Optimal sensitometric curves of Kodak EDR2 film for dynamic intensity modulated radiation therapy verification
Division of Radiation Oncology, Department of Radiology,
Faculty of Medicine, King Chulalongkorn Memorial Hospital, Bangkok, Thailand
Purpose: To investigate the optimal sensitometric
curves of extended dose range (EDR2) radiographic film in terms of depth, field
size, dose range and processing conditions for dynamic intensity modulated
radiation therapy (IMRT) dosimetry verification with 6 MV X-ray beams.
Materials and methods: A Varian Clinac 23 EX linear
accelerator with 6 MV X-ray beam was used to study the response of Kodak EDR2
film. Measurements were performed at depths of 5, 10 and 15 cm in MedTec
virtual water phantom and with field sizes of 2x2, 3x3, 10x10 and 15x15 cm2.
Doses ranging from 20 to 450 cGy were used. The film was developed with the
Kodak RP X-OMAT Model M6B automatic film processor. Film response was measured
with the Vidar model VXR-16 scanner. Sensitometric curves were applied to the
dose profiles measured with film at 5 cm in the virtual water phantom with
field sizes of 2x2 and 10x10 cm2 and compared with ion chamber data.
Scanditronix/Wellhofer OmniProTM IMRT software was used for the evaluation of
the IMRT plan calculated by Eclipse treatment planning.
Results: Investigation of the reproducibility and
accuracy of the film responses, which depend mainly on the film processor, was
carried out by irradiating one film nine times with doses of 20 to 450 cGy. A
maximum standard deviation of 4.9% was found which decreased to 1.9% for doses
between 20 and 200 cGy. The sensitometric curves for various field sizes at
fixed depth showed a maximum difference of 4.2% between 2x2 and 15x15 cm2
at 5 cm depth with a dose of 450 cGy. The shallow depth tended to showed a
greater effect of field size responses than the deeper depths. The
sensitometric curves for various depths at fixed field size showed slightly
different film responses; the difference due to depth was within 1.8% for all
field sizes studied. Both field size and depth effect were reduced when the
doses were lower than 450 cGy. The difference was within 2.5% in the dose range
from 20 to 300 cGy for all field sizes and depths studied. Dose profiles
measured with EDR2 film were consistent with those measured with an ion
chamber. The optimal sensitometric curve was acquired by irradiating film at a
depth of 5 cm with doses ranging from 20 to 450 cGy with a 3�3 cm2
multileaf collimator. The optimal sensitometric curve allowed accurate determination
of the absolute dose distribution. In almost 200 cases of dynamic IMRT plan
verification with EDR2 film, the difference between measured and calculated
dose was generally less than 3% and with 3 mm distance to agreement when using
gamma value verification.
Conclusion: EDR2 film can be used for accurate
verification of composite isodose distributions of dynamic IMRT when the
optimal sensitometric curve has been established. � 2008 Biomedical
Imaging and Intervention Journal. All rights reserved.
Keywords: IMRT treatment plan verification, EDR2 film,
The dosimetric verification of intensity modulated
radiation therapy (IMRT) requires an accurate delivery of the radiation dose.
Radiographic film is a popular dosimeter for determining the two-dimensional
dose distribution. It has a good spatial resolution which is due to the small
grain size and the small aperture of the light beam of the densitometer.
However, the film response for photon beams is not considered to be an accurate
dosimeter due to the variation of the film response with the energy, depth and
field size. In the IMRT beam, the fluence spectrum varies significantly across
a single fluence map from the combined in-field and outer-penumbral areas of
beamlets. Also the field sizes vary from field to field and from patient to
patient. The increase in the low energy photon spectrum in the penumbral region
as depth increases can cause a significant increase in film response due to the
photoelectric effect of silver bromide in the photographic emulsion . To use
film in dosimetry verification of the IMRT plan, Kodak Extended Dose Range 2
(EDR2) film was introduced. It is a very low speed, fine grained film. The
silver content of EDR2 film is about one-half that of Kodak XV2 film so the
sensitivity of the film is lower. The EDR2 film can be exposed to a dose of at
least 300 cGy . The decrease in the emulsion thickness of EDR2 films should
decrease the dependence of the film response with depth, field size and energy.
Zhu et al.  and Esthappan et al.  showed the lower response of EDR2 film
with depth, field size and energy compared to XV film. However, for smaller
fields of less than 4�4 cm2 which are important in IMRT, few data are available
for the response of EDR2 film. The other problem associated with film dosimetry
is the significant effect of processing conditions, namely type of processor,
processing time and temperature . The scanner type also has an effect when
reading the optical density. The solution to using film as a good verification
tool is to reduce the error associated with the dependence on depth, field size
and environmental conditions of the processor and scanner.
Even though other two-dimensional tools have been
introduced for IMRT verification such as Gafchromic External Beam Therapy (EBT)
film, two-dimensional (2D) diode array or ionisation array, limitations are
still observed. The variation in the spatial uniformity of the EBT film limits
the accuracy and precision of the results . The disadvantage of the 2D diode
array is the spatial resolution, even though new software with a stepper
platform can deliver multiple beams with the diode array in different positions
providing more measurement points that can be superimposed to give a higher
resolution measurement array . However, film is superior in terms of higher
resolution which is advantageous for IMRT dosimetry verification.
In this study, the sensitometric curves of EDR2 film were
investigated for field sizes of 2�2, 3�3, 10�10 and 15�15 cm2 at
depths of 5, 10 and 15 cm to select the optimal depth and field size for
dosimetric film calibration for dynamic IMRT verification. The idea was to
irradiate a single calibration film with one depth and one field size for the
range of doses used in the IMRT plan. The error due to the processing
conditions and the quality of the densitometry system would be examined and
Materials and Methods
Preparation of densitometer and film processor
The Vidar VXR-16 automatic film scanning densitometer with
Scanditronix/Wellhofer OmniProTM IMRT software was calibrated with a Kodak step
wedge film to define the relationship between the densitometer signal and the
net optical density. The film scanner operates with a resolution of 142 dots
per inch (0.179 mm/pixel) and a depth of 16 bits. The special step wedge film
was delivered from the manufacturer with an optical density range from 0.04 to
3.65. The reference density value for each step of the step wedge film was
entered into the automatic film scanning densitometer and the graph of the
signal versus the net optical density was plotted.
������� To reduce the effect of film processor conditions,
daily quality control of the Kodak RP X-OMAT Model M6B automatic film processor
has been performed with a Kodak process control sensitometer. This sensitometer
consists of a stable light source, timer, diffusion panel, optical step wedge
and pressure plate to eliminate the air gap between the film contacts during
exposure. When a film is exposed on both sides to light from the device, the
optical step wedge provides a series of light intensities. The film is
processed and curves are plotted of step number versus optical density. For
routine processor monitoring, three measurements of optical density should be
made . One measurement is the base plus fog density and should be made in a
region of the film that is not exposed to the light. The second measurement is
obtained in a region of the wedge image where the optical density is near 1.0
over the base plus fog density. This is referred to as the speed index. The
third measurement should be made in a region of the wedge image where the
density is about 0.25 and 2.0 over the base plus fog density. The density
difference of these steps is denoted as the contrast index. Moreover, the
temperature of the developer solution should be noted. To establish the average
baseline values for these variations, measurements were taken for five films.
The tolerance values were set so that the measurements of these parameters for
the quality assurance (QA) film were compared with the reference and tolerance
values of: Speed Index 1.18 � 0.2, Contrast Index 1.85 � 0.2, base+fog 0.18 �
0.03, and Temperature Index 34.8 � 0.3 to confirm the consistency of the
processor performance. When the daily QA film measurement had been performed
with the parameter values within the tolerance limits, then the dosimetry film
could be developed.
Reproducibility of results with film
The effect of film processor and the reproducibility of
results with film were investigated by irradiating each film at the depth of
maximum dose (dmax), in MedTec virtual water phantom using 3x3 cm2
field size with the dose ranging between 20 and 450 cGy at 100 cm
source-to-axis distance (SAD). This study has been performed nine times. The
sensitometric curves were plotted on the same graph to illustrate the deviation
of the responses of the films developed at the different times.
Irradiation of film and sensitometric curves
The dose-response curves of the Kodak EDR2 film to 6 MV
X-ray beams from a Varian Clinac 23EX linear accelerator were studied for
depths of 5, 10 and 15 cm and field sizes of 2�2 cm2, 3�3 cm2,
10�10 cm2 and 15�15 cm2 with delivered doses of 20-450
cGy. Small and moderate field sizes were chosen to investigate the film
responses. The maximum dose of the composite IMRT plan could be raised to 400
cGy, so doses of 20-450 cGy were selected for the study. The 25.4�30.5 cm2
EDR2 films were irradiated in virtual water phantom; the films were placed at
100 cm SAD, and sandwiched between virtual water phantom slabs. Each phantom
slab had dimensions of 30�30 cm2 with various thicknesses. The
backscatter layers were kept at 20 cm. The EDR2 films were oriented normal to
the central axis of the beam.
The optical densities along the central axis were measured
using the film scanner. The net optical densities were obtained by subtracting
the optical density corresponding to base+fog. Sensitometric curves were
plotted as a function of net optical density versus dose for the set of fixed
depths and various field sizes and the set of fixed field sizes and various
depths for each energy studied.
Measurement of beam profile
Beam profiles were measured with films in MedTec virtual
water phantom for field sizes of 2x2 and 10x10 cm2. This phantom
agrees with water within 0.5% for 6 MV X-ray beams. The films were placed
between slabs of virtual water phantom at 5 cm depth. The central axis of the
beam was perpendicular to the surface of the phantom. The beam profiles were
plotted by applying the sensitometric curve to convert the optical density to
dose and by normalising doses at the off-axis points to the dose at the central
axis. Then the beam profiles at 5 cm depth measured with the
Scanditronix/Wellhofer CC13 0.13 cc ion chamber in the Scanditronix/Wellhofer
3D water phantom system were compared with beam profiles measured with film at
the same field size.
Optimal calibration curve and verification of clinical IMRT plan for the
After analysing the data, the optimal field size and depth
were obtained and used to construct a calibration curve to convert the optical
density to dose. The aim was to irradiate a single film with many doses at one
definite field size and depth for each set of IMRT plan verifications. To
acquire the correct doses of the small field having a few centimetres of space
between each other in the same film, the individual doses delivered to small
regions at the optimal depth were measured with the Scanditronix/Wellhofer
model CC13 0.13 cc ionisation chamber and Scanditronix/Wellhofer model DOSE1
electrometer in virtual water phantom. Then the monitor unit for the delivered
dose was calculated and used to irradiate the calibration film. The treatment
plan for each IMRT patient was transferred to the virtual water phantom and
exposed as the composite field in the coronal plane, at optimal depth close to
the isocentre of the patient. All fields were at the same zero gantry angle.
This treatment plan used the same beam fluences, energies and monitor units as
in the patient plan. The phantom with EDR2 film was subsequently irradiated.
Then the absolute isodose distribution in each IMRT plan was compared with the
calculated isodose distribution from Eclipse treatment planning using
Scanditronix/Wellhofer OmniProTM IMRT software.
Calibration of densitometer
The relationship between the signal of the
analogue-to-digital converter (ADC) and the optical density from the step wedge
is shown in Figure 1. This relationship was almost linear for optical densities
between 0.2 and 3.0. Saturation of the signal began at an optical density of
about 3.0. This result indicates that the optical density of the film used
should not be more than 3.0.
Reproducibility of results with film
The reproducibility of results with film is shown in Table
1 for nine measurements with doses ranging from 20 to 450 cGy and the
sensitometric curves are shown in Figure 2. The fitted line represents the
average for all data. The percent standard deviations increased with higher doses
to a maximum of 4.9%. Good reproducibility and accuracy of results were
obtained with film when the irradiated doses were between 20 and 200 cGy; the
reproducibility was within 1.9%.
Response of film with fixed depths and varying field sizes
Sensitometric curves were plotted as a function of net
optical density versus dose for field sizes of 2�2 cm2, 3�3 cm2,
10�10 cm2 and 15�15 cm2 with fixed depths of 5, 10 and 15
cm as shown in Figures 3a, 3b and 3c, respectively. The marker points in the
figure represent measured data points and the solid lines illustrate the fitted
All of the sensitometric curves for fixed depth and with
various field sizes showed the difference in the film response with field size
when the dose was increased. The effect of field size tended to be greater at
shallow depths (5 cm) than at deeper depths (15 cm). The maximum film response
differences between 2x2 and 15�15 cm2 fields at a dose of 450 cGy
were within 4.2%, for the fixed depths studied.
Response of film with fixed field sizes and varying depths
Sensitometric curves for depths of 5, 10 and 15 cm with
fixed field sizes of 2�2 cm2, 3�3 cm2,
10�10 cm2 and 15�15 cm2 are shown in
Figures 4a, 4b, 4c and 4d respectively. The curves showed less difference of
film response between the different depths for all fixed field sizes studied
compared with the effect of different field sizes for fixed depths. The maximum
film response differences between 5 and 15 cm depth at a dose of 450 cGy were
within 1.8%, for the fixed field sizes studied.
Comparisons of beam profiles between film and ionisation
chamber measurements for field sizes of 2�2 and 10�10 cm2 at 5 cm
depth are shown in Figures 5a and 5b, respectively. The doses for the film were
obtained from the sensitometric curves and these were then normalised to the
dose at the central axis. The dose beam profiles measured with the film were
superimposed on the dose beam profile measured with the ionisation chamber in
the water phantom. The agreement of profiles for small and large field sizes
was within 2 mm. These results agreed with other studies [3,4].
Optimal film calibration curve for IMRT dosimetry verification
The goal of film calibration is to convert a film density
value obtained from an actual beam measurement into an accurate tissue dose
value. As IMRT field size and fluence maps vary from field to field, from
treatment site to treatment site, and from patient to patient, there is no
simple way of modelling such variability of fluence maps within a phantom and
incorporating such a model in a calibration procedure. Therefore, in general,
the use of a relatively small field size, such as 6�6 cm2 or 7�7 cm2,
for calibration is recommended .
For our study, after the dosimetric properties of EDR2
film had been investigated, a small field size of 3�3 cm2 was
selected using multileaf collimators to generate regions for doses from 20 to
450 cGy on a single sheet of film. The film was placed in the virtual water
phantom at an isocentre of 5 cm depth perpendicular to the beam. These
parameters were taken to be the optimal conditions for the film calibration
curves for six X-ray beams. The 3�3 cm2 field was chosen due to the
reliable dose measurement when using the 0.13 cc ion chamber for output
measurement. However, 3�3 cm2 and 2�2 cm2 field sizes
gave sensitometric curves which were not significantly different over the range
of doses and depths studied. Small fields were chosen because most of the IMRT
fields are composed of small fields and a significant difference in film response
was found when compared with the larger fields of 10�10 and 15�15 cm2.
The 5 cm depth was selected because it was close to the isocentre depth of head
and neck cancer which is mostly treated in this institute. The beam profile for
a small field measured with film at 5 cm depth agreed with that measured with
the ionisation chamber.
Verification of clinical IMRT plan
The calibration curve was measured for the dose range of
20 to 450 cGy for every IMRT plan verification. An example of verification of the
IMRT plan in the virtual water phantom for composite fields at the same gantry
angle of zero degrees was demonstrated using Scanditronix/Wellhofer OmniProTM
IMRT software by looking at the absolute dose distribution, beam profile and
gamma evaluation  as shown in Figure 6.
The fluence map obtained from film in Figure 6a looks
visually similar to the fluence map from Eclipse calculation in Figure 6b. The
cross-hair in both fluence maps is the reference point for alignment. The
isodose distribution in Figure 6c shows the overlap between the solid line of
the EDR2 film measurement and the dotted line of the calculation. The gamma
evaluation in Figure 6d is shaded if the gamma values are more than 1 with the
criteria of 3% dose difference and 3 mm distance to agreement. The profile in
Figure 6e shows a reasonable match between measurement and calculation apart
from the penumbral region where the planning system model underestimated the
dose. The measured film profile shows better resolution than the calculated
profile because the resolution of film was 0.2 mm while the treatment planning
was 2 mm.
During the period October 2005-May 2007, about 200 IMRT
plans for nasopharynx, lung, prostate and other cancers were verified by EDR2
film. Most of the plans showed good agreement between the measured and
calculated dose in the central region but some higher doses were observed from
film at the edge of the beams. This result illustrates the greater response of
film in the low energy area and also that the algorithm calculated by treatment
planning was underestimated. The gamma values were mostly less than 1 except
the area near the edge of the field. The five dose points from film at
positions (0, 0), (0, 2), (0, -2), (2, 0) and (-2, 0) for 42 IMRT plans were
read , and the mean difference from calculation of these points was 2.4%
with a standard deviation of 1.07. The result demonstrated the accuracy of IMRT
dose distribution using EDR2 film which implied good film calibration curves.
Discussion and Conclusions
Film can be used for the determination of relative or
absolute dose distributions if a suitable calibration curve is available. In
principle, absolute dose values are obtained if the film sensitivity is
identical for all films. But in a real situation, the film sensitivity is
rarely identical due to the variation in emulsion coating. The same batch of
film should be used. The processing conditions affect the film response, and
investigation of the processor equipment type, chemicals, processing time and
temperature should be considered . There are also variations in film
densitometer measurements which lead to inaccuracies in optical density
readings. However, film sensitivity is dependent on energy, depth, and field
size, and the conversion of dose is not a simple procedure. To determine a
suitable calibration curve for converting optical density to absolute dose,
these parameters need to be studied so that the optimal calibration curve will
give the correct dose value in all patients in the IMRT field.
The EDR2 films used in this study came from the same batch
to reduce the deviation of sensitivity due to variation in emulsion coating
thickness. A small difference in film optical density response would be
obtained for repeated irradiation and for films developed at different times if
the complete daily QA had not been performed before developing film dosimetry.
For accurate determination of the absolute dose distribution in the IMRT plan
verification, the doses given to the film should be in the order of 200 cGy so
that we can measure the absolute dose with an accuracy within 2.0%. At the same
time, using EDR2 film for the range of depths and field sizes investigated
here, our results indicated that for megavoltage photon beams, sensitometric
curves are slightly dependent on field size and depth of calibration. The
difference in film response due to field size and depth was within 2.5% for
doses from 20 to 300 cGy. Chetty and Charland  reported comparable results
for the variation of optical density in the order of 2-3% for field sizes of 3x3
cm2 and 10x10 cm2 at dmax values of 5 and 15 cm. This
implies that the field size and depth have a slight effect on the response of
the film. The reduction in silver content and smaller grain size of EDR2 film
reduced the energy dependence problem. The results were confirmed by Yeo and
Kim  who mentioned that EDR2 film is less sensitive to scattered low energy
A calibration curve was generated in a single sheet to
convert film optical density to dose with exposures from 20 to 450 cGy, 3x3 cm2
field size, 5 cm depth and 100 cm SAD. The advantage of choosing these
parameters as a single film is because it can be irradiated and developed at
the same time so there is no effect of film development.
If a composite IMRT plan was selected such that the
average daily fraction dose was in the order of 200-300 cGy, the difference
between measured and calculated dose was generally less than 3%. On the other
hand, if the average dose went up to 300-400 cGy, the difference could increase
up to 6% because the effect of field size was dominant at doses higher than 300
cGy and also because of the inaccuracy of the densitometer readings at higher
optical densities. However, the dose selected to calibrate the film must be high
enough to cover the maximum dose in the IMRT plan. The dose range would be
adjusted according to the absolute isodose distribution in the planning
because, for each calibrated film set, only eight levels of dose could be
irradiated. The response of the film with the actual gantry angle showed
agreement with the calculation in the same direction as the beam at zero gantry
angle . The point dose measured by the ionisation chamber at the same time
as film measurement showed good agreement. For 200 IMRT plans, we were quite
successful in IMRT verification with film. Based on our experience, we believe
that EDR2 film is the tool of choice for IMRT plan verification.
We would like to thank Puangpen Tangboonduangjit, PhD from
the Division of Radiotherapy, Department of Radiology, Ramathibodi Hospital,
Mahidol University, Bangkok, Thailand for her advice during writing the
manuscript. The research was supported by the Ratchadapiseksompotch Fund,
Faculty of Medicine, Chulalongkorn University, Bangkok, Thailand.
Figure 1 Densitometer calibration curve.
Figure 2 Sensitometric curve for nine measurements with a 3�3 cm2 field, dmax at 100 cm SAD.
Figure 3 Sensitometric curves for varying field sizes of 2�2 cm2, 3�3 cm2, 10�10 cm2 and 15�15 cm2 with fixed depths of (a) 5 cm, (b) 10 cm and (c) 15 cm.
Figure 4 Sensitometric curves for varying depths of 5, 10 and 15 cm with fixed field sizes of (a) 2�2 cm2, (b) 3�3 cm2, (c) 10�10 cm2 and (d) 15�15 cm2.
Figure 5 Dose profile measured with EDR2 film in solid water phantom and an ion chamber in water phantom for (a) 2�2 cm2 field and (b) 10�10 cm2 field at 5 cm depth.
Figure 6 Verification of dose distribution calculated by Eclipse treatment planning and measured by EDR2 film for 6 MV X-ray beams: (a) Fluence map from film measurement; (b) Fluence map from Eclipse calculation; (c) Absolute isodose distribution comparison; (d) Gamma value verification and (e) Profile comparison between EDR2 film and Eclipse treatment planning.
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|Received 3 July 2007; received in revised form 1 November
2007, accepted 17 November 2007
Correspondence: Division of Radiation Oncology, Department of Radiology, Faculty of Medicine, King Chulalongkorn Memorial Hospital, Bangkok, Thailand. Tel.: +662-256-4334; Fax: +662-256-4590; E-mail: firstname.lastname@example.org (Sivalee Suriyapee).
Please cite as: Suriyapee S, Pitaxtarnin NN, Oonsiri S, Jumpangern C, Israngkul Na Ayuthaya I,
Optimal sensitometric curves of Kodak EDR2 film for dynamic intensity modulated radiation therapy verification, Biomed Imaging Interv J 2008; 4(1):e4
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