- Email: [email protected]
FBX aqueous chemical dosimeter for measurement of dosimetric parameters
Applied Radiation and Isotopes 69 (2011) 399–402
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Technical note
FBX aqueous chemical dosimeter for measurement of dosimetric parameters O. Moussous a,n, T. Medjadj a, M. Benguerba b a b
Centre de Recherche Nucle´aire d’Alger (CRNA), 02 Boulevard Frantz Fanon B.P. 399, 16000 Alger, Algeria Faculte´ de Physique, Universite´ des Sciences et de la Technologie Houari-Boumedie ne USTHB, Alger, Algeria
a r t i c l e in fo
abstract
Article history: Received 19 June 2010 Received in revised form 16 September 2010 Accepted 1 October 2010
We investigated the ferrous sulphate–benzoic acid–xylenol orange (FBX) aqueous chemical dosimeter for measurement of dosimetric parameters such as the output factor, backscatter factor and lateral beam profiles for different square fields sizes for 60Co g-rays. A water phantom was employed to measure these parameters. An ionization chamber (IC) was used for calibration and comparison. A comparison of the resulting measurements with an ionization chamber’s measured parameters showed good agreement. We thus believe that the tissue equivalent FBX dosimetry system can measure the dosimetric parameters for 60Co with reasonable accuracy. & 2010 Elsevier Ltd. All rights reserved.
Keywords: FBX dosimeter Dosimetric parameters Cobalt-60 Ionization chamber
1. Introduction
2. Materials and method
An aqueous chemical dosimeter known as an FBX dosimeter was developed for low level dose measurements as a modification of the Fricke system by Gupta and co-workers (Gupta, 1973; Gupta and Gomathy, 1974; Gupta et al, 1978). This dosimeter is derived from the standard Fricke solution, plus benzoic acid and orange xylenol. In this system the absorbed doses are measured in terms of the oxidation yield of ferric ions produced by irradiation. The ferric ions form a color complex with xylenol orange which is measured spectrophotometrically at 548 nm. One of the important properties of this dosimeter is its energy and dose rate independent response for X-rays and g-rays from 33 to 1250 keV (Gupta, 1973). This dosimetric system has been studied for various applications in radiotherapy such as percentage depth dose measurements, output calibration in teletherapy with photons and electrons, brachytherapy source calibration and in vivo dosimetry (Gupta et al., 1986, 1982, 1992; Madhvanath et al., 1976; Semwal et al., 2005). In this work we used the FBX dosimeter and the 0.6 cm3 IC to study the dosimetric parameters related to a 60Co g-ray beam for different field sizes.The lateral beam profiles, backscatter factor and the output factor were measured in a water phantom for incident g-rays produced by a 60Co Eldorado 78 therapy unit. Our goal in this investigation was to demonstrate whether the tissue equivalent FBX dosimeter could be used in dosimetric parameters’ measurements as an alternative to commonly used systems such as ionization chambers.
2.1. FBX dosimeter
n
Corresponding author. Tel.: + 213 21434444; fax: +213 21 424380. E-mail address: [email protected] (O. Moussous).
0969-8043/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2010.10.001
The FBX dosimeter consists of a sealed glass ampoule filled with FBX aqueous solution. The solution was prepared from analytical reagent (AR) grade chemicals and high purity water (18.2 MO) following the method described by Gupta et al. (1982) to achieve the following composition: 0.2 mol m 3 ferrous ammonium sulphate, 5.0 mol m 3 benzoic acid and 0.20 mol m 3 xylenol orange (XO) in 40.0 mol m 3 sulphuric acid. The dosimeter ampoules have the following dimensions: inner diameter 10.6 mm, height 31 mm (referred to the top surface of the liquid in the ampoule) and Pyrex wall thickness 0.5 mm. Therefore the volume of the dosimetric solution is about 2 cm3. The ampoule shape was specifically designed to make it easier to fill and to seal without damaging the solution. The ampoule shape also makes it possible to irradiate the dosimeters in vertical beams without any air bubbles in the cylindrical part of the ampoule.
2.2. Evaluation of molar extinction coefficient It was reported in earlier studies that the molar extinction coefficient of the ferric–XO complex in the FBX dosimeter is not the same for different manufacturing companies of XO and it is necessary to establish separate molar extinction coefficients for a batch of XOs used (Gupta and Narayan, 1985). In the present study, AR grade XO from Fluka was used. Molar extinction coefficient of the complex was evaluated by measuring variation in optical absorbance with the concentration of the complex. The ferric ion solution was prepared by dissolving crystalline ferric
400
O. Moussous et al. / Applied Radiation and Isotopes 69 (2011) 399–402
ammonium sulphate, about 0.2 mol m 3, in 40.0 mol m 3of sulphuric acid. From this sample, solutions of different known concentrations were prepared by successive dilution with a water solution of sulphuric acid, benzoic acid and xylenol orange in the proportions mentioned above for the FBX solution. 2.3. Calibration curve The calibration of the dosimeters was performed in 60Co g-rays using an Eldorado therapy 78 unit at a dose rate of about 0.5 Gy min 1 to water. The dosimeters were placed in a water phantom 30 30 30 cm3 in volume, with their geometrical center positioned at a reference depth of 5 g cm 2, with a distance–source–surface (SSD) of 80 cm and a field size of 10 10 cm2 at the phantom surface. The output dose rate to water was measured previously using a 0.6 cm3 reference ionization chamber WDIC70 #141 connected to a PTW UNIDOS 10002 electrometer. The reference chamber was calibrated in terms of absorbed dose to water at the International Atomic Energy Agency reference laboratory. The calibration coefficient (ND,w) established at the conditions T¼ 20 1C and P¼101.325 kPa, is thus traceable to the Bureau International des Poids et Mesures (International Bureau of Weights and Measures). The calibration curve was obtained by irradiating individually filled ampoules at known dose values with a range of doses from 1 to 5 Gy. For each selected dose, four FBX dosimeters were irradiated and their absorbance values averaged. 2.4. Signal stability The pre- and post-irradiation stability of the solutions was studied with respect to time. The post-irradiation stability of the FBX dosimeter was tested by measuring the absorbance of the irradiated FBX solution immediately after irradiation and then on a longer term. The irradiated solutions were kept after irradiation under normal laboratory conditions in the dark.
2.7. Beam profile Beam profiles were measured in a water phantom type MedTec with a volume of 40 40 40 cm3 at a reference depth of 5 g cm2, SSD of 80 cm for an absorbed dose of 2 Gy for 5 5 cm2 and 10 10 cm2 field size using the FBX and the IC dosimeters. The FBX ampoules were placed horizontally (in amounts of 9 and 16, to provide the expected field size lengths for the measurements) on a polymethyl methacrylate (PMMA) slab, which in turn was placed in a water phantom. The long axes of the ampoules were placed perpendicular to the long axis of the PMMA slab. Three measurements were taken for each point of profile point, whose readings were averaged and normalized for the maximum absorbed dose value (in the center of the field). The same geometrical conditions were used for the ionization chamber. 2.8. Backscatter factor Backscatter factor (BSF) measurements were carried out with FBX dosimeters and with the 0.6 cm3 IC for an absorbed dose of 2 Gy, an SSD of 80 cm and for field sizes of 5 5 cm2, 7 7 cm2, 8 8 cm, 10 10 cm2, 12 12 cm2, 15 15 cm2 and 20 20 cm2. The dose measurements were performed by first placing the dosimeters freein-air at the SSD of 80 cm from the source. The output of the 60Co source in terms of air kerma dose rate at this SSD was 1 Gy min 1. This air kerma dose rate was previously determined using the reference chamber cited above, calibrated in terms of air kerma at the International Atomic Energy Agency reference laboratory. Additional measurements were made with the phantom in place. The water phantom type Med-Tec with a volume of 40 40 40 cm3 for the vertical beam was used in this work as scattering material. The phantom was placed on the irradiation bench of the cobalt unit at the SSD of 80 cm from the beam focus such that the center of the field coincided with the geometric center of the dosimeters. The BSF was defined as the ratio of the on-phantom measurements divided by the free-in-air measurement.
3. Results and discussion 2.5. Spectrophotometric analysis 3.1. Absorption spectrum Absorbance readings of FBX solution are carried out by a Varian Cary 100 UV–vis double beam spectrophotometer with a resolution of 0.0001 optical density units (ODU). In this work absorbance readings are taken at a selected wavelength with a bandwidth of 1.5 nm, using a quartz micro cuvette having a path length of 10 mm and an optical window of 4 mm. The absorption spectra of the FBX solution were measured scanning over the wavelength range between 300 and 600 nm to observe the absorption peak.
Representative spectra of unirradiated and irradiated FBX dosimeter solution with 60Co g-rays are shown in Fig. 1. It can be seen that the difference spectrum shows a very broad maximum centered at about 540 nm. However, since in the sensitive range of interest spectra lie around 548 nm, this value was selected for further analysis of the experimental data. 3.2. Molar extinction coefficient
2.6. Output factor Output factor (OF) measurements were obtained in a water phantom 30 30 30 cm3 in volume with FBX dosimeters and with the 0.6 cm3 IC for an absorbed dose of 2 Gy, an SSD of 80 cm and depth in water of 5 g cm2. Single FBX dosimeters were placed in water and centered in the square field sizes of 5 5 cm2, 7 7 cm2, 8 8 cm2, 10 10 cm2, 12 12 cm2, 15 15 cm2 and 20 20 cm2. Three of such FBX dosimeters were irradiated in the same conditions and their absorbed dose values averaged for each field size. The irradiation conditions with the ionization chamber were the same cited. The output factor is expressed as the ratio of the absorbed dose at the center of a selected field size to that at the center of a larger reference field size (10 10 cm2), both measured at the reference dose depth in the same medium (IAEA, 2000).
The molar extinction coefficient, e ,was measured at 548 nm and found to be 14740750 M 1 cm 1 . Three independent sets of experiments were carried out and the error was calculated using standard deviations of the mean. 3.3. Calibration curve Fig. 2 shows the net absorbance, DA, of the irradiated samples acquired as a function of the absorbed dose to water, Dw, in the interval of 1–5 Gy. The net absorbance of the irradiated samples was obtained by subtracting the absorbance from the control unirradiated samples. A linear regression through the points gave a correlation coefficient, R2, of 0.9999 and a differential absorbance sensitivity (slope) of (0.08370.00016) Gy 1. The radiation
O. Moussous et al. / Applied Radiation and Isotopes 69 (2011) 399–402
401
solutions at dose of 3 Gy are given in Table 1. As can be seen by this table, there is no significant thermal oxidation rate for up to 1 week. 3.5. Output factor The output factors measured with our FBX dosimeters and with the ionization chamber are shown in Fig. 3. As can be seen by this figure, the FBX dosimeter gives readings that are consistent with the ionization chamber measurements. The maximum difference between the relative values measured by both systems was found to be within 70.5%. 3.6. Field profile
Fig. 1. The change in the absorption spectrum of an irradiated and unirradiated (FBX) dosimeter, respectively. The values 0 and 4 Gy were measured with air as the reference. The difference spectrum has a broad peak centered at about 540 nm.
The FBX and ionization chamber profile behaviors for 60Co are shown in Fig. 4. According to anterior protocols (IAEA, 2004; ICRU, 1976) the field profile evaluations can be done considering these three parameters: homogeneity, symmetry and penumbra. The accepted tolerance for homogeneity is r2% of the difference between the lowest and the highest values. In our measurements differences of less than 1.02% were inferred for both field sizes. The accepted tolerance for symmetry is r2% of the difference between the left and the right values in 80% of the size, related to value at the central axis. In our measurements differences of less than 1.30% were inferred for both field sizes. The accepted tolerance for the penumbra is r2 mm, distance differences of the points at 80% and 20% in the region where the dose gradient is evaluated. In this study was observed a maximum Table 1 The variation in absorbances of an unirradiated and irradiated (3 Gy) FBX solutions with time.
Fig. 2. FBX calibration curve for 60Co energy photons, 10 10 cm2, 80 cm SSD at 5 cm depth in water. The relative standard deviation of the mean is 0.3%.
Time Unirradiated (days)
FBX solution absorbance values
Irradiated FBX solution absorbance values
0 4 5 6 7
0.1043 0.1045 0.1045 0.1047 0.1055
0.2525 0.2526 0.2530 0.2534 0.2544
chemical yield G(Fe + 3) value, which is a product of this slope and e, the molar extinction coefficient, was evaluated and found to be (55.9670.2) 10 7 mol J 1. The G value obtained in the present investigation is comparable within experimental error with the previously reported value by Gupta and Nilekani (1998). The linear interval tested is consistent with the results cited in the literature (Gupta et al., 1982) and is adequate for radiotherapy purposes. According to the results obtained the useful dose range for FBX dosimeter can be considered to be 1–5 Gy. The basic data set that follows is relative to each dosimeter response at a given position and depth, and did not use the calibration curve.
3.4. Signal stability Unirradiated and irradiated FBX solutions were found to be stable with pre- and post-irradiation storage time of about one week. However, the unirradiated solution was freshly prepared, air saturated, stabilized for about an hour and used within 1 day. Variations in absorbance of the unirradiated and irradiated
Fig. 3. Output factor versus square field size for 60Co at 100 cm SSD and reference depth. The relative standard deviation of the mean is about 0.5% for FBX and 0.02% for 0.6 cm3 IC.
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also remarked that when the field size increases the BSF increases slightly. This could be attributed to the increased number of backscattered particles back from the phantom as the field size increased (Grosswendit, 1990).
4. Conclusion
Fig. 4. Field size profiles from FBX and ionization chamber dosimeters with 100 cm SSD for cobalt energy photons at 5 cm depth in water. The relative standard deviation of the mean is about 0.5% for FBX and 0.02% for 0.6 cm3 IC.
The FBX due to its water-equivalent composition and its energy-independent response for X-rays and g-rays is expected to be an adequate dosimeter for radiotherapy, which can be confirmed through the dosimetric parameters of field profile, output and backscatter factor. In this work, these parameters were obtained through the FBX dosimeter and compared to the ionization chamber results. The present study confirms that the FBX gives results that are consistent with the expectations from the ionization chamber. Considering the dosimetric parameters’ results obtained, one can consider the FBX as an adequate dosimeter for conventional field size measurements. In this way we can conclude that this dosimeter can be used in general radiotherapy control through the dosimetric parameters’ measurements.
References
Fig. 5. The effect of field size on the backscatter factor from the water phantom for 60 Co. The relative standard deviation of the mean is about 0.6% for FBX and 0.02% for 0.6 cm3 IC.
deviation of 1 mm between the two positions of the FBX and ionization chamber obtained for 5 5 cm2 and 10 10 cm2 field sizes, respectively. 3.7. Backscatter factor Backscatter factor measurements were carried out using our FBX system. This was intercompared with similar measurements made using the ionization chamber under identical conditions. There is good agreement between the BSF measured by both systems using a water phantom as scattering material, as seen in Fig. 5. The maximum difference between the relative values measured by both systems was found to be within 75%. It was
Gupta, B.L., 1973. Low-level dosimetric studies with the FeSO4–benzoic acid– xylenol orange system. In: Dosimetry in Agriculture, Industry, Biology and Medicine, Proceedings of a Symposium Organized by the IAEA [International Atomic Energy Agency] with the Participation of WHO [World Health Organization], 17–21 April 1972, IAEA, Vienna, Austria, pp. 421–432. Gupta, B.L., Gomathy, K.R., 1974. Consistency of ferrous sulfate–benzoic acid– xylenol orange dosimeter. Int. J. Appl. Radiat. Isot. 25 (11–12), 509–513. Gupta, B.L., Bhat, R.M., Gomathy, K.R., Suseela, B., 1978. Radiation chemistry of the ferrous sulphate–benzoic acid–xylenol orange dosimeter. Radiat. Res. 75, 269–277. Gupta, B.L., Kini, U.R., Bhat, R.M., Madhvanath, U., 1986. Intercomparison of ionization and chemical dosimetry techniques for central axis depth-dose measurements from cobalt-60 teletherapy machines. Med. Phys. Bull. 11 (3 and 4), 28–31. Gupta, B.L., Kini, U.R., Bhat, R.M., Madhvanath, U., 1982. Use of the FBX dosimeter for the calibration of cobalt-60 and high energy teletherapy machines. Phys. Med. Biol. 27 (2), 235–245. Gupta, B.L., Narayan, G.R., Bhat, R.M., Kini, U.R., Ramaswamy, R.S., Suseela, B., 1992. Use of the FBX dosimeter for the quality assurance of 60Co and high-energy teletherapy machines by mail. Phys. Med. Biol. 37 (11), 2095–2102. Gupta, B.L., Narayan, G.R., 1985. G(Fe3 + ) values in the FBX dosimeter. Phys. Med. Biol. 30, 337–340. Gupta, B.L., Nilekani, S.R., 1998. Ferrous ion oxidation by H, OH and H2O2 in aerated FBX dosimetry system. Radiat. Phys. Chem. 53, 643–650. Grosswendit, B., 1990. Dependence of the photon BSF for water on source to phantom distance and irradiation field size. Phys. Med. Biol. 35, 1233–1245. IAEA, 2000. Absorbed dose determination in external beam radiotherapy: an international code of practice for dosimetry based on standards of absorbed dose to water. Technical Reports Series 398 (International Atomic Energy Agency). IAEA, 2004. Commissioning and Quality Assurance of Computerized Planning Systems for Radiation Treatment of Cancer. Technical Reports Series 430, Vienna, International Atomic Energy Agency. ICRU, 1976. Determination of absorbed dose in a patient irradiated by beams or gamma rays in radiotherapy procedures, ICRU Report 24, International Commission on Radiation Units and Measurements, Washington, USA. Madhvanath, U., Kini, U.R., Gupta, B.L., 1976. Calibration of brachytherapy sources using ferrous sulfate–benzoic acid–xylenol orange dosimeter. Int. J. Appl. Radiat. Isot. 27 (8), 443–446. Semwal, M.K., Bansal, A.K., Thakur, P.K., Vidyasagar, P.B., 2005. In-vivo (entrance) dose measurements in external beam radiotherapy with aqueous FBX dosimetry system. J. Med. Phys. 30 (1), 32–35.
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Technical note
FBX aqueous chemical dosimeter for measurement of dosimetric parameters O. Moussous a,n, T. Medjadj a, M. Benguerba b a b
Centre de Recherche Nucle´aire d’Alger (CRNA), 02 Boulevard Frantz Fanon B.P. 399, 16000 Alger, Algeria Faculte´ de Physique, Universite´ des Sciences et de la Technologie Houari-Boumedie ne USTHB, Alger, Algeria
a r t i c l e in fo
abstract
Article history: Received 19 June 2010 Received in revised form 16 September 2010 Accepted 1 October 2010
We investigated the ferrous sulphate–benzoic acid–xylenol orange (FBX) aqueous chemical dosimeter for measurement of dosimetric parameters such as the output factor, backscatter factor and lateral beam profiles for different square fields sizes for 60Co g-rays. A water phantom was employed to measure these parameters. An ionization chamber (IC) was used for calibration and comparison. A comparison of the resulting measurements with an ionization chamber’s measured parameters showed good agreement. We thus believe that the tissue equivalent FBX dosimetry system can measure the dosimetric parameters for 60Co with reasonable accuracy. & 2010 Elsevier Ltd. All rights reserved.
Keywords: FBX dosimeter Dosimetric parameters Cobalt-60 Ionization chamber
1. Introduction
2. Materials and method
An aqueous chemical dosimeter known as an FBX dosimeter was developed for low level dose measurements as a modification of the Fricke system by Gupta and co-workers (Gupta, 1973; Gupta and Gomathy, 1974; Gupta et al, 1978). This dosimeter is derived from the standard Fricke solution, plus benzoic acid and orange xylenol. In this system the absorbed doses are measured in terms of the oxidation yield of ferric ions produced by irradiation. The ferric ions form a color complex with xylenol orange which is measured spectrophotometrically at 548 nm. One of the important properties of this dosimeter is its energy and dose rate independent response for X-rays and g-rays from 33 to 1250 keV (Gupta, 1973). This dosimetric system has been studied for various applications in radiotherapy such as percentage depth dose measurements, output calibration in teletherapy with photons and electrons, brachytherapy source calibration and in vivo dosimetry (Gupta et al., 1986, 1982, 1992; Madhvanath et al., 1976; Semwal et al., 2005). In this work we used the FBX dosimeter and the 0.6 cm3 IC to study the dosimetric parameters related to a 60Co g-ray beam for different field sizes.The lateral beam profiles, backscatter factor and the output factor were measured in a water phantom for incident g-rays produced by a 60Co Eldorado 78 therapy unit. Our goal in this investigation was to demonstrate whether the tissue equivalent FBX dosimeter could be used in dosimetric parameters’ measurements as an alternative to commonly used systems such as ionization chambers.
2.1. FBX dosimeter
n
Corresponding author. Tel.: + 213 21434444; fax: +213 21 424380. E-mail address: [email protected] (O. Moussous).
0969-8043/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2010.10.001
The FBX dosimeter consists of a sealed glass ampoule filled with FBX aqueous solution. The solution was prepared from analytical reagent (AR) grade chemicals and high purity water (18.2 MO) following the method described by Gupta et al. (1982) to achieve the following composition: 0.2 mol m 3 ferrous ammonium sulphate, 5.0 mol m 3 benzoic acid and 0.20 mol m 3 xylenol orange (XO) in 40.0 mol m 3 sulphuric acid. The dosimeter ampoules have the following dimensions: inner diameter 10.6 mm, height 31 mm (referred to the top surface of the liquid in the ampoule) and Pyrex wall thickness 0.5 mm. Therefore the volume of the dosimetric solution is about 2 cm3. The ampoule shape was specifically designed to make it easier to fill and to seal without damaging the solution. The ampoule shape also makes it possible to irradiate the dosimeters in vertical beams without any air bubbles in the cylindrical part of the ampoule.
2.2. Evaluation of molar extinction coefficient It was reported in earlier studies that the molar extinction coefficient of the ferric–XO complex in the FBX dosimeter is not the same for different manufacturing companies of XO and it is necessary to establish separate molar extinction coefficients for a batch of XOs used (Gupta and Narayan, 1985). In the present study, AR grade XO from Fluka was used. Molar extinction coefficient of the complex was evaluated by measuring variation in optical absorbance with the concentration of the complex. The ferric ion solution was prepared by dissolving crystalline ferric
400
O. Moussous et al. / Applied Radiation and Isotopes 69 (2011) 399–402
ammonium sulphate, about 0.2 mol m 3, in 40.0 mol m 3of sulphuric acid. From this sample, solutions of different known concentrations were prepared by successive dilution with a water solution of sulphuric acid, benzoic acid and xylenol orange in the proportions mentioned above for the FBX solution. 2.3. Calibration curve The calibration of the dosimeters was performed in 60Co g-rays using an Eldorado therapy 78 unit at a dose rate of about 0.5 Gy min 1 to water. The dosimeters were placed in a water phantom 30 30 30 cm3 in volume, with their geometrical center positioned at a reference depth of 5 g cm 2, with a distance–source–surface (SSD) of 80 cm and a field size of 10 10 cm2 at the phantom surface. The output dose rate to water was measured previously using a 0.6 cm3 reference ionization chamber WDIC70 #141 connected to a PTW UNIDOS 10002 electrometer. The reference chamber was calibrated in terms of absorbed dose to water at the International Atomic Energy Agency reference laboratory. The calibration coefficient (ND,w) established at the conditions T¼ 20 1C and P¼101.325 kPa, is thus traceable to the Bureau International des Poids et Mesures (International Bureau of Weights and Measures). The calibration curve was obtained by irradiating individually filled ampoules at known dose values with a range of doses from 1 to 5 Gy. For each selected dose, four FBX dosimeters were irradiated and their absorbance values averaged. 2.4. Signal stability The pre- and post-irradiation stability of the solutions was studied with respect to time. The post-irradiation stability of the FBX dosimeter was tested by measuring the absorbance of the irradiated FBX solution immediately after irradiation and then on a longer term. The irradiated solutions were kept after irradiation under normal laboratory conditions in the dark.
2.7. Beam profile Beam profiles were measured in a water phantom type MedTec with a volume of 40 40 40 cm3 at a reference depth of 5 g cm2, SSD of 80 cm for an absorbed dose of 2 Gy for 5 5 cm2 and 10 10 cm2 field size using the FBX and the IC dosimeters. The FBX ampoules were placed horizontally (in amounts of 9 and 16, to provide the expected field size lengths for the measurements) on a polymethyl methacrylate (PMMA) slab, which in turn was placed in a water phantom. The long axes of the ampoules were placed perpendicular to the long axis of the PMMA slab. Three measurements were taken for each point of profile point, whose readings were averaged and normalized for the maximum absorbed dose value (in the center of the field). The same geometrical conditions were used for the ionization chamber. 2.8. Backscatter factor Backscatter factor (BSF) measurements were carried out with FBX dosimeters and with the 0.6 cm3 IC for an absorbed dose of 2 Gy, an SSD of 80 cm and for field sizes of 5 5 cm2, 7 7 cm2, 8 8 cm, 10 10 cm2, 12 12 cm2, 15 15 cm2 and 20 20 cm2. The dose measurements were performed by first placing the dosimeters freein-air at the SSD of 80 cm from the source. The output of the 60Co source in terms of air kerma dose rate at this SSD was 1 Gy min 1. This air kerma dose rate was previously determined using the reference chamber cited above, calibrated in terms of air kerma at the International Atomic Energy Agency reference laboratory. Additional measurements were made with the phantom in place. The water phantom type Med-Tec with a volume of 40 40 40 cm3 for the vertical beam was used in this work as scattering material. The phantom was placed on the irradiation bench of the cobalt unit at the SSD of 80 cm from the beam focus such that the center of the field coincided with the geometric center of the dosimeters. The BSF was defined as the ratio of the on-phantom measurements divided by the free-in-air measurement.
3. Results and discussion 2.5. Spectrophotometric analysis 3.1. Absorption spectrum Absorbance readings of FBX solution are carried out by a Varian Cary 100 UV–vis double beam spectrophotometer with a resolution of 0.0001 optical density units (ODU). In this work absorbance readings are taken at a selected wavelength with a bandwidth of 1.5 nm, using a quartz micro cuvette having a path length of 10 mm and an optical window of 4 mm. The absorption spectra of the FBX solution were measured scanning over the wavelength range between 300 and 600 nm to observe the absorption peak.
Representative spectra of unirradiated and irradiated FBX dosimeter solution with 60Co g-rays are shown in Fig. 1. It can be seen that the difference spectrum shows a very broad maximum centered at about 540 nm. However, since in the sensitive range of interest spectra lie around 548 nm, this value was selected for further analysis of the experimental data. 3.2. Molar extinction coefficient
2.6. Output factor Output factor (OF) measurements were obtained in a water phantom 30 30 30 cm3 in volume with FBX dosimeters and with the 0.6 cm3 IC for an absorbed dose of 2 Gy, an SSD of 80 cm and depth in water of 5 g cm2. Single FBX dosimeters were placed in water and centered in the square field sizes of 5 5 cm2, 7 7 cm2, 8 8 cm2, 10 10 cm2, 12 12 cm2, 15 15 cm2 and 20 20 cm2. Three of such FBX dosimeters were irradiated in the same conditions and their absorbed dose values averaged for each field size. The irradiation conditions with the ionization chamber were the same cited. The output factor is expressed as the ratio of the absorbed dose at the center of a selected field size to that at the center of a larger reference field size (10 10 cm2), both measured at the reference dose depth in the same medium (IAEA, 2000).
The molar extinction coefficient, e ,was measured at 548 nm and found to be 14740750 M 1 cm 1 . Three independent sets of experiments were carried out and the error was calculated using standard deviations of the mean. 3.3. Calibration curve Fig. 2 shows the net absorbance, DA, of the irradiated samples acquired as a function of the absorbed dose to water, Dw, in the interval of 1–5 Gy. The net absorbance of the irradiated samples was obtained by subtracting the absorbance from the control unirradiated samples. A linear regression through the points gave a correlation coefficient, R2, of 0.9999 and a differential absorbance sensitivity (slope) of (0.08370.00016) Gy 1. The radiation
O. Moussous et al. / Applied Radiation and Isotopes 69 (2011) 399–402
401
solutions at dose of 3 Gy are given in Table 1. As can be seen by this table, there is no significant thermal oxidation rate for up to 1 week. 3.5. Output factor The output factors measured with our FBX dosimeters and with the ionization chamber are shown in Fig. 3. As can be seen by this figure, the FBX dosimeter gives readings that are consistent with the ionization chamber measurements. The maximum difference between the relative values measured by both systems was found to be within 70.5%. 3.6. Field profile
Fig. 1. The change in the absorption spectrum of an irradiated and unirradiated (FBX) dosimeter, respectively. The values 0 and 4 Gy were measured with air as the reference. The difference spectrum has a broad peak centered at about 540 nm.
The FBX and ionization chamber profile behaviors for 60Co are shown in Fig. 4. According to anterior protocols (IAEA, 2004; ICRU, 1976) the field profile evaluations can be done considering these three parameters: homogeneity, symmetry and penumbra. The accepted tolerance for homogeneity is r2% of the difference between the lowest and the highest values. In our measurements differences of less than 1.02% were inferred for both field sizes. The accepted tolerance for symmetry is r2% of the difference between the left and the right values in 80% of the size, related to value at the central axis. In our measurements differences of less than 1.30% were inferred for both field sizes. The accepted tolerance for the penumbra is r2 mm, distance differences of the points at 80% and 20% in the region where the dose gradient is evaluated. In this study was observed a maximum Table 1 The variation in absorbances of an unirradiated and irradiated (3 Gy) FBX solutions with time.
Fig. 2. FBX calibration curve for 60Co energy photons, 10 10 cm2, 80 cm SSD at 5 cm depth in water. The relative standard deviation of the mean is 0.3%.
Time Unirradiated (days)
FBX solution absorbance values
Irradiated FBX solution absorbance values
0 4 5 6 7
0.1043 0.1045 0.1045 0.1047 0.1055
0.2525 0.2526 0.2530 0.2534 0.2544
chemical yield G(Fe + 3) value, which is a product of this slope and e, the molar extinction coefficient, was evaluated and found to be (55.9670.2) 10 7 mol J 1. The G value obtained in the present investigation is comparable within experimental error with the previously reported value by Gupta and Nilekani (1998). The linear interval tested is consistent with the results cited in the literature (Gupta et al., 1982) and is adequate for radiotherapy purposes. According to the results obtained the useful dose range for FBX dosimeter can be considered to be 1–5 Gy. The basic data set that follows is relative to each dosimeter response at a given position and depth, and did not use the calibration curve.
3.4. Signal stability Unirradiated and irradiated FBX solutions were found to be stable with pre- and post-irradiation storage time of about one week. However, the unirradiated solution was freshly prepared, air saturated, stabilized for about an hour and used within 1 day. Variations in absorbance of the unirradiated and irradiated
Fig. 3. Output factor versus square field size for 60Co at 100 cm SSD and reference depth. The relative standard deviation of the mean is about 0.5% for FBX and 0.02% for 0.6 cm3 IC.
402
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also remarked that when the field size increases the BSF increases slightly. This could be attributed to the increased number of backscattered particles back from the phantom as the field size increased (Grosswendit, 1990).
4. Conclusion
Fig. 4. Field size profiles from FBX and ionization chamber dosimeters with 100 cm SSD for cobalt energy photons at 5 cm depth in water. The relative standard deviation of the mean is about 0.5% for FBX and 0.02% for 0.6 cm3 IC.
The FBX due to its water-equivalent composition and its energy-independent response for X-rays and g-rays is expected to be an adequate dosimeter for radiotherapy, which can be confirmed through the dosimetric parameters of field profile, output and backscatter factor. In this work, these parameters were obtained through the FBX dosimeter and compared to the ionization chamber results. The present study confirms that the FBX gives results that are consistent with the expectations from the ionization chamber. Considering the dosimetric parameters’ results obtained, one can consider the FBX as an adequate dosimeter for conventional field size measurements. In this way we can conclude that this dosimeter can be used in general radiotherapy control through the dosimetric parameters’ measurements.
References
Fig. 5. The effect of field size on the backscatter factor from the water phantom for 60 Co. The relative standard deviation of the mean is about 0.6% for FBX and 0.02% for 0.6 cm3 IC.
deviation of 1 mm between the two positions of the FBX and ionization chamber obtained for 5 5 cm2 and 10 10 cm2 field sizes, respectively. 3.7. Backscatter factor Backscatter factor measurements were carried out using our FBX system. This was intercompared with similar measurements made using the ionization chamber under identical conditions. There is good agreement between the BSF measured by both systems using a water phantom as scattering material, as seen in Fig. 5. The maximum difference between the relative values measured by both systems was found to be within 75%. It was
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