one could use a two-dimensional system

that illuminates all or most of the film, while an imaging system measures light transmission

from multiple points at once. Similar dependence on the light source and absorbance spectrum

of the film exists but the resolution is only dependent on the pixel size of the imaging system. In

addition, since no scanning is required, the processing time is greatly reduced3. Positioning

accuracy of the resulting image is dependent on regularity of the “imaging grid”, or the

faithfulness of the resulting image to the actual dimensions and spatial position of the object.

This can be examined by scanning known grid patterns and comparing the image to known

dimensions.

V. Medical Uses of Radiochromic Film

Niroomand-Rad et al. outline some of the uses of RCF for medical dosimetric

applications3. Beta-particle emitting applicators, namely 90Sr and 90Y have been used for

ophthalmic malignancies since 1978. Radiochromic film was used to as part of a standard

calibration procedure by providing dose-rate profiles along the surface of these sources29. Prior

to the standardization of this calibration, dose measurement discrepancies of up to 50% were

recorded3.

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The steep dose gradients around a brachytherapy source lend themselves well to

measurement with RCF. The spatial resolution of RCF in regions of high dose gradient is

superior to other detectors, such as TLDs, which may miss large changes in dose over a small

area. The spatial dose distribution around a high activity (370 GBq) 192Ir source was effectively

measured using RCF30.

When the radius of a radiotherapy beam is less than 1 cm, as is typical in stereotactic

radiosurgery, the dose along the central axis has been shown to decrease due to a process

known as electron disequilibrium. This process can lead to measurement artifacts when the

detector is too large3. Both radiographic and radiochromic film have been used to resolve dose

distribution for these narrow beams. McLaughlin et al. have used RCF to measure the dose

characteristics from fields with sizes down to 4 mm with accuracy within 2% compared to the

dose calculated by computational planning systems31.

Radiochromic film has also been demonstrated to be an effective dosimeter for clinical

proton beams3. Gafchromic MD-55-2 has been shown to have a linear dose response in the

region from 10-100 Gy with the exception of film irradiated in the Bragg peak location. Here,

there is a 5-10% underrepresentation of the deposited dose. One study using RCF to verify

dose delivered to a phantom from proton Bragg peak stereotactic radiosurgery confirmed the

prescribed dose within 5%3.

VI. Conclusion

Radiochromic film is an effective radiation dosimeter with spatial extent and resolution

superior to that of ion chambers and with less energy dependence that silver halide films. In

addition, changes in optical density can be recorded without any chemical processing.

Combining those characteristics with near tissue equivalence and the ability to immerse it in

water, radiochromic film is an excellent tool in medical radiation dosimetry. For all of its positive

attributes, RCF has some drawbacks as well: RCF must be calibrated for absolute dose,

energy-dependent response exists in the keV range, OD can be affected by inhomogeneities in

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the manufacture of the film, and the additional step of scanning the film introduces its own

source of error. The numerous advantages of RCF warrant further development of the

dosimetric system, with the hopes of reducing or eliminating some of the current uncertainties

associated with its use.

VII. References

1 G. Knoll, Radiation Detection and Measurement, 3 ed. (John Wiley & Sons, Inc., Ann Arbor, Michigan, 2000).

2 C.G. Soares, "Radiochromic film dosimetry," Radiat. Meas. 41, S100-S116 (2006). 3 A. Niroomand-Rad, C.R. Blackwell, B.M. Coursey, K.P. Gall, J.M. Galvin, W.L.

McLaughlin, A.S. Meigooni, R. Nath, J.E. Rodgers, C.G. Soares, "Radiochromic film dosimetry: Recommendations of AAPM Radiation Therapy Committee Task Group 55," Med. Phys. 25, 2093-2115 (1998).

4 S. Devic, "Radiochromic film dosimetry: Past, present, and future," Phys. Medica 27, 122-134 (2011).

5 M.J. Butson, P.K.N. Yu, T. Cheung, P. Metcalfe, "Radiochromic film for medical radiation dosimetry," Materials Science & Engineering R-Reports 41, 61-120 (2003).

6 F. Khan, The Physics of Radiation Therapy, 4 ed. (Lippincott Williams & Wilkins, Baltimore, MD, 2010).

7 M. Bazioglou, J. Kalef-Ezra, "Dosimetry with radiochromic films: a document scanner technique, neutron response, applications," Applied Radiation and Isotopes 55, 339-345 (2001).

8 B.C. Ferreira, M.C. Lopes, M. Capela, "Evaluation of an Epson flatbed scanner to read Gafchromic EBT films for radiation dosimetry," Phys. Med. Biol. 54, 1073-1085 (2009).

9 M.A. Stevens, J.R. Turner, R.P. Hugtenburg, P.H. Butler, "High-resolution dosimetry using radiochromic film and a document scanner," Phys. Med. Biol. 41, 2357-2365 (1996).

10 M. Fuss, E. Sturtewagen, C. De Wagter, D. Georg, "Dosimetric characterization of GafChromic EBT film and its implication on film dosimetry quality assurance," Phys. Med. Biol. 52, 4211-4225 (2007).

11 S. Devic, N. Tomic, S. Aldelaijan, F. DeBlois, J. Seuntjens, M.F. Chan, D. Lewis, "Linearization of dose-response curve of the radiochromic film dosimetry system," Med. Phys. 39, 4850-4857 (2012).

12 S. Devic, J. Seuntjens, G. Hegyi, E.B. Podgorsak, C.G. Soares, A.S. Kirov, I. Ali, J.F. Williamson, A. Elizondo, "Dosimetric properties of improved GafChromic films for seven different digitizers," Med. Phys. 31, 2392-2401 (2004).

13 J.F. Dempsey, D.A. Low, S. Mutic, J. Markman, A.S. Kirov, G.H. Nussbaum, J.F. Williamson, "Validation of a precision radiochromic film dosimetry system for quantitative two-dimensional imaging of acute exposure dose distributions," Med. Phys. 27, 2462-2475 (2000).

14 R.D.H. Chu, G. Vandyk, D.F. Lewis, K.P.J. Ohara, B.W. Buckland, F. Dinelle, "GAFCHROMIC DOSIMETRY MEDIA - A NEW HIGH-DOSE, THIN-FILM ROUTINE DOSIMETER AND DOSE MAPPING TOOL," Radiat. Phys. Chem. 35, 767-773 (1990).

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15 M.J. Butson, J.N. Mathur, P.E. Metcalfe, "Radiochromic film as a radiotherapy surface-dose detector," Phys. Med. Biol. 41, 1073-1078 (1996).

16 S. Pai, L.E. Reinstein, G. Gluckman, Z.G. Xu, T. Weiss, "The use of improved radiochromic film for in vivo quality assurance of high dose rate brachytherapy," Med. Phys. 25, 1217-1221 (1998).

17 Ashland, "Gafchromic Self-Developing Dosimetry Films," (2013). 18 S. Devic, N. Tomic, C.G. Soares, E.B. Podgorsak, "Optimizing the dynamic range

extension of a radiochromic film dosimetry system," Med. Phys. 36, 429-437 (2009). 19 T.A.D. Brown, K.R. Hogstrom, D. Alvarez, K.L. Matthews, K. Ham, J.P. Dugas, "Dose-

response curve of EBT, EBT2, and EBT3 radiochromic films to synchrotron-produced monochromatic x-ray beams," Med. Phys. 39, 7412-7417 (2012).

20 A. Sarfehnia, I. Kawrakow, J. Seuntjens, "Direct measurement of absorbed dose to water in HDR Ir-192 brachytherapy: Water calorimetry, ionization chamber, Gafchromic film, and TG-43," Med. Phys. 37, 1924-1932 (2010).

21 L.J. van Batturn, D. Hoffmans, H. Piersma, S. Heukelom, "Accurate dosimetry with GafChromic (TM) EBT film of a 6 MV photon beam in water: What level is achievable?," Med. Phys. 35, 704-716 (2008).

22 A.S. Meigooni, M.F. Sanders, G.S. Ibbott, S.R. Szeglin, "Dosimetric characteristics of an improved radiochromic film," Med. Phys. 23, 1883-1888 (1996).

23 Y.M. Zhu, A.S. Kirov, V. Mishra, A.S. Meigooni, J.F. Williamson, "Quantitative evaluation of radiochromic film response for two-dimensional dosimetry," Med. Phys. 24, 223-231 (1997).

24 E.R. Giles, P.H. Murphy, "Measuring skin dose with radiochromic dosimetry film in the cardiac catheterization laboratory," Health Physics 82, 875-880 (2002).

25 J.A. Sayeg, C.W. Coffey, W.L. McLaughlin, "LOW-ENERGY X-RAY DOSIMETRIC STUDIES WITH GAF CHROMIC RADIATION DETECTORS," Medical Physics (Woodbury) 18, 653 (1991).

26 S. Devic, J. Seuntjens, E. Sham, E.B. Podgorsak, C.R. Schmidtlein, A.S. Kirov, C.G. Soares, "Precise radiochromic film dosimetry using a flat-bed document scanner," Med. Phys. 32, 2245-2253 (2005).

27 H. Ohuchi, "High sensitivity radiochromic film dosimetry using an optical common-mode rejection and a reflective-mode flatbed color scanner," Med. Phys. 34, 4207-4212 (2007).

28 R.R. Mayer, F.W. Ma, Y. Chen, R.I. Miller, A. Belard, J. McDonough, J.J. O'Connell, "Enhanced dosimetry procedures and assessment for EBT2 radiochromic film," Med. Phys. 39, 2147-2155 (2012).

29 C.G. Soares, "CALIBRATION OF OPHTHALMIC APPLICATORS AT NIST - A REVISED APPROACH," Med. Phys. 18, 787-793 (1991).

30 P.J. Muench, A.S. Meigooni, R. Nath, W.L. McLaughlin, "PHOTON ENERGY-DEPENDENCE OF THE SENSITIVITY OF RADIOCHROMIC FILM AND COMPARISON WITH SILVER-HALIDE FILM AND LIF TLDS USED FOR BRACHYTHERAPY DOSIMETRY," Med. Phys. 18, 769-775 (1991).

31 W.L. McLaughlin, C.G. Soares, J.A. Sayeg, E.C. McCullough, R.W. Kline, A. Wu, A.H. Maitz, "THE USE OF A RADIOCHROMIC DETECTOR FOR THE DETERMINATION OF STEREOTAXIC RADIOSURGERY DOSE CHARACTERISTICS," Med. Phys. 21, 379-388 (1994).

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VIII. Appendix Figure 1. Plot of net optical density as a function of dose for MD-55-2 radiochromic film. From

The Physics of Radiation Therapy, 4th ed., Khan, F.6

Figure 2. Absorption spectrum of Gafchromic EBT2 radiochromic film before (orange curve) and after (green curve) exposure to 2.0 Gy x-rays. From Ashland® website17.