- Email: [email protected]
Effect of perchloric acid on the performance of the Fricke xylenol gel dosimeter
Applied Radiation and Isotopes 113 (2016) 66–69
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Effect of perchloric acid on the performance of the Fricke xylenol gel dosimeter M.I. El Gohary a, Y.S. Soliman b,n, E.A. Amin c, M.H. Abdel Gawad a, O.S. Desouky b a
Biophysics Branch, Physics Department, Faculty of Science, Al-Azhar University, Nasr City, Cairo, Egypt National Center for Radiation Research and Technology, Atomic Energy Authority (AEA), P. O. Box 8029, Nasr City, Cairo, Egypt c Medical Physics Unit, Ain-Shams Hospital, Ain-Shams University, El Abbasia, Cairo, Egypt b
H I G H L I G H T S
Perchloric acid enhances the radiation sensitivity of the ferrous xylenol orange gel. The gel, either with or without the acid, shows a linear dose response in the range 1–15 Gy. Storage environment factors need to be well controlled to minimize dose errors.
art ic l e i nf o
a b s t r a c t
Article history: Received 15 October 2015 Received in revised form 24 April 2016 Accepted 24 April 2016 Available online 26 April 2016
The conventional ferrous xylenol orange (XO) gel (FXG) dosimeter is being wildly investigated for radiotherapy dose measurements. Upon irradiation, its color turns red due to oxidation of Fe2 þ into Fe3 þ , which forms a complex with xylenol orange. The effect of perchloric acid (PCA) on the dosimetric properties of the gel in the dose range of 1–15 Gy was investigated using visual spectrophotometry. FXGPCA responds to radiation dose linearly and exhibits higher radiation sensitivity than the conventional gel dosimeter. PCA in a concentration of 20 mM enhances the radiation sensitivity 44%. Stability of the absorbances of both the gels during storage under various conditions was investigated, and the uncertainty of dose measurements was estimated. & 2016 Elsevier Ltd. All rights reserved.
Keywords: Dosimetry Radiochromic gel Radiotherapy
1. Introduction Radiation therapy uses chemical dosimeters of many types (Khan, 2010). Among them are mixtures of ferrous ions with xylenol orange (XO) (Davies and Baldock, 2008); acrylate monomers (Hiroki et al., 2013); radiochromic dyes, such as leuco crystal violet (LCV) (Babic et al., 2009); Gafchromic films (Devic, 2011; Farah et al., 2014) and gelatin containing Ag þ (Soliman, 2014). One of the common dosimeters in this category is based on the Fricke system and contains, in addition to the ferrous ions, xylenol orange in gelatin (FXG) (Fricke and Morse, 1927; Fricke and Hart, 1966; Core et al., 1984; Galante et al., 2008). It exhibits excellent water and tissue equivalence below 100 keV of photon energy (Keall and Baldock, 1999). Upon a γ-ray exposure, ferrous ions in the gel get oxidized into ferric ions, which form bonds with XO; this process is accompanied by a change of the color of the gel n
Corresponding author. E-mail address: [email protected] (Y.S. Soliman).
http://dx.doi.org/10.1016/j.apradiso.2016.04.024 0969-8043/& 2016 Elsevier Ltd. All rights reserved.
(Bero et al., 2001; Davies and Baldock, 2008). The radiation sensitivity of this system is high, and the absorbance of such dosimeters grows linearly with the dose up to 25 Gy (Davies and Baldock, 2008). However, oxidation of Fe2 þ continues even after the end of irradiation, which shifts the dose response function with time. Various additives were previously proposed to improve the radiation sensitivity of FXG to radiotherapeutic doses (Davies and Baldock, 2008; Pirani et al., 2009; Jin et al., 2012). In this work, we studied effects of perchloric acid (PCA) on the radiation sensitivity and dosimetric performance of the conventional FXG. We have found that PCA in the FXG gel decreases the dependence of the absorbance of the Fe2 þ -XO complex on the acid concentration and decreases the sensitivity of the materials to added components described by Gay and Gebicki (2002). We studied the stability of the dose response of both the gel types (FXG and FXG-PCA) under various environmental conditions. In addition, the overall uncertainty of dose measurements in a radiotherapeutic γ-ray facility with both the dosimeters was estimated.
M.I. El Gohary et al. / Applied Radiation and Isotopes 113 (2016) 66–69
67
2. Materials and methods 2.1. Gel dosimeter preparation Four batches of FXG solutions were prepared, each containing 4% (wt) of gelatin (300 Bloom Gelatin from porcine skin, SigmaAldrich) and 0, 10, 20 or 30 mM of perchloric acid (Aldrich, 70% ACS Reagent Grade). The mixtures also contained 50 mM of sulfuric acid, 1 mM of ferrous ammonium sulfate hexahydrate (Aldrich) and 0.1 mM of the ferric ion indicator XO (Oxford Laboratory, India). All the mixtures were stirred magnetically at 30 °C then transferred into polymethyl methacrylate (PMMA) capped cuvettes (1 1 4.5 cm3) and allowed to form transparent gels overnight in a refrigerator at 6 °C. 2.2. Irradiation and measurement procedures The prepared gels were exposed to γ-rays from a 60Co radiotherapy unit (PHOENIX Theratrons, MDS Nordion, Canada) at room temperature. The dose rate of the unit was measured according to the IAEA TRS 398 protocol with a calibrated ionization chamber (IAEA, 2006). We irradiated the gels in the cuvettes positioned into a cubic wax phantom (10 10 10 cm3). The gels were irradiated with four fields at the gantry angles of 0, 90, 180 and 270°, a fixed collimator angle (0°) and a fixed square field size (10 10 cm2) at a source-to-surface-distance (SSD) of 80 cm. These conditions had previously been used in the output calibration; the dose rate at the center of the phantom was reported as 1.2 Gy/min. The optical absorbances of the cuvettes (1 cm light path) were measured with a double-beam SPECORDs spectrophotometer (Analytik Jena AG, Jena, Germany) in the wavelength range of 300– 600 nm. Absorbances at 570 nm were used in the analysis.
Fig. 1. Dose response functions of the FXG dosimeters containing different concentrations of PCA (570 nm).
3. Results and discussion 3.1. Dose response functions and radiation sensitivity Fig. 1 shows dose response curves for FXGs with various PCA concentrations in the dose range of 1–15 Gy. The dependences of the absorbances on the dose are linear with correlation coefficients (r2) of 0.9996, 0.9910, 0.9998, and 0.9998 for the gels with 0, 10, 20, and 30 mM PCA, respectively. Fig. 2 shows the slope of linear dose response function (radiation dose sensitivity) as a function of the PCA concentration in the gel. Addition of PCA increases the radiation sensitivity of the gel, and the increase can be described by a quadratic polynomial function with r2 ¼0.9999. The radiation sensitivity reaches 0.0915 Gy 1 at 20 mM PCA and decreases slightly at higher concentrations of PCA. An increase in the PCA concentration from 0 to 20 mM enhances the sensitivity 44%. These results show that PCA in the concentration range 0– 30 mM improves the radiation sensitivity and the linearity of the dose response of the conventional gel. 3.2. Pre- and post-irradiation color stability Figs. 3 and 4 show the net absorbances at 570 nm as functions of the storage time for unirradiated and irradiated conventional and PCA-containing gels, respectively. Both gels were stored at room temperature ( 25 °C) in the dark and under laboratory light, as well as at 10 °C in a refrigerator. The unirradiated and irradiated conventional gels (Fig. 3) stored at 10 °C exhibit good stability with an increase of their responses within only 1–4% during the initial 24 h of storage, depending on the absorbed dose. However, their absorbances increased significantly with the time
Fig. 2. Radiation dose sensitivity of the FXG dosimeters as a function of the PCA concentration in the dose range of 1–15 Gy.
of storage at room temperature, both in the dark and in the light. The increases after 24 h of storage were in the ranges of (13–25)% and (20–30)% for the gels stored in the dark and in the light, respectively. The absorbances of unirradiated PCA-FXG gels (Fig. 4) stored at 10 °C grew with time initially and tended to stabilize after 48 h of storage. By contrast, the absorbances of the gels stored at room
68
M.I. El Gohary et al. / Applied Radiation and Isotopes 113 (2016) 66–69
Fig. 3. Pre- and post-irradiation absorbance stability of the conventional FXG dosimeter stored under various conditions.
temperature, either in the dark or in the light, increased significantly. The absorbances of irradiated PCA-containing gels decrease (2–8)%, depending on absorbed dose, during the first 24 h of storage at 10 °C. On the contrary, the absorbances of the gels stored at room temperature in the dark or in the light increase (21–70)% in 24 h of storage. The growth of the absorbances with time for both the gel types shows that oxidation of Fe2 þ into Fe3 þ continues. Thus, in order to increase the shelf life of the unirradiated gels, it is strongly recommended to store them in the dark at 10 °C. Otherwise, they must be used immediately after preparation. In addition, it is strongly recommended to put gels into a refrigerator immediately after their irradiation to slow down the oxidation of Fe þ 2 into Fe þ 3 and the natural increase in the absorbance. Otherwise, the absorbances of the gel dosimeters must be measured immediately after the irradiation.
4. Uncertainty of the estimated doses There are various factors contributing to the overall uncertainty, such as uncertainties of the calibration of the radiotherapy unit, of the homogeneity of the dosimeter batch, of the absorbance measurement, of the calibration curve fit, and of the
Fig. 4. Pre- and post-irradiation absorbance stability of the FXG-PCA dosimeter stored under various conditions.
temporal stability of the absorbance (Table 1). Some of the uncertainty contributions are discussed in the footnotes of Table 1. The others can be summarized as follows. The standard uncertainty of calibration of the ionization chamber reported in the certificate of calibration from IBA Dosimetry GmbH, Germany was 1.1%. The ionization chamber was used to calibrate the radiotherapy units with the overall uncertainty of 1.2%. The gel batch uniformity was tested by irradiating four sets of gel samples to various doses, measuring them under identical conditions and analyzing the results statistically (Sharpe and Miller, 2009; Soliman, 2014). The estimated uncertainties were 0.95% and 0.72% for the conventional and the PCA-containing gel dosimeter, respectively. The standard uncertainties of the calibration curves were calculated from the residuals (Sharpe and Miller, 2009; Soliman, 2014). The best linear fits were found with a commercial program Table Curve 2D (Version 5.01, SYSTAT Software, Inc., San Jose, CA, USA), and the standard uncertainties of the fits were found using the formula (Sharpe and Miller, 2009)
u=
Σ (residuals )2 , nd − nc
M.I. El Gohary et al. / Applied Radiation and Isotopes 113 (2016) 66–69
Table 1 Uncertainty budget for radiotherapeutic dose measurements with the conventional FXG and the PCA-FXG gel dosimeters in the dose range of 1–15 Gy. Uncertainty component
Type of uncertainty
Uncertainty, % conventional FXG
PCA-FXG
Radiation source calibration Other irradiation componentsa Spectrophotometer sensitivity variationsb Reproducibility of measurementsc Batch in inhomogeneity Calibration curve fit Temporal stability of absorbance Combined standard uncertainty (uc), 1s Expanded uncertainty (2r)
B
1.12
1.12
B
0.33
0.33
A
0.11
0.11
A
0.25
0.28
A A A
0.95 1.41 0.16
0.72 2.05 0.24
2.09
2.50
4.18
5.00
69
dosimeter in the irradiation unit under actual irradiation conditions (ISO/ASTM, 51,707, 2004). Storage conditions are important for both the gel types; they can result in a significant error of a determined dose if not taken into account properly.
5. Conclusion Both gel types can be useful for dose measurements in radiotherapy if the irradiation conditions, environmental factors and measurement procedures are well standardized. Addition of 20 mM PCA to the conventional FXG increases the radiation sensitivity 44%. The absorbances of both the gels increase significantly with time after irradiation; thus, the time of spectrophotometric measurements during calibration and dose measurements must be standardized to minimize the errors in dose monitoring. The overall uncertainties of dose measurements with the conventional FXG and FXG-PCA were found to be 4.18% and 5.0% at the 95% confidence level, respectively.
a
Geometry, source decay correction, timer setting and field nonuniformity. Estimated from absorbance measurements of an irradiated gel at 570 nm 100 times with the gel cuvette intact in the sample position. c Estimated from absorbance measurements of an irradiated gel at 570 nm 100 times with the gel cuvette taken out and put back into the sample holder after each measurement. b
where nd is the number of dosimeters and nc is the number of coefficients in the selected fitting function. The uncertainties found from the very good fits (r2 E0.999) were 1.41% and 2.05% for the conventional and the PCA-containing gel dosimeter, respectively. In the calculations, all individual dose points were taken into account (not averages of the replicates for the same dose point). The uncertainty of dose measurements based on a leastsquares fit can be reduced by increasing the number of calibration doses and the number of replicate dosimeters irradiated to each of them (Nagy et al., 2002). The instability of the gel absorbance after irradiation is yet another factor that affects the overall uncertainty of dose measurements in radiation therapy. The average increase of the absorbances of the gels stored in the dark at 10 °C was estimated to be 3.4% per day of storage after irradiation, or 0.142% per hour. So, post-irradiation absorbance stability is the main contributor to the overall uncertainty. It is assumed here that the intervals between the irradiations and absorbance measurements are the same for the calibration and test dosimeters. If the absorbances are measured during the first five hours of refrigerated storage and the uncertainty of the duration of the period is 72 h, the corresponding contribution to the overall uncertainty for the conventional FXG dosimeters will be 0.164% (1s) according to the following formula for the rectangular distribution (Sharpe and Miller, 2009; Soliman, 2014):
u(1σ ) =
(2 × 0. 142) =0. 164%. 3
The corresponding value for the FXG-PCA dosimeters is 0.246%. The overall uncertainty in dose measurements using the conventional and PCA gels in the dose range 1–15 Gy is 4.18% and 5.0%, respectively (2s, 95% confidence level) (Table 1). The uncertainty could be reduced by increasing the number of the replicate dosimeter cuvettes during calibration (Nagy et al., 2002) and by calibrating the gel dosimeter along with a transfer/reference
References Babic, S., Battista, J., Jordan, K., 2009. Radiochromic leuco dye micelle hydrogels: II. Low diffusion rate leuco crystal violet gel. Phys. Med. Biol. 54, 6791–6808. Bero, M.A., Gilboy, W.B., Glover, P.M., 2001. Radiochromic gel dosemeter for threedimensional dosimetry. Radiat. Phys. Chem. 61, 433–435. Davies, J.B., Baldock, C., 2008. Sensitivity and stability of the Fricke-gelatin xylenol orange gel dosimeter. Radiat. Phys. Chem. 77, 690–696. Devic, S., 2011. Radiochromic film dosimetry: past, present, and future. Phys. Med. 27, 122–134. Farah, N., Francis, Z., Abboud, M., 2014. Analysis of the EBT3 Gafchromic film irradiated with 6 MV photons and 6 MeV electrons using reflective mode scanners. Phys. Med. 30, 708–712. Fricke, H., Hart, E.J., 1966. Attix, F.H., Roesch, W.C., Tochilin, E. (Eds.), 2nd ed. Radiation Dosimetry 2. Academic, New York, pp. 167–239. Fricke, H., Morse, S., 1927. The chemical action of roentgen rays on dilute ferrosulphate solutions as a measure of dose. Am. J. Roentgenol. Radium Ther. Nucl. Med. 18, 430–432. Galante, A.M.S., Cervantes, H.J., Cavinatoa, C.C., et al., 2008. MRI study of radiation effect on Fricke gel solutions. Radiat. Meas. 34, 550–553. Gay, C.A., Gebicki, J.M., 2002. Perchloric acid enhances sensitivity and reproducibility of the ferric–xylenol orange peroxide assay. Anal. Biochem. 304, 42–46. Hiroki, A., Sato, Y., Nagasawa, N., Ohta, A., Seito, H., Yamabayashi, H., Yamamoto, T., Taguchi, M., Tamada, M., Kojima, T., 2013. Preparation of polymer gel dosimeters based on less toxic monomers and gellan gum. Phys. Med. Biol. 58, 7131–7141. IAEA, 2006. Absorbed Dose Determination in External Beam Radiotherapy: An International Code of Practice for Dosimetry Based on Standards of Absorbed Dose to Water (TRS398), Version 12. IAEA, Vienna, p. 179. ISO/ASTM 51707, 2004. Standard Guide for Estimating Uncertainties in Dosimetry for Radiation Processing. Annual Book of ASTM Standards. ASTM International, West Conshohocken, PA. Jin, C., Chen, J., Yang, L., Luo, W., Wu, G., Zha, Y., 2012. Effect of DMSO on the sensitivity and diffusion of FPGX gel dosimeter. Radiat. Phys. Chem. 81, 879–883. Keall, P., Baldock, C., 1999. A theoretical study of the radiological properties and water equivalence of Fricke and polymer gels used for radiation dosimetry. Australas. Phys. Eng. Sci. Med. 22, 85–91. Khan, F.H., 2010. Physics of Radiation Therapy, 4th ed. Lippincott Williams & Wilkins, pp. 125–133. Nagy, V., Sholom, S.V., Chumak, V.V., Desrosiers, M.F., 2002. Uncertainties in alanine dosimetry in the therapeutic dose range. Appl. Radiat. Isot. 56, 917–929. Pirani, L.F., De Oliveira, L.N., Petchevist, P.C., Moreira, M.V., De Almeida, A., 2009. New chemical Fricke gel radiation dosimeter. J. Radioanal. Nucl. Chem. 280 (2), 259–264. Sharpe, P., Miller, A., 2009. Guidelines for the Calibration of Routine Dosimetry Systems for Use in Radiation Processing. NPL Rep. CIRM 29. Teddington, UK, National Physical Laboratory. ISSN 1369-6793. Soliman, Y.S., 2014. Gamma-radiation induced synthesis of silver nanoparticles in gelatin and its application for radiotherapy dose measurements. Radiat. Phys. Chem. 102, 60–67.
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Effect of perchloric acid on the performance of the Fricke xylenol gel dosimeter M.I. El Gohary a, Y.S. Soliman b,n, E.A. Amin c, M.H. Abdel Gawad a, O.S. Desouky b a
Biophysics Branch, Physics Department, Faculty of Science, Al-Azhar University, Nasr City, Cairo, Egypt National Center for Radiation Research and Technology, Atomic Energy Authority (AEA), P. O. Box 8029, Nasr City, Cairo, Egypt c Medical Physics Unit, Ain-Shams Hospital, Ain-Shams University, El Abbasia, Cairo, Egypt b
H I G H L I G H T S
Perchloric acid enhances the radiation sensitivity of the ferrous xylenol orange gel. The gel, either with or without the acid, shows a linear dose response in the range 1–15 Gy. Storage environment factors need to be well controlled to minimize dose errors.
art ic l e i nf o
a b s t r a c t
Article history: Received 15 October 2015 Received in revised form 24 April 2016 Accepted 24 April 2016 Available online 26 April 2016
The conventional ferrous xylenol orange (XO) gel (FXG) dosimeter is being wildly investigated for radiotherapy dose measurements. Upon irradiation, its color turns red due to oxidation of Fe2 þ into Fe3 þ , which forms a complex with xylenol orange. The effect of perchloric acid (PCA) on the dosimetric properties of the gel in the dose range of 1–15 Gy was investigated using visual spectrophotometry. FXGPCA responds to radiation dose linearly and exhibits higher radiation sensitivity than the conventional gel dosimeter. PCA in a concentration of 20 mM enhances the radiation sensitivity 44%. Stability of the absorbances of both the gels during storage under various conditions was investigated, and the uncertainty of dose measurements was estimated. & 2016 Elsevier Ltd. All rights reserved.
Keywords: Dosimetry Radiochromic gel Radiotherapy
1. Introduction Radiation therapy uses chemical dosimeters of many types (Khan, 2010). Among them are mixtures of ferrous ions with xylenol orange (XO) (Davies and Baldock, 2008); acrylate monomers (Hiroki et al., 2013); radiochromic dyes, such as leuco crystal violet (LCV) (Babic et al., 2009); Gafchromic films (Devic, 2011; Farah et al., 2014) and gelatin containing Ag þ (Soliman, 2014). One of the common dosimeters in this category is based on the Fricke system and contains, in addition to the ferrous ions, xylenol orange in gelatin (FXG) (Fricke and Morse, 1927; Fricke and Hart, 1966; Core et al., 1984; Galante et al., 2008). It exhibits excellent water and tissue equivalence below 100 keV of photon energy (Keall and Baldock, 1999). Upon a γ-ray exposure, ferrous ions in the gel get oxidized into ferric ions, which form bonds with XO; this process is accompanied by a change of the color of the gel n
Corresponding author. E-mail address: [email protected] (Y.S. Soliman).
http://dx.doi.org/10.1016/j.apradiso.2016.04.024 0969-8043/& 2016 Elsevier Ltd. All rights reserved.
(Bero et al., 2001; Davies and Baldock, 2008). The radiation sensitivity of this system is high, and the absorbance of such dosimeters grows linearly with the dose up to 25 Gy (Davies and Baldock, 2008). However, oxidation of Fe2 þ continues even after the end of irradiation, which shifts the dose response function with time. Various additives were previously proposed to improve the radiation sensitivity of FXG to radiotherapeutic doses (Davies and Baldock, 2008; Pirani et al., 2009; Jin et al., 2012). In this work, we studied effects of perchloric acid (PCA) on the radiation sensitivity and dosimetric performance of the conventional FXG. We have found that PCA in the FXG gel decreases the dependence of the absorbance of the Fe2 þ -XO complex on the acid concentration and decreases the sensitivity of the materials to added components described by Gay and Gebicki (2002). We studied the stability of the dose response of both the gel types (FXG and FXG-PCA) under various environmental conditions. In addition, the overall uncertainty of dose measurements in a radiotherapeutic γ-ray facility with both the dosimeters was estimated.
M.I. El Gohary et al. / Applied Radiation and Isotopes 113 (2016) 66–69
67
2. Materials and methods 2.1. Gel dosimeter preparation Four batches of FXG solutions were prepared, each containing 4% (wt) of gelatin (300 Bloom Gelatin from porcine skin, SigmaAldrich) and 0, 10, 20 or 30 mM of perchloric acid (Aldrich, 70% ACS Reagent Grade). The mixtures also contained 50 mM of sulfuric acid, 1 mM of ferrous ammonium sulfate hexahydrate (Aldrich) and 0.1 mM of the ferric ion indicator XO (Oxford Laboratory, India). All the mixtures were stirred magnetically at 30 °C then transferred into polymethyl methacrylate (PMMA) capped cuvettes (1 1 4.5 cm3) and allowed to form transparent gels overnight in a refrigerator at 6 °C. 2.2. Irradiation and measurement procedures The prepared gels were exposed to γ-rays from a 60Co radiotherapy unit (PHOENIX Theratrons, MDS Nordion, Canada) at room temperature. The dose rate of the unit was measured according to the IAEA TRS 398 protocol with a calibrated ionization chamber (IAEA, 2006). We irradiated the gels in the cuvettes positioned into a cubic wax phantom (10 10 10 cm3). The gels were irradiated with four fields at the gantry angles of 0, 90, 180 and 270°, a fixed collimator angle (0°) and a fixed square field size (10 10 cm2) at a source-to-surface-distance (SSD) of 80 cm. These conditions had previously been used in the output calibration; the dose rate at the center of the phantom was reported as 1.2 Gy/min. The optical absorbances of the cuvettes (1 cm light path) were measured with a double-beam SPECORDs spectrophotometer (Analytik Jena AG, Jena, Germany) in the wavelength range of 300– 600 nm. Absorbances at 570 nm were used in the analysis.
Fig. 1. Dose response functions of the FXG dosimeters containing different concentrations of PCA (570 nm).
3. Results and discussion 3.1. Dose response functions and radiation sensitivity Fig. 1 shows dose response curves for FXGs with various PCA concentrations in the dose range of 1–15 Gy. The dependences of the absorbances on the dose are linear with correlation coefficients (r2) of 0.9996, 0.9910, 0.9998, and 0.9998 for the gels with 0, 10, 20, and 30 mM PCA, respectively. Fig. 2 shows the slope of linear dose response function (radiation dose sensitivity) as a function of the PCA concentration in the gel. Addition of PCA increases the radiation sensitivity of the gel, and the increase can be described by a quadratic polynomial function with r2 ¼0.9999. The radiation sensitivity reaches 0.0915 Gy 1 at 20 mM PCA and decreases slightly at higher concentrations of PCA. An increase in the PCA concentration from 0 to 20 mM enhances the sensitivity 44%. These results show that PCA in the concentration range 0– 30 mM improves the radiation sensitivity and the linearity of the dose response of the conventional gel. 3.2. Pre- and post-irradiation color stability Figs. 3 and 4 show the net absorbances at 570 nm as functions of the storage time for unirradiated and irradiated conventional and PCA-containing gels, respectively. Both gels were stored at room temperature ( 25 °C) in the dark and under laboratory light, as well as at 10 °C in a refrigerator. The unirradiated and irradiated conventional gels (Fig. 3) stored at 10 °C exhibit good stability with an increase of their responses within only 1–4% during the initial 24 h of storage, depending on the absorbed dose. However, their absorbances increased significantly with the time
Fig. 2. Radiation dose sensitivity of the FXG dosimeters as a function of the PCA concentration in the dose range of 1–15 Gy.
of storage at room temperature, both in the dark and in the light. The increases after 24 h of storage were in the ranges of (13–25)% and (20–30)% for the gels stored in the dark and in the light, respectively. The absorbances of unirradiated PCA-FXG gels (Fig. 4) stored at 10 °C grew with time initially and tended to stabilize after 48 h of storage. By contrast, the absorbances of the gels stored at room
68
M.I. El Gohary et al. / Applied Radiation and Isotopes 113 (2016) 66–69
Fig. 3. Pre- and post-irradiation absorbance stability of the conventional FXG dosimeter stored under various conditions.
temperature, either in the dark or in the light, increased significantly. The absorbances of irradiated PCA-containing gels decrease (2–8)%, depending on absorbed dose, during the first 24 h of storage at 10 °C. On the contrary, the absorbances of the gels stored at room temperature in the dark or in the light increase (21–70)% in 24 h of storage. The growth of the absorbances with time for both the gel types shows that oxidation of Fe2 þ into Fe3 þ continues. Thus, in order to increase the shelf life of the unirradiated gels, it is strongly recommended to store them in the dark at 10 °C. Otherwise, they must be used immediately after preparation. In addition, it is strongly recommended to put gels into a refrigerator immediately after their irradiation to slow down the oxidation of Fe þ 2 into Fe þ 3 and the natural increase in the absorbance. Otherwise, the absorbances of the gel dosimeters must be measured immediately after the irradiation.
4. Uncertainty of the estimated doses There are various factors contributing to the overall uncertainty, such as uncertainties of the calibration of the radiotherapy unit, of the homogeneity of the dosimeter batch, of the absorbance measurement, of the calibration curve fit, and of the
Fig. 4. Pre- and post-irradiation absorbance stability of the FXG-PCA dosimeter stored under various conditions.
temporal stability of the absorbance (Table 1). Some of the uncertainty contributions are discussed in the footnotes of Table 1. The others can be summarized as follows. The standard uncertainty of calibration of the ionization chamber reported in the certificate of calibration from IBA Dosimetry GmbH, Germany was 1.1%. The ionization chamber was used to calibrate the radiotherapy units with the overall uncertainty of 1.2%. The gel batch uniformity was tested by irradiating four sets of gel samples to various doses, measuring them under identical conditions and analyzing the results statistically (Sharpe and Miller, 2009; Soliman, 2014). The estimated uncertainties were 0.95% and 0.72% for the conventional and the PCA-containing gel dosimeter, respectively. The standard uncertainties of the calibration curves were calculated from the residuals (Sharpe and Miller, 2009; Soliman, 2014). The best linear fits were found with a commercial program Table Curve 2D (Version 5.01, SYSTAT Software, Inc., San Jose, CA, USA), and the standard uncertainties of the fits were found using the formula (Sharpe and Miller, 2009)
u=
Σ (residuals )2 , nd − nc
M.I. El Gohary et al. / Applied Radiation and Isotopes 113 (2016) 66–69
Table 1 Uncertainty budget for radiotherapeutic dose measurements with the conventional FXG and the PCA-FXG gel dosimeters in the dose range of 1–15 Gy. Uncertainty component
Type of uncertainty
Uncertainty, % conventional FXG
PCA-FXG
Radiation source calibration Other irradiation componentsa Spectrophotometer sensitivity variationsb Reproducibility of measurementsc Batch in inhomogeneity Calibration curve fit Temporal stability of absorbance Combined standard uncertainty (uc), 1s Expanded uncertainty (2r)
B
1.12
1.12
B
0.33
0.33
A
0.11
0.11
A
0.25
0.28
A A A
0.95 1.41 0.16
0.72 2.05 0.24
2.09
2.50
4.18
5.00
69
dosimeter in the irradiation unit under actual irradiation conditions (ISO/ASTM, 51,707, 2004). Storage conditions are important for both the gel types; they can result in a significant error of a determined dose if not taken into account properly.
5. Conclusion Both gel types can be useful for dose measurements in radiotherapy if the irradiation conditions, environmental factors and measurement procedures are well standardized. Addition of 20 mM PCA to the conventional FXG increases the radiation sensitivity 44%. The absorbances of both the gels increase significantly with time after irradiation; thus, the time of spectrophotometric measurements during calibration and dose measurements must be standardized to minimize the errors in dose monitoring. The overall uncertainties of dose measurements with the conventional FXG and FXG-PCA were found to be 4.18% and 5.0% at the 95% confidence level, respectively.
a
Geometry, source decay correction, timer setting and field nonuniformity. Estimated from absorbance measurements of an irradiated gel at 570 nm 100 times with the gel cuvette intact in the sample position. c Estimated from absorbance measurements of an irradiated gel at 570 nm 100 times with the gel cuvette taken out and put back into the sample holder after each measurement. b
where nd is the number of dosimeters and nc is the number of coefficients in the selected fitting function. The uncertainties found from the very good fits (r2 E0.999) were 1.41% and 2.05% for the conventional and the PCA-containing gel dosimeter, respectively. In the calculations, all individual dose points were taken into account (not averages of the replicates for the same dose point). The uncertainty of dose measurements based on a leastsquares fit can be reduced by increasing the number of calibration doses and the number of replicate dosimeters irradiated to each of them (Nagy et al., 2002). The instability of the gel absorbance after irradiation is yet another factor that affects the overall uncertainty of dose measurements in radiation therapy. The average increase of the absorbances of the gels stored in the dark at 10 °C was estimated to be 3.4% per day of storage after irradiation, or 0.142% per hour. So, post-irradiation absorbance stability is the main contributor to the overall uncertainty. It is assumed here that the intervals between the irradiations and absorbance measurements are the same for the calibration and test dosimeters. If the absorbances are measured during the first five hours of refrigerated storage and the uncertainty of the duration of the period is 72 h, the corresponding contribution to the overall uncertainty for the conventional FXG dosimeters will be 0.164% (1s) according to the following formula for the rectangular distribution (Sharpe and Miller, 2009; Soliman, 2014):
u(1σ ) =
(2 × 0. 142) =0. 164%. 3
The corresponding value for the FXG-PCA dosimeters is 0.246%. The overall uncertainty in dose measurements using the conventional and PCA gels in the dose range 1–15 Gy is 4.18% and 5.0%, respectively (2s, 95% confidence level) (Table 1). The uncertainty could be reduced by increasing the number of the replicate dosimeter cuvettes during calibration (Nagy et al., 2002) and by calibrating the gel dosimeter along with a transfer/reference
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