CHEMICAL DOSIMETRY OF GAMMACELL WITH FERROUS SULFATE

Size: px

Start display at page:

Download "CHEMICAL DOSIMETRY OF GAMMACELL WITH FERROUS SULFATE"

Melissa Johnston
5 years ago
Views:

1 2009 International uclear Atlantic Conferce - IAC 2009 Rio de Janeiro,RJ, Brazil, September27 to October 2, 2009 ASSOCIAÇÃO BRASILEIRA DE EERGIA UCLEAR - ABE ISB: CHEMICAL DOSIMETRY OF GAMMACELL WITH FERROUS SULFATE Carlos Austerlitz 1, Vanesa Panettieri 2, Diana Campos 1, Maria Clara Ferreira 1, Sidi Bhabib 1 and Alfredo Lopes Filho 3 1 East Carolina University 600 Moy Blvd, Greville, C, USA camposc@ecu.edu 2 Clatterbridge Ctre for Oncology Clatterbridge road Bebington CH63 4JY Wirral, UK vpanettieri@yahoo.com 3 Comissão acional de Energia uclear, BR Av. Prof. Luiz Freire, 200, Recife, PE, Brazil alopesfulho@correios.net.br ABSTRACT The influce of Compton scatter radiation from a Gammacell-220 on the ferric-ion yield [G(Fe +3 )] was determined for the Fricke dosimetry. Monte Carlo simulations were performed using the PEELOPE code to obtain the photon spectrum of a 60 Co ordion teletherapy unit at a depth of 2 cm in a 50x50x50 cm 3 cubic water tank. Published values of G(Fe +3 ) were fitted by a third order polynomial and the resulting equation was used to determine a mean chemical yield of such unit. The same procedure was performed over the spectrum of a Gammacell-220 published by the ASTM. The mass-ergy absorption coefficit for the Fricke solution was weighted over the Gammacell-220 spectrum and compared against the ordion spectrum. The ratio betwe the mean chemical yields was used to determine the influce of the Compton scatter radiation on the value of G(Fe +3 ). The ratio of the mass-ergy absorption coefficit of water to Fricke solution was used to convert the absorbed dose in the Fricke solution to absorbed dose to water. From the results obtained it was concluded that the dosimetry of a Gammacell-220 with the Fricke dosimeter may overestimate the measured absorbed dose to water by a factor of 1.01 due to changes in the G(Fe +3 ) value. Differces in the mean ergy of the spectra can lead to large errors in the isodoses curves of samples irradiated with both equipmts. To establish evidce that the radiation process will provide the desired results, the knowledge of the radiation spectrum is needed. Alternatively, whever it is possible, the dosimetry should be performed by positioning the capsule with the Fricke solution inside of dummy samples. 1. ITRODUCTIO The spectrum of a 60 Co from a radiotherapy unit, a gammacell or an industrial radiation processing, include two photopeaks of gamma ray emission and Compton scatter radiation. The

2 amount of Compton scatter radiation depds on the design of such irradiation devices and the position in which a detector or a medium are being exposed. The absorbed dose to water with the ferrous sulfate dosimeter, for the ergy of the 60 Co is calculated using the radiation yield of ferric ion, G(Fe )[1, 2]. This factor has be determined with filled ampoules with Fricke solution irradiated at known absorbed dose values at a depth of a water tank (e.g. 5 gcm -2 ) with a teletherapy unit. The ferrous sulfate dosimeter has be widely used to perform chemical dosimetry of Gammacells[3-6] loaded with 60 Co radioactive sources using the convtional G(Fe ) value, e.g., ICRU 17[7]. The differces in the shape and intsity of the Compton scatter of these units are expected to lead to error in the calculation of the absorbed dose determined with the Fricke dosimeter. This work has had the aim to evaluate the influce of the Compton scatter radiation from a Gammacell-220 on G(Fe +3 ) for the Fricke dosimetry. 2. METHODOLOGY Monte Carlo (MC) simulations were performed using the PEELOPE code[8, 9] with the peasy package to obtain the photon spectrum of a 60 Co ordion radiotherapy unit at a depth of 2 cm in a 50x50x50 cm 3 cubic water tank (source to axis distance equal to 80 cm). The set of G(Fe +3 ) values recommded by the ICRU[7, 10] together with the G(Fe +3 ) for the 192 Ir published by A.O. Frege[11] were fitted by a third order polynomial. The resulting equation was used to weight the corresponding G(Fe +3 ) value over the ordion spectrum to determine the mean chemical yield s value, G [( Fe )] RD, by means of Eqn. 1. The same procedure was performed over a typical spectrum of a Gammacell-220 (no filter) tak from the ASTM[12] to derive a mean G [( Fe )] GMC for the irradiator using the Eqn. 1. The mean ergy, E, of the spectra was calculated by the Eqn. 2, and the mean mass-ergy absorption coefficit, ( µ, of the F Fricke solution was calculated using the Eqn. 3. G( Fe ) E = 1.40 E = 0.05 G( Fe ) E E MeV E MeV [1] E E E E [2] ( µ F ( µ E E E [3]

3 where, [ ( / d( ] dφ is the photon fluce ergy per ergy interval and E is the photon ergy interval. The mean yield values, G [( Fe )] GMC and G [( Fe )] RD, were compared to determine the influce of the Gammacell-220 Compton scatter on the ICRU G(Fe +3 ). For this purpose, it was assumed that the G(Fe +3 ) value recommded by the ICRU for the 60 Co ergy was determined with a spectrum radiation similar to that of the ordion unit. The mean photon ergy from both spectra were utilized to characterize the spectra, and the ratio of the mass-ergy absorption coefficit of water to Fricke solution was used to convert the absorbed dose in the Fricke solution to absorbed dose to water. 3. RESULTS AD DISCUSSIO Figure 1 shows the curve of the yields values as a function of the photon ergy. The curve fitted by a third order polynomial resulted in the coefficits; x 0 = , x 1 = , x 2 = , x 3 = with a linear correlation coefficit equal to r 2 = Figures 2 and 3 show the relative differtial photon fluce as a function of the photon ergy for the ordion and the Gammacell-220, respectively. The ratio betwe the mean weighted G(Fe +3 ) values for the ordion and the Gammacell-220 differ by a factor of It means that the absorbed dose to water in the Fricke solution, capsulated in a 0.5 cm thick tissue equivalt vial, irradiated with a Gammacell-220 unit (Eqn. 4[1]), increases by the same factor,i.e., D A = 3 G( F ). d. ρ. ε F + e where A is the net absorbance at the optimum wavelgth (302 to 304 nm), ρ is the dsity of the dosimetric solution, d is the optical pathlgth and ε is the molar linear absorption coefficit of the ferric ions. The mean photon ergy for the ordion and the Gammacell-220 spectra were MeV and MeV, respectively. [4] G(Fe ) [ion/100 ev) Coefficits: b[0] = b[1] = b[2] = b[3] = r ² = Photon ergy (kev) Figure 1. Depdce of (GFe ) with the photon ergy

4 Compared with the ordion spectrum, the Gammacell-220 spectrum has a relatively high amount of low Compton scatter radiation. It lowers not only the mean photon ergy of the spectrum but also the G(Fe +3 ) value. Hce, as expected, this change in the shape of the spectrum does increase the uncertainty in the absorbed dose to the Fricke solution (assuming the ICRU s G(Fe +3 ) value is valid for a photon spectrum close to that of the ordion unit). 40 Relative number of photons ordion Photon ergy (MeV) Figure 2. The spectrum of the ordion unit. 1.0 Relative number of photons Gammacell Photon ergy (MeV) Figure 3. Typical spectrum of a typical Gammacell-220 without filter[12]. Following the recommdations of the ICRU Report 64[13], the absorbed dose to water, D W, can be determined from the mean absorbed dose in the Fricke solution, D F, using the Eqn. 5: D W = ( µ W F PW, D, F F [5]

5 where, (, µ, is the ratio of the mass-ergy absorption coefficit of water to Fricke W F solution and P W, is the correction factor of the perturbation introduced by the dosimeter vessel. F This later factor is of no concern in this work. As can be se in Figure 4, for photon ergy betwe 0.1 MeV and 1.25 MeV the values of the mass absorption coefficit for the Ferrous sulfate (Standard Fricke[14]) are almost equal to that of water. Changes in the value of the mean photon ergy will not significantly affect the conversion absorbed dose from the Fricke solution to that of liquid water wh this dosimeter is being used to determine absorbed dose to water delivered by both machines. For example, the ratios of µ for 1.15 MeV (ordion spectrum) and MeV (Gammacell-220 ( W, F spectrum) are x 10-2 cm 2 g -1. In this case, the differce in the absorbed dose assessed with the Fricke dosimeter is caused by the change in the G(Fe +3 ) value. However, if the Fricke dosimeter is irradiated with the same number of photons with both equipmt, there will be a differce in the absorbed dose. Assuming a value of the mass- ergy absorption coefficit of the water for the ordion unit of x 10-2 cm 2 g -1 (1.15 MeV) and x 10-2 cm 2 g -1 (0.818 MeV) for the Gammacell-220, the ratio ( µ is The resulting overall differce in the W, F absorbed dose in the Fricke solution irradiated with the Gammacel-220 is the product of 1.01 by 0.93, which is evertheless, from the output dosimetry point of view this effect only makes the Gammacell-220 irradiator less efficit wh comparable with the ordion unit. 1 Mass ergy absorption coefficit (cm 2 /g) 0.1 Water Fricke Photon ergy (MeV) Figure 4. Mass-ergy absorption coefficits for Fricke solution and liquid water[14]. The ergy absorbed to water determined with the Fricke solution is relatively indepdt of the ratio ( µ. Conversely, it does not mean that the isodoses curves of a sample irradiated W, F with the ordion and gammacell units will be same. The mass-attuation coefficit for the blood are 7.72 x 10-2 cm 2 g -1 (0.818 MeV) and 6.55 x 10-2 cm 2 g -1 (1.15 MeV). Using this numbers, 5

6 a 3-cm thickness blood sample attuates about 4% less the ordion spectrum wh compared with that of the Gammacell-220. Besides that, dosimetry of 60 Co gamma chamber has be performed with ferrous sulphate-bzoic acid-xylol stored in large plastic bottles (2-cm diameter and 3-cm height[3]). The average dose effect has be not considered in such work, which lower the value of the absorbed dose assessed with the Fricke dosimeter. The specification of the output of a gammacell irradiator determined with the Fricke dosimeter requires not only the value of the absorbed dose but also the knowledge of its spectrum. Alternatively, the Fricke dosimeter can be placed inside of a dummy sample to determine the behavior of the isodoses curves and doses that will be delivered to the sample. 4. COCLUSIOS The Compton scatter from a Gammacell-220 spectrum has be compared with that from a ordion teletherapy unit to determine their effects on the Fricke dosimeter response. It was concluded that the dosimetry of a Gammacell-220 with the Fricke dosimeter may overestimate the measured absorbed dose to water by a factor of 1.01 due too changes in the G(Fe +3 ) value. A negligible differce was observed in the ratio of ( µ to convert the absorbed dose in the W, F Fricke solution to that of water. However, the differces in the mean ergy of the spectra can lead to large errors in the isodoses curves inside of samples irradiated with both equipmts. To establish evidce that the radiation process will provide the desired results, the knowledge of the radiation spectrum is needed. Alternatively, whever possible, the dosimetry should be performed by positioning the capsule with the Fricke solution inside of dummy samples. REFERECES 1. AST, American ational Standard, Standard Practice for Using the Fricke Referce- Standard Dosimetry System. American Society for Testing and Materials, 2005(Designation: E ). 2. IAEA, Dosimetry for food irradiation. International Atomic Energy Agcy, Technical Reports Series no Upadhyay, S.., Dosimetry of Cobalt 60 Gamma Chamber. Defce Scice Journal, (3): p Chung, W.H., Polymer gamma dosimeter. Journal of Appliied Polymer Scice, (7): p MEHTA, K., High-dose Standardization Service of the IAEA. Appl. Radiat. Isot., (11/12): p Emanuela Bortolin, C.B., Arcangelo Calicchia, Angelo Alberti, Piergiorgio Fuochi and Sandro Onori, Irradiated herbs and spices detection: light-induced fading of the photostimulated luminescce response. International Journal of Food Scice and Technology, : p ICRU, Radiation Dosimetry: X-rays Originated At Pottials of 5 to 150 kv. International Commission on Radiation Units and Measuremts, ICRU Report 17, Panettieri, V., et al., Monte Carlo simulation of MOSFET detectors for high-ergy photon beams using the PEELOPE code. Phys Med Biol, (1): p

7 9. Panettieri, V., J. Sempau, and P. Andreo, Chamber-quality factors in 60Co for three planeparallel chambers for the dosimetry of electrons, protons and heavier charged particles: PEELOPE Monte Carlo simulations. Phys Med Biol, (21): p ICRU, Radiation dosimetry: X-rays and gamma-rays with maximum photon ergies betwe 0.6 and 50 MeV. International Commission on Radiation Units and Measuremts, ICRU Report 14, Frege, A.O., Calibration of the ferrous sulfate dosimeter by ionometric and calorimetric methods for radiations of a wide range of ergy. Radiat Res, 1967(31, 2): p ASTM, Standard Practice for Minimizing Dosimetry Errors in Radiation Hardness Testing of Silicon Electronic Devices Using Co-60 Sources. American Society for Testing and Materials, 2005(Designation: E ). 13. ICRU, Dosimetry of High-Energy Photon Beams based on Standards of Absorbed dose International Commission on Radiation Units and Measuremts, ICRU Report 64, IST, Tables of X-ray Mass Attuation Coefficits and Mass Energy-absorption Coefficits. ational Institute of Standards and Technology, 1996(ISRIR 5632). 7

MODELLING A GAMMA IRRADIATION PROCESS USING THE MONTE CARLO METHOD

2011 International Nuclear Atlantic Conference - INAC 2011 Belo Horizonte, MG, Brazil, October 24-28, 2011 ASSOCIAÇÃO BRASILEIRA DE ENERGIA NUCLEAR - ABEN ISBN: 978-85-99141-04-5 MODELLING A GAMMA IRRADIATION