Electron absorbed dose measurements in LINACs by thermoluminescent dosimeters

Applied Radiation and Isotopes 83 (2014) 210–213

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Electron absorbed dose measurements in LINACs by thermoluminescent dosimeters J. Rodríguez Cortés a,b,n, R. Alvarez Romero a, J. Azorín Nieto c, T. Rivera Montalvo a a

Centro de Investigación en Ciencia Aplicada y Tecnología Avanzada-Legaria, IPN. Av. Legaria 694, 11500 México D.F., Mexico Hospital General de México, Dr. Balmis 148, Col. Doctores, 06726 México D.F., Mexico c Universidad Autónoma Metropolitana-Iztapalapa, Av. San Rafael Atlixco 186, 09340 México D.F., Mexico b

H I G H L I G H T S

   

Experimental results of ZrO2 irradiated by high energy electron beam. Dosimetric characteristics of CaSO4:Dy were obtained under high energy electron effect. Absorbed dose in electron beam was determined by TL phosphors. Absorbed dose could be measured by TL phosphors and the results suggest that phosphors are good candidate for absorbed dose determining.

art ic l e i nf o

a b s t r a c t

Available online 21 June 2013

In this work, electron absorbed doses measurements in radiation therapy (RT) were obtained. Radiation measurements were made using thermoluminescent dosimeters of calcium sulfate doped with dysprosium (CaSO4:Dy) and zirconium oxide (ZrO2). TL response calibration was obtained by irradiating TLDs and a Farmer cylindrical ionization chamber PTW 30013 at the same time. Each TL material showed a typical glow curve according to each material. Both calcium sulfate doped with dysprosium and zirconium oxide exhibited better light intensity to high energy electron beam compared with lithium fluoride. TL response as a function of absorbed dose was analyzed. TL response as a function of high energy electron beam was also studied. & 2013 Published by Elsevier Ltd.

Keywords: Electron beam dosimetry LINACs Thermoluminescence CaSO4:Dy

1. Introduction Radiation oncology employs ionizing radiation in the treatment of cancerous cells. Two methods (Teletherapy and Brachytherapy) are used to deliver the ionizing radiation to the target volume (NCRP, 2005). A quality assurance program ensures that all treatment facilities used in radiotherapy are properly checked for accuracy or consistency that all irradiations facilities are functioning according to the manufacturer specifications (IAEA, 1987). Dosimetry deals with methods for the quantitative determination of the absorbed delivered by medical electron linear accelerator are used in radiotherapy treatment (AAPM, 1999; IAEA, 2000). Different types of radiation detectors are used currently for the measurement of absorbed dose which include ionization chamber, diodes, film, and thermoluminescent dosimeters. Thermoluminescent dosimetry has an increasing demand for dosimetry of the beams of particles because they are increasingly used for

n Corresponding author at: Centro de Investigación en Ciencia Aplicada y Tecnología Avanzada-Legaria, IPN. Av. Legaria 694, 11500 México D.F., Mexico. E-mail address: [email protected] (J. R. Cortés).

0969-8043/$ - see front matter & 2013 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.apradiso.2013.06.013

diagnostic and therapeutic purposes, mainly for quality assurance (Rivera, 2012). Although TL dosimetry has a small sector in clinical dosimetry, it has been largely used for quality assurance of clinical dosimetry by postal dose inter-comparison. Dosimetry and quality assurance protocols recently in use recommend a number of parameters for qualifying beam quality for heterogeneous X-rays generated by medical LINACs. The choice of parameters for qualifying beam quality is governed by the objective of the given task. The parameter d80 measured under the electroncontamination-free condition as recommended by AAPM (1999) and ICRU (1984) for D10 will be a better choice for verifying manufacturer-specified nominal X-ray-beam energy and determining shielding thickness, as it shows an increasing nature with increasing nominal beam energy. However, for determining the beam-quality conversion factor for beam dosimetry purposes, protocol-specific beam-quality indicator should be measured and used. Dosimetry data generated following a standard technique at the hospital site should be used for patient dosimetry, irrespective of the difference in stated and measured energy of the X-ray beam (IAEA, 2000). High electron beams from accelerators are widely used for irradiation in various fields of basic researches, and industrial or

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medical applications. In medical applications mainly in radiation therapy dosimetry must be known to ensure that the prescribed absorbed dose is delivered to the target volume in the patient (IAEA, 1987; Svensson and Brahme, 1986). High electron beam dosimetry is performed in order to determine absorbed doses to patients who are exposed to ionizing radiation, either for therapy or diagnosis (Kron, 1995). Therefore, absorbed dose is a quantity of fundamental interest, and may be specified in human body for any type of ionizing radiation. Traditionally, absorbed dose determination with TLD in radiation therapy has been mainly restricted to personal dosimetry and local investigations, in clinical practice, the main detectors used for absorbed dose measuring are ionizing chambers and semiconductors. The aim of the present work is to measure absorbed dose in a high energy electron beams facilities by means TL phosphors of CaSO4:Dy and ZrO2.

2. Experimental details Samples used for this investigation were ZrO2 and CaSO4:Dy, both phosphors were obtained by the precipitation method as reported in previous works (Azorín et al., 1980, 1984; Rivera et al., 2006,). The polycrystalline powders obtained were further annealed at 900 1C for 2 h under an oxygen atmosphere. In order to facilitate handling, samples were made in pellet form. Pressing the thermoluminescent material and polytetrafluoroethylene (PTFE) in the mixed ratio (2:1) at room temperature, pellets of 5 mm diameter and 0.8 mm thickness were obtained. Then the sinterization process was carried out at a temperature slightly lower than that of the PTFE melting, using a technique described in previous works (Azorín et al., 1993; Azorín, 1990). Before exposure of the phosphors (CaSO4:Dy+PTFE and ZrO2+PTFE) to the gamma radiation, they were annealed at 300 1C during 30 min for CaSO4:Dy; meanwhile ZrO2+PTFE samples were annealed at a thermal treatment of 300 1C for 10 min in order to erase all possible remaining information. The thermoluminescence of the phosphors in pellet form was checked by irradiation in a gamma radiation source of Co-60 with an absorbed dose of 100 cGy and the thermoluminescent glow curve was analyzed. For determine absorbed dose and others dosimetric characteristics of phosphors under high energy electron beam effect, all samples were irradiated with clinical high energy electron beams using an accelerator Siemens model Primus, with different doses at energies of 6, 9, 12, 15 and 18 MeV using a radiation field size of 10  10 cm2. A solid water phantom (SWP) was used to maintain the TLD positioned at the center of the 10  10 cm2 radiation field formed at the phantom (30  30  30 cm3) surface, located at 1 m of the electron beam focus, at the depth of maximum ionization, R100, of the height energy, E, electron beam incident in the phantom material under standard practice dosimetry irradiation, (AAPM TG-51). Calibration measurements were carried out using a Farmer cylindrical ionization chamber PTW 30013 was positioned according to the IAEA TRS 398 (IAEA 2000). The studied dosimetric characteristics were: thermoluminescent (TL) glow curve, fading of the TL response, TL response as a function of electrons dose, reproducibility of the dosimetric response and energy dependence of the TL response. Thermoluminescent readings were carried out by using an analyzer Harshaw model 3500 connected to a PC in order to store and to analyze the glow curves, digitizing both TL and temperature signals by means of two channels of an RS232C interface. The linear heating rate of the TL analyzer was kept at 10 1C/s for all the TL readings, which were made integrating the signal from 50 up to 350 1C. All TL measurements were made in a nitrogen atmosphere

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in order to reduce the thermal noise from the heating planchet of the TL reader.

3. Results and discussion According to the literature data (Azorín et al., 1993; Palacios et al., 2012), the mean TL peak, must appear between 190 1C and 250 1C. Fig. 1 shows the thermoluminescence emission of both TL phosphors under electron beam irradiated. This figure shows the TL glow curve of CaSO4:Dy+PTFE (denoted by dashed line) irradiated by high energy electron beams with a nominal energy of 6 MeV, at an absorbed dose of 100 cGy. As it can be seen in this figure, thermoluminescent glow curve of CaSO4:Dy+PTFE is composed of two peaks centered at around at 150 and 240 1C respectively. Meanwhile, thermoluminescent emission of ZrO2+PTFE (denoted by solid line) irradiated with high energy electron beams with a nominal energy of 6 MeV, at an absorbed dose of 100 cGy, TL glow curve of ZrO2+PTFE exhibited just one peak centered at around 240 1C. The reproducibility is very important when measuring electrons absorbed dose values. This parameter was determined by irradiating 20 detectors, during 10 times, at the same experimental conditions. Fig. 2 shows TL intensity of TL materials irradiated with 6 MeV electrons as a function of various cycles processing. The reproducibility of TL readings of CaSO4:Dy irradiated by high energy electron beams was 7 3%. The reproducibility of TL readings of ZrO2+PTFE under electron beams was 74.0% after 10 measurements. Other TL characteristics which is related with absorbed dose, is the fading, this is the TL response remaining in the phosphor at different post irradiation times. A group of TL chips was irradiated at an absorbed dose of 100 cGy with high energy electron beams

Fig. 1. TL glow curves of phosphors irradiated with 6 MeV electrons beams.

Fig. 2. Reproducibility of TL response of phosphors electron beam irradiated with 6 MeV.

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also reported as a function of electron beam energy. In this figure it can be seen that the energy dependence of TL response of both phosphors for electrons with nominal energies above 6 MeV (6, 9, 12, 15 and 18 MeV) varies little. Correction factor value obtained for 4 MV electron beam was almost 2%. However, for clinical application 6 MV electron beam correction factor was 1.06%; the value is less than that reported by Wang and Rogers, (2009). As it can be seen in this figure that the electron beam dose measured at 18 MeV dependence is varying for about 1.04 this could attribute to the absorbed dose mixed radiation (photons and neutrons). The mean factor correction value obtained was also less than that reported by the literature (Robar et al., 1996). The TLD system could be useful for practical clinical electron beam calibration as an additional method calibration. Fig. 3. TL response of phosphors as a function of electron dose.

4. Conclusion Considering the purpose of developing materials that is TL dosimetry of ionizing radiation and now to high-energy electrons, this material is useful in determining the absorbed dose from this type of radiation. The results obtained in this investigation proved the applicability of the phosphors to measure absorbed doses delivered from radiation therapy beams. The wide linearity range (from 25 up to 100 Gy) of the before mentioned phosphors and the well-defined TL glow curve, are the main advantages of these dosimeters, which speak in favor of their application in high energy electron beam therapy for determining the absorbed dose. The results obtained during this work suggest that ZrO2 and CaSO4:Dy could be used as good candidates for absorbed dose calculation in high energy electron beam outputs. Fig. 4. TL response of CaSO4:Dy+PTFE and ZrO2+PTFE as a function of electron beam energy.

and the readout procedure was performed each different day from 2 to up to 60 days after irradiation. Between irradiations, the dosimeters were stored at typical room conditions. After 60 days fading of the phosphors showed a fading value of 4.5% compared with that of the value obtained just after irradiation. The fading phenomenon at RT in ZrO2+PTFE was studied for periods that varied from 2 to 60 days after irradiation. Then, the fading as a function of storage time in ZrO2 can be related to effects of transferring charge from deep traps to the shallow traps that can become emptied at RT by isothermal decay. Thus, if the ZrO2 pellets can be used in routine dosimetry, the fading must be taken into account in the determination of absorbed doses. TL response of the materials under investigation varying the absorbed dose can be seen in Fig. 3. This figure shows the TL response of phosphors as a function of electron absorbed dose was linear in the range from 25 to 10,000 cGy (see Fig. 3). For each dose value, eight dosimeters were irradiated simultaneously in the standard conditions with the 6 MeV electron beam. The figure shows an increase of TL response as the electron dose is increased. In this figure it is observed that as the absorbed dose increases the TL response is also increased. CaSO4:Dy phosphors show better sensitivity as high energy electron beam absorbed dose is increased. In the low energy region in both TL phosphors their response was similar. Concerning ZrO2 response to high dose is low may due to high luminescent center competition. It is well known that CaSO4:Dy detector has a specific response curve as a function of photon energy compared with TL response of TLD-100. However, TL response of this phosphor under electron beam dependence is not reported yet. TL response of CaSO4:Dy +PTFE as a function of high energy electron beams is observed in Fig. 4 (denoted by dashed line). In this figure TL response of ZrO2 is

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