VII Radiation Physics & Protection Conference, 27-30 November 2004,Ismailia-Egypt
EG0500343 Dosimetric Measurements Using Radiographic Film and Ionization Chamber in Radiotherapy Ahmed L. El-Attar, A.H. El-kamel, M. A. Hefni Physics Department, Faculty of Science, Assiut University, Assiut, Egypt Moamen M.O. Moustafa Radiotherapy Department, South Egypt Cancer Institute, Assiut University, Assiut, Egypt
ABSTRACT
The aim of the present study is to improve the efficiency of lateral scattering filters in radiographic film dosimetry at high-energy and compare it with ionization chamber (I.C.). This enables the use of radiographic film as a dosimeter characterized by many advantages over I.C. especially in dynamic radiation fields. In the present study, open radiation fields (4×4, 10×10, 15×15 and 20×20) cm2 and wedged fields (of 15°, 30°, 45° and 60° wedge angles) are chosen. Central axis and off-axis doses measured by I.C. are considered as reference measurements for radiographic films with and without lateral scattering filters. This study revealed agreement of film measurements (using lead filters) with I.C. measurements within ±3%.
Key Words: Radiographic film, Ionization chamber, Dosimetry, Linac, Wedge, static field, Dynamic field, Phantoms, Radiotherapy.
INTRODUCTION
Dosimetry of radiotherapy treatment beams is a very important procedure because successful radiation therapy requires an accurate delivery of dose to a cancerous volume of tissue. It is believed that a decrease of 10% - 15% in dose delivery will result in a decrease in the chance of cure by a factor of 2 or 3, while an increase in dose will similarly increase the chance of irreversible damage [1]. Photon beam dosimetry in radiotherapy requires an accurate three-dimensional in-phantom dose measurement with a relatively good spatial resolution and a reasonable measurement time. In addition to the properties required for normal static-field (constant field size) measurement, dynamic-field photon beam dosimetry requires simultaneous integration of doses at multiple points in a phantom. The reason is that the beam collimator of a dynamic-field treatment constantly moves to create a desirable dose distribution in a patient (or a phantom) [2]. Existing method of dosimetry for the quality assurance of radiotherapy treatment includes the use of equipment such as ionization chambers, thermoluminescent dosimeters and diode detectors. Ionization chambers are universally regarded as the standard dosimeter for calibration and dose measurement for radiation therapy. Ionization chamber in water phantom system has been recommended for the isodose distribution measurement. However they have some shortcomings in their usage. Measurement with an ionization chamber provides only selective information with poor spatial resolution in addition measurement time is relatively long [3, 4].
For dynamic-field dosimetry, a large array of ionization chamber must be used to simultaneously measure the doses at various positions in a phantom. In addition to the economical disadvantage, simultaneously placing a large number of ionization chambers in a phantom also alters the dose distribution being measured. This argument about ionization chambers also applies to the thermoluminescent dosimeters and diode detectors. Radiographic film is potentially the ideal detector for determining dose distribution for dynamic beams and for studying combinations of stationary beams treated sequentially (e.g. gap dosimetry). Both of these situations are difficult to measure using conventional water phantom dosimetry system, since the dose distribution changes with time [5]. So
٦٧٥ VII Radiation Physics & Protection Conference, 27-30 November 2004,Ismailia-Egypt
film has many desirable features that make it a good candidate dosimeter for radiotherapy photon beams (both static and dynamic fields).
EXPERIMENTAL TECHNIQUE
The present experimental work describes the performance of radiographic film under different conditions – lateral scatter filter, physical and virtual wedge – at high energy and compares it with the ionization chamber as a reference. For this reason open radiation fields (4×4, 10×10, 15×15 and 20×20) cm2 and wedged fields (of 15°, 30°, 45° and 60° wedge angles) are chosen. Percentage depth dose (PDD) and off-axis doses ratio (OAR) measured by ionization chamber are considered as reference measurements for radiographic films with and without lateral scattering filters.
The apparatus used in this work consists mainly of the source of radiation – linear accelerator – ionization chamber and radiographic film to measure the dose.
Source of Radiation:-
All experiments were performed applying medical linear accelerator (Siemens Mevatron MD2). The linear accelerator (Linac) accelerates electrons by high-frequency electromagnetic waves to high energies through a linear tube. The accelerating voltage is 6 and/or 15 MV. The high-energy electron beam strikes a target to produce collimated photon beam with effective average energies 2 and/or 5 MeV respectively. The Linac is calibrated for both energies to deliver 100 cGy per 100 monitor units (MU) for field size (10 × 10) cm2 at source to surface distance (SSD) 100 cm and depth of maximum dose (dmax). The calibration is done using the Netherlands code of practice with the equation:-
D = M × N × C K w,u (1) where D is the absorbed dose, M is the electrometer reading (charge) corrected for air temperature and pressure, ion recombination and polarity effects, NK is the air kerma calibration factor for the ionization chamber (the ionization chamber correction factor) and Cw,u is the air to water conversion factor which depends on radiation energy.
Detectors:-
Two different types of radiation detectors are used in this study, ionization chamber (Farmer type) as a gas filled detector and radiographic film as a solid state detector. For the relative dosimetry (PDD and OAR) we used two ionization chambers having an internal volume of 0.125 cm3, one as sample and another as a reference. The Kodak X-Omat V radiographic films are commonly applied. These are relatively low-speed films. The emulsion is coated on both sides of the plastic base.
Phantoms:-
For ionization chamber measurements a water phantom (model 3112) was used. This water phantom is a large tank 60 × 60 cm2 in length and width and 70 cm in depth. It has a movable remote- controlled device carrying the ionization chamber operated by a computer outside the treatment room. For the radiographic film measurements a solid phantom was used. It consists of a custom designed set of (30 × 30) cm2 blocks of polystyrene (density = 1.03 g/cm3, virtual water) forming a total thickness of 26 cm.
Dosimetry System:-
The dosimetry system (DynaScan, CMS Associates Inc.) is a computerized control system intended to help automatic collection of the linear accelerator radiation therapy beam characteristic
٦٧٦ VII Radiation Physics & Protection Conference, 27-30 November 2004,Ismailia-Egypt
data. It provides utilities for displaying, plotting and modifying scan data. This system consists mainly of an operator computer with software to control the measurements.
A. Ionization Chamber Reference Doses Measurements of Static Fields:-
For every field size (4 × 4, 10 × 10, 15 × 15 and 20 × 20) cm2 open (without using wedge), or wedged (angels 15˚, 30˚, 45˚ and 60˚) the PDD and profiles (OAR) at depths of dmax, 5, 10 and 20 cm were measured for both 6 and/or 15 MV. The percentage depth dose is defined as follows
ionization PDD = d × 100% (2) ionization d max where d is the depth of interest and dmax is the depth of maximum dose (reference depth). These dose distribution measurements are considered as the reference data.
B. Radiographic Film Measurements Doses in Static Fields (Unmodified Solid Phantom)
In the unmodified solid phantom i.e. without using the lateral scattering filters, the films were placed between slabs of solid phantom and rotate the gantry (the head of the Linac) to 270º. Approximately 2 cm of film was left above the phantom surface. This method produces a well-defined beam entrance line on the film [5].
For each experiment the phantom was leveled, where the SSD is 100 cm. In this case the experiment was repeated, as with the ionization chamber, for every field size open or wedged. PDD and profiles (OAR) at depths of dmax, 5, 10 and 20 cm were measured.
• Film Calibration
Radiation detectors can be used as dosimeters if their response is converted and calibrated to dose reading. For (OD – D) (optical density - dose) calibration process, different films from the same box were irradiated by zero (un irradiated film) – 10 – 20 – 30 – 40 – 50 – 60 – 70 – 80 – 90 - 100 monitor unit (MU), for field size (10×10) cm2, SSD 100 cm, and depth 5 cm. The measured value of zero (un- irradiate) film is subtracted from the value of the other doses. Good dosimetric results can be obtained if control films are irradiated with small to moderate field sizes (up to about 15 × 15 cm2), at moderate depths (up to 15 cm) [6]. To convert the MU readings to dose the dose of irradiation delivered to the control films was measured by the ionization chamber. The dose to each film was then plotted as a function of net optical density and the resulting calibration graph was used to convert optical density to dose. This curve may be called calibration curve, sensitometric curve or H & D (Hunter & Driffield) curve [7].
• Film Exposure Techniques
For all experimental films, the film on its ready back paper was sandwiched within the phantom slabs and the phantom was placed onto the external therapy machine couch. For irradiation, with the beam central axis parallel to the plane of the film, the gantry is rotated to 270º but for perpendicular irradiation, the beam central axis is normal to the plane of the film; the gantry was 0º where the beam was directed towards the floor.
Kodak X-Omat RP processor, with developer temperature fluctuations by less than ±1 ºC for all films and 90 second automatics procession was used. The optical density (OD) is a measure of relative transmission on a logarithm basis. For example, a material measured an OD = 1 means that the light transmission between the source of light and detector is decreased to 10% of its original value (without the material).
٦٧٧ VII Radiation Physics & Protection Conference, 27-30 November 2004,Ismailia-Egypt
The optical density was measured using laser densitometer model CMS-1710 with the following specifications: The source is a laser diode operating at 655 ± 25 nm wave-length, the laser spot size at the plates is 0.25 ± 0.05 mm in diameter and the positioning resolution is 0.25 mm in both axis with an accuracy of 0.025 mm. The output of densitometer is directly proportional to the optical density.
C. Radiographic Film Using The Lateral Scattering Filter Measurements in Static Fields (Modified Solid Phantom)
The modified solid phantom was designed to prevent scattered low-energy photons, which cause over-response, from reaching the film. A lead foil with thickness of 0.25 mm was placed in the phantom parallel to the film and equidistant, 5 mm, from the film plane on both sides. Using this modified solid phantom, as in the case of unmodified solid phantom and ionization chamber, for field sizes (4×4, 10×10, 15×15 and 20×20) cm2 open and wedged, angels 15˚, 30˚, 45˚ and 60˚, the PDD and profiles (OAR) at depths of dmax, 5, 10 and 20 cm were measured.
D. Radiographic Film Using The Lateral Scattering Filter in Dynamic Fields (Virtual Wedge Data Measurements)
Virtual wedge is an alternative wedging system uses no physical wedge and produces angled isodose lines by scanning one of the collimator jaws across the field while the beam is on. This dynamic method dose not results in hardening or attenuation of the beam. Using the modified solid phantom, profiles at depths of dmax, 10 and 20 cm for different wedge angles 15º, 30º, 45º and 60˚ for field sizes (4×4, 10×10 and 20×20) cm2 were measured. Comparison of doses delivered to some selected points measured using the ionization chamber and those measured by radiographic film in modified solid phantom were made. For off-axis points, we select distances ±1 cm for field size (4×4) cm2, distances ± 2.5 cm for (10×10) cm2 and distances ±5 and ±7 cm for (20×20) cm2.
RESULTS AND DISCUSSION
A. Dose versus density. The results for the field size 10 × 10 cm2, SSD 100 cm and depth 5 cm control curve shown on figure (1) emphasize the importance of the relation between the dose and the density. Ideally this graph should be a straight line at low doses, but was found to be slightly nonlinear for this film type and densitometer combination. Therefore, for this study a simple fit to third order polynomial equation yielded acceptable results for maximum doses of 90 cGy or less. The choice of 10 × 10 cm2 at SSD 100 cm and 5 cm depth for all calibration films is based on the fact that further reduction in field size or increase in depth did not affect the film sensitivity curve. It seems apparent that this set of conditions represents the film response to the higher primary beam energy with minimal film sensitivity enhancement that would result from the lower scatter energies.
٦٧٨ VII Radiation Physics & Protection Conference, 27-30 November 2004,Ismailia-Egypt
120
6 MV 15 MV
100 Poly. (6 MV) Poly. (15 MV) y = -4E-09x3 + 3E-05x2 + 0.0354x + 0.4068
80
60 Dose (cGy) Dose
40
y = -3E-09x3 + 3E-05x2 + 0.0324x + 0.3722 20
0 0 200 400 600 800 1000 1200 1400 1600 Optical density (ADCU)
Figure (1): -Characteristic curve for film calibration to dose. The ADCU value refers to the internal ADC (analog to digital converter) unit associated with the measurement scale of the optical density, which is between 0 and 4095.
B. Comparison between unmodified and modified solid water phantom.
a. For open fields:-
The percentage depth doses for -6MV- open fields (4×4) cm2 are drown in figure (2). While the dose profiles measured at depths dmax, 5, 10 and 20 cm as example is drown in figure (3).
120 6MV X-ray Open 4cm x 4cm At Different Depths
6MV X-ray Open 4cm x 4cm 120 dmaxcm 100 100 5 cm 80 80 10 cm 60 60
Relative Dose (%) Dose Relative 40 40 20 cm IC Percentage Depth Dose (%) Dose Depth Percentage Film Without Filter 20 20 Film With Filter 0 -5 -4 -3 -2 -1 0 1 2 3 4 5
0 Distance From Central Ray (cm) 0 2 4 6 8 101214161820 IC Film Without Filter Film With Filter Depth (cm)
Figure (2): -Energy 6MV percentage Figure (3): -Energy 6MV dose profiles depth dose for open field size (4×4) cm2. for open field size (4×4) cm2 at depths dmax, 5, 10 and 20cm.
In addition, for -15MV- the percentage depth doses and dose profiles at depths dmax, 5, 10 and 20 cm for open field size (4×4) cm2 as example are shown in figures (4), (5).
dmaxcm
٦٧٩ VII Radiation Physics & Protection Conference, 27-30 November 2004,Ismailia-Egypt
120 120 15MV X-ray Open 4cm x 4cm At Different Depths 15 MV X-ray Open 4cm x 4cm 100 100 5 cm
80 80 10 cm 60
60 Relative Dose (%) Dose Relative 40 20 cm 40 IC IC 20
Percentage DepthPercentage Dose (%) Film Without Filter Film Without Filter 20 Film With Filter Film with Filter 0 -5-4-3-2-1012345 Distance From Central Ray (cm) 0 02468101214161820 Depth (cm) Figure (4): -Energy 15MV percentage Figure (5): -Energy 15MV dose profiles 2 depth dose for open field size (4×4) cm2. for open field size (4×4) cm at depths dmax, 5, 10 and 20cm.
The resulting percentage depth dose curve measured from film for -6MV- was compared to the actual percentage depth dose measured with the ionization chamber in water phantom. Relative to the ionization chamber measurements, (and apart from the surface dose which is affected by the finite diameter of the ionization chamber) the film percentage depth doses are higher by a range from 0-7% for field (4×4) cm2, 0-9% for field (10×10) cm2 and (15×15) cm2 and from 0-10% for field (20×20) cm2.
On the other hand for beam energy of -15MV- the film percentage depth doses are higher by a range from 0-2% for field (4×4) cm2, 0-9% for fields (10×10) cm2, (15×15) cm2 and from 0-11% for field (20×20) cm2. It is to be noted that the range of over-response increases with increasing of depth and field size.
In order to address the over-response of film to very low energy scattered photons, lead foil of thickness 0.25 mm was placed in the phantom parallel to the film plane. This over-response was reduced to reach 3% maximum for both -6MV- and -15MV-.
Off-axis photon spectra are known to differ from the spectrum on the beam axis especially in the penumbra region. This effect could influence the accuracy of dose profiles measured with film. The results show a good overall agreement between ionization chamber measurements and film with filter measurements. Small differences occur in the steepness of penumbra and for points beyond the geometrical field edge. The penumbra measured with film is generally somewhat steeper than the ionization chamber penumbra; this difference in penumbra width is due to the finite diameter of the ionization chamber [8].
Burch et al., 1997 [5] reported a 17% over-response between film without using scattering filter and ionization chamber central axis depth dose measurements for field (6×6) cm2 and 65% for (25×25) cm2. However, using the lateral scatter filtering technique, less than 4% difference was observed for those field sizes. 0The present work is more consistent showing over-response of 10 % without filter and 3 % with filter.
b. For wedged fields:-
b.1 Physical wedges (Lead wedge angle 15º fields) :-
The measured percentage depth doses and dose profiles for -6MV- lead wedge 15° field 2 size (4×4) cm at depths of dmax, 5, 10 and 20 cm as example are shown in figures (6), (7) respectively. Relative to the ion chamber measurements, the films percentage depth doses for -
dmaxcm
٦٨٠ VII Radiation Physics & Protection Conference, 27-30 November 2004,Ismailia-Egypt
6MV- are higher by a range from 0-4% for field sizes (4×4) cm2 and (10×10) cm2 and from 0- 9% for field sizes (15×15) cm2 and (20×20) cm2.
120 120 6MV X-ray Wedge 15 4cm x 4cm At Different Depths
100 100 6MV X-ray Wedge 15 4cm x 4cm 5 cm
80 80 10 cm 60 60
40 40 (%) Dose Relative 20 cm IC
Percentage Depth Dose (%) Dose Depth Percentage IC Film Without Filter 20 20 Film With Filter Film Without Filter Film With Filter 0 0 -5 -4 -3 -2 -1 0 1 2 3 4 5 0 2 4 6 8 10 12 14 16 18 20 Distance From Central Ray (cm) Depth (cm) Figure (7): -Energy 6MV dose profiles Figure (6): -Energy 6MV percentage for wedge angle 15° field size (4×4) cm2 depth dose for wedge angle 15° field size at depths d , 5, 10 and 20cm. (4×4) cm2. max
On the other hand, for -15MV- the percentage depth doses and dose profiles at depths dmax, 5, 10 and 20 cm for lead wedge 15° field size (4×4) cm2 as example are shown in figures (8), (9) respectively. The film percentage depth doses are higher by a range from 0-3% for field (4×4) cm2, 0-4% for field (10×10) cm2, 0-5% for field (15×15) cm2 and from 0-8% for field (20×20) cm2. It is also to be noted that the range of over-response increases with increasing depth and field size. d cm 120 120 max 15MV X-ray Wedge 15 4cm x 4cm 15MV X-ray Wedge 15 4cm x 4cm At Different Depths 100 100 5 cm 80 80
60 60 10 cm
40 IC 40 Relative Dose (%) Dose Relative
Film Without Filter IC 20 cm 20 20 Percentage Depth Dose (%) Depth Percentage Film With Filter Film Without Filter Film With Filter 0 0 0 2 4 6 8 101214161820 -5 -4 -3 -2 -1 0 1 2 3 4 5 Depth (cm) Distance From Central Ray (cm)
Figure (9): -Energy 15MV dose profiles for Figure (8): -Energy 15MV percentage depth 2 dose for wedge angle 15° field size (4×4) wedge angle 15° field size (4×4) cm at cm2. depths dmax, 5, 10 and 20cm.
Using the lateral scatter filter with this wedge angle -15°- fields the over-response is reduced to be within 3% maximum for both -6MV- and -15MV-. There is nice agreement between ionization chamber measurements and film with filter measurements.
b.2 Analysis of virtual wedge (VW) results:-
The lateral scattering filters tested in case of open and physical wedged fields are used in virtual wedge (alternative wedging system uses no physical wedge and produces angled isodose lines by scanning one of the collimator jaws across the field while the beam is on). Figures (10) and (11) show the results of dose profiles measurements by film using filter and at selected points by the ionization chamber for -6MV- and -15MV- virtual wedge 15° and 30° respectively for field size (4×4) cm2 as example.
٦٨١ VII Radiation Physics & Protection Conference, 27-30 November 2004,Ismailia-Egypt
120 120 Film at Dmax Film at Dmax 6MV Virtual wedge 15 15MV Virtual wedge 30 Film at 10 cm Film at 10 cm 4cm x 4cm Film at 20 cm Film at 20 cm 4cm x 4cm 100 I.C. 100 I.C.
80 80
60 60
40 Relative(%) Dose 40 Relative Dose (%)
20
20
0 -5 -4 -3 -2 -1 0 1 2 3 4 5
0 Distance From Central Ray (cm) -5 -4 -3 -2 -1 0 1 2 3 4 5 Diatance From Central Ray (cm) Figure (11): -Dose profiles at different Figure (10): -Dose profiles at different depths for -15MV- field size (4×4) cm2 depths for -6MV- field size (4×4) cm2 for for virtual wedge 30°. virtual wedge 15°.
These results were found to be coincident with those measured by ionization chamber (within ≤ 3%) at chosen points for each field size and depth, as shown in figures (10) and (11).
It is to be noted that, the ionization chamber measurements are very difficult, need hard effort, and long time. The time of measurements using the ionization chamber depends upon the number of selected points. The radiation dose at each point in any given field is measured individually using the ionization chamber. But using the radiographic film the distribution of doses can be measured at once. For example, the time taken to measure the doses at six selected point for field size (4×4) cm2 by ionization chamber is higher than the corresponding measurements using the radiographic film by six times.
CONCLUSIONS
This study revealed a coincidence of film measurements (using lead filters) with I.C. measurements with a maximum variation of 3%.
It can be concluded that:-
) Good dosimetric results are obtained if films from the same batch are irradiated with (10×10) cm2 field size at 5cm depth using 0.25 mm lead filter. This result is in agreement with Claudia et al. [6], if films from the same batch are irradiated with small to moderate field sizes (up to about (15×15) cm2) at moderate depths (up to about 15 cm) using a single calibration curve.
) The film over-response dependents on field size, depth of interest, energy and use of physical wedge filter or virtual wedge.
) This over-response is significantly reduced to acceptable level (≤ 3%) using lateral scattering filter method.
) Once a radiographic film is calibrated as indicated above. The films from the same batch can be used for estimating doses delivered to a patient under the same conditions.
REFERENCES
(1) Hendrickson F. R.: "Precision in radiation oncology," Int. J. Radiat. Oncol. Biol., Phys. 8, Pp. 311-312, 1982.
٦٨٢ VII Radiation Physics & Protection Conference, 27-30 November 2004,Ismailia-Egypt
(2) Yeo I. J., Chris Wang C.K., and Burch S. E.: "A filtration method for improving film dosimetry on photon radiation therapy," Med. Phys. 24(12), Pp. 1943-1953, December 1997. (3) AAP Task Group 21: "A Protocol for the determination of absorbed dose from high energy photon and electron beams," Med. Phys. 10, 741 1983. (4) ICRU 23. : "Measurement of absorbed dose in a phantom irradiated by a single beam of X or gamma rays" (ICRU Publications, Washington D. C.), Pp. 91-149 1973. (5) Burch S. E., et al.: "A new approach to film dosimetry for high energy photon beams: Lateral Scatter Filtering," Med. Phys. 24(5), Pp. 775-783, May 1997. (6) Claudia, D., Basil, S., Jean-Clude, R., et al: "Variation of sensitometric curve of radiographic films in high energy photo beams", Med. Phys., 28 (6): 966-974, 2001. (7) Khan F: The Physics of Radiation Therapy, 2nd ed. Baltimore, Williams & Wilkins, 1994. (8) Metcalfe P., Kron T., Elliott A., Wong T. and Hoban P.: "Dosimetry of 6 MV X-ray beam penumbra", Med. Phys. 20(5): Pp. 1439-1445. 1993.
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