Dosimetric Characteristics of Standard and Micro MOSFET Dosimeters as In-vivo Dosimeter for Clinical Electron Beam

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1 Journal of the Korean Physical Society, Vol. 55, No. 6, December 2009, pp Dosimetric Characteristics of Standard and Micro MOSFET Dosimeters as In-vivo Dosimeter for Clinical Electron Beam Jin-Beom Chung Department of Radiation Oncology, Seoul National University Bundang Hospital, Seongnam Jeong-Woo Lee Department of Radiation Oncology, Konkuk University Hospital, Seoul Tae-Suk Suh, Doo-Hyun Lee and Bo-Young Choe Research Institute of Biomedical Engineering, The Catholic University of Korea, Seoul Yeon-Sil Kim Department of Radiation Oncology, The Catholic University of Korea, Seoul Jae-Sung Kim and In-Ah Kim Department of Radiation Oncology, Seoul National University Bundang Hospital, Seongnam Kyoung-Sik Choi Department of Radiation Oncology, SAM Anyang Hospital, Anyang Sung-Joon Ye Department of Radiation Oncology, Seoul National University Hospital Seoul and Institute of Radiation Medicine, Medical Research Center, Seoul National University, College of Medicine, Seoul (Received 30 June 2009, in final form 17 September 2009) The metal oxide semiconductor field effect transistor (MOSFET) dosimeters have recently become commercially available. The purpose of this study was to investigate the fundamental dosimetric characteristics of MOSFET dosimeters for clinical electron beams and to compare standard MOS- FET and micro MOSFET dosimeters. In this study, five identical standard MOSFET (TN-502-RD) and micro MOSFET (TN-502-RDM) dosimeters were used for measurements. All measurements, with the exception of angular dependence, were performed in a slab-shaped PMMA phantom. For determining the angular dependence of MOSFET dosimeters, a cylindrically shaped PMMA phantom was used. Both MOSFET dosimeters showed excellent linearity against doses measured in the dose range of cgy for a electron beam of 9 MeV. The reproducibility of all MOSFETs, excepted one standard MOSFET, was less than ±2 %. The dose-rate dependence of the two types MOSFET was within ±3 %. However, for the angular dependence, standard and micro MOSFETs show remarkable differences relative to gantry angles. This study shows the dosimetric characteristic of standard and micro MOSFET dosimeters for clinical electron beams. Both MOSFET dosimeters are suitable for dosimetry of electron beams in the energy range of 6 20 MeV. However, the dose verification of radiation therapy using multidirectional electron beam treatments allows for better use of micro MOSFETs which have a reduced directional dependence compared to standard MOSFETs. PACS numbers: a, Jj Keywords: MOSFET dosimeter, Dosimetric characteristics, Electron beam DOI: /jkps suhsanta@catholic.ac.kr

2 Dosimetric Characteristics of Standard and Micro MOSFET Dosimeters Jin-Beom Chung et al I. INTRODUCTION As in-vivo dosimetry is the most direct and independent method for monitoring the dose delivered to patients, it is recommended by various national and international organizations. In conventional radiotherapy, ionization chambers, radiographic films, and thermoluminescent dosimeters (TLD) are the three commonly used dose measurement tools. However, an ionization chamber has difficulty in accurate measuring a large dose gradient across the ion chamber volume. Radiographic film and TLD also require careful calibration and post processing procedures. The use of TLD is time consuming (typically 24 h) for a routine dosimetry. Therefore, it is necessary to explore a new method for dose verification [1 3,8,10,16]. The metal oxide semiconductor field-effect transistor (MOSFET) dosimeter has been introduced as an alternative to the TLD. The dosimeter is small and wireless and is designed to be permanently implanted in vivo. The use of MOSFET dosimeters has several advantages, such as multiple point dose measurements, real time readout, and immediate reuse. MOSFET dosimeters provide information on the accumulated dose, which cannot be reset. However, its use is limited by the change of its sensitivity with use and its reduced lifetime (one can measure until a 200 mv cumulative signal, which is equal to Gy upon the precision mode applied). MOSFET dosimeters have been shown to be useful in current radiotherapy practice. Ramaseshan et al [4] reported the clinical use of a MOSFET dosimeter for patient dose verification. Quach et al. [7] used a MOSFET dosimeter and TLD to measure skin dose on a chest wall phantom, with a 6 MV beam. Chuang et al. [8] demonstrated the use of a MOSFET in IMRT QA. Best et al [18] also used it for in-vivo patient dosimetriy during total body irradiation (TBI) with 6 MV photons. The design and the characteristics of the MOSFET have improved significantly over the past few years. In a previously report, we investigated the dosimetric characteristics (dose linearity, reproducibility, and dose rate dependence) of standard and micro MOSFET dosimeters for clinical photon beams with a homemade phantom [17]. However, few studies have estimated the dosimetric characteristics of standard and micro MOS- FET dosimeters for clinical electron beam in the literature. Therefore, our objective of this study was to compare a characterization between a standard MOSFET dosimeter of a earlier version and a new improved micro MOSFET dosimeter for clinical electron beam irradiations. II. MATERIALS AND METHODS Table 1. Dosimeter and bias supply combinations used in this study. MOSFET dosimeter Standard Micro MOSFET MOSFET (TN-502RD) (TN-502RDM) Package thickness (mm) 2 1 Active detection area (mm 2 ) <0.04 <0.04 Dose range (Gy) >1 >1 Bias supply Standard Standard Sensitivity (mv/cgy) 1 1 Fig. 1. MOSFET dosimeter system. The system consists of the reader, the bias box, and the MOSFET dosimeter with a phantom. 1. MOSFET Dosimeter System The MOSFET dosimeter is a miniature silicon transistor, which has an excellent spatial resolution and offers very little attenuation of the beam due to its small size [5, 6]. Irradiation generates electron-hole pairs within the gate oxide (SiO 2 ) of the MOSFET. The gate voltage across the two MOSFETs is +1 V and +15 V while the MOSFETs are being irradiated. Charges trapped in the gate oxide, especially close to the SiO 2 /Si interface, induce a shift of the device threshold voltage (V th ), which has been shown to be a linear function of the received dose [6, 8, 9, 14]. For radiation dosimetry the threshold voltage is often the voltage required across the gate before a set current is reached (10 µa) between the source and the drain. The TN502RD (standard MOSFET) and TN502RDM (micro MOSFET) model of MOSFET dosimeters were used in this study (Table 1). The MOS- FET dosimeters are connected to a bias supply, which is coupled to the reader (MOSFET AutoSense Reader, Thomson and Nielsen Electronics, Canada) as in Fig. 1. Five MOSFET dosimeters can be connected to one bias supply box, each with a 20 cm long, 2.5 mm wire and 0.4 mm thick cable. The reader has 4 bias boxes connected to it at the same time, thus making it possible to read up to 20 MOSFET dosimeters at the same time. The dual bias supply provides a choice of high or low sensitivity response, depending on the type of measurement. For this study, all our measurements were performed under standard sensitivity (1 mv/cgy) bias. It is recom-

3 Journal of the Korean Physical Society, Vol. 55, No. 6, December 2009 mended that the MOSFET dosimeters be connected to a 5 V standard bias supply box and that the reader be warmed up for approximately 60 minutes before use. The physical characteristics of MOSFET dosimeters and the principle of dual bias detector operation are described in great detail elsewhere [11 13]. 2. Phantom System For dosimetry characteristics of standard MOSFET and micro MOSFET dosimeter, the phantom was constructed from regular square poly methyl methacrylate (PMMA) slabs (ρ = 1.14) with cm 2 cross section and a 1.0 cm thickness for each slab. A 1.0 cm thick slab was drilled with five holes, which hold five MOS- FET dosimeters. The slab phantom shown was used for calibration and characterization measurements such as reproducibility, linearity, dose rate dependence, energy dependence, and field size dependence of standard and micro MOSFET dosimeters. A cylindrical shaped PMMA phantom was also used for measuring the angular dependence. Table 2. Calibration factor of standard and micro MOS- FET dosimeters for a 9 MeV electron beam. MOSFET dosimeter type Dose Average reading CF (cgy) (mv) (mv/cgy) Standard MOSFET Average ± 0.76 mv/cgy Micro MOSFET Average ± 0.66 mv/cgy 3. Experiment For electron energies of 6 20 MeV, a dose of 1.0 Gy was calibrated for 100 MU delivered at a 100 cm sourceto skin distance (SSD), at the reference depth of each energy level with cm 2 electron cone applicators. This provides the user with a millivolt (mv) to centigray (cgy) conversion. Typical values are approximately 1 mv/cgy for standard sensitivity. For calibration and reproducibility, all five MOSFETs of standard and micro MOSFET dosimeters were repeatedly exposed to 100 MU five times on the surface of the phantom using the Varian Clinac 21EX medical linear accelerator. The five MSOFET dosimeters were individually calibrated. The responses were averaged at the end of each exposure. For all measurements, the flat surfaces of the MOSFET dosimeters were kept facing toward the beam. The dose-linearity measurement was conducted with doses ranging from 50 cgy up to 600 cgy in the established calibration setup for a 9 MeV electron beam under reference conditions. The effect of dose rate on the MOSFET dosimeter response was studied for different dose rate levels ranging from 100 to 600 MU/min for a 16 MeV electron beam under reference measurement conditions. The angular dependence was evaluated for a cylindrical phantom with a radius of 0.6 cm. The angular dependence of the MOSFET dosimeter was investigated by positioning the geometric center of the dosimeter in a phantom at the isocenter, both perpendicular and parallel to the gantry rotational axis. The response of the an- Fig. 2. Both dosimeters response consistency measured in a 12 MeV electron beam with a cm 2 electron cone applicator. The data points are averages of five readouts for each dosimeter. The error bars represent the standard deviation for five signals for each dosimeter. gular dependence was studied for a delivery beam ranging from 0 and 360 degrees gantry angles in steps of 90 degrees. A dose of 1.0 Gy was delivered in a cylindrical phantom. III. RESULTS AND DISCUSSION 1. Dose Reproducibility At 9 MeV, the average calibration factor as in Table 2 was ± 0.76 and ± 0.66 mv/cgy for standard and micro MOSFET dosimeter, respectively. It was found to have a maximum 1.4 % variation. The reproducibility of the MOSFET dosimeters was also checked and an excellent consistency was obtained for repetitive measurements. Figure 2 shows the re-

4 Dosimetric Characteristics of Standard and Micro MOSFET Dosimeters Jin-Beom Chung et al Fig. 3. Linearity response of the MOSFET dosimeter for doses ranging from 50 to 600 cgy measured in a 9 MeV electron beam with a cm 2 electron cone applicator. producibility on the surface of the phantom for a 300MU/min dose rate of a 12 MeV electron beam at 100 MU irradiations. The data points are averages of five readouts for each MOSFET dosimeter. The measured dose difference in percent was within ± 2 %. The micro MOSFET dosimeters had better reproducibility than the standard MOSFET dosimeters. No fading effect was observed for the dosimeters at the beginning of their lifetime. However, the fading effect was observed when the dosimeter was removed from the bias supply and reconnected after some time. A previous report [11] has described that 3 % fading was observed within the first 5 h following irradiation. Thus, all MOSFETs were read between 3 min and 10 min post irradiation to reduce the fade effects. 2. Dose Linearity Figure 3 shows the linearity line of standard and micro MOSFET dosimeters for a clinical 9 MeV electron beam. Both MOSFET dosimeters responses in terms of mv are plotted against the actual dose delivered, as shown in Fig. 3. TN505RD and TN502RDM type MOS- FET dosimeters are linear up to 600 cgy, as shown by the excellent linear response (0.3 % at 1 SD) with R values of and , respectively. The difference between the two dosimeters was not statistically significant for a 9 MeV electron beam. Both MOSFET dosimeters showed that the linearity or dynamic range was the same as that of an extended dose range film (EDR, Kodak). The linearity of both dosimeters in the dose range cgy of a electron beam is comparable to that of an ionization chamber (a plane-parallel type PTW 34001) in a similar range. The results also indicate that the minimum dose that can be measured in relation to the sensitivity is around 50 cgy, where the percent standard deviation falls below 1 % because the MOSFET response was measured with a dose lower than 100 cgy delivered for calibration. Fig. 4. Dose rate dependence of the MOSFET dosimeter for different dose rates from 100 to 600 MU/min in a 16 MeV electron beam with a cm 2 electron cone applicator. Fig. 5. Angular dependence of standard MOSFET and micro MOSFET dosimeters measured in a 20 MeV electron beam with a cm 2 electron cone applicator. The error bars represent the standard deviation for five signals for each dosimeter. 3. Dose Rate Dependence Figure 4 shows the average dose rate dependence of both MOSFET dosimeters at different dose rates ranging from 100 to 600 MU/min. The results were normalized to the MOSFET response at 300 MU/min. For different dose rates, both MOSFET dosimeters responses indicated a difference of less than ±3 %, which was also the reproducibility. No measurable effect was observed by changing the dose rate in the range from 100 to 600 MU/min, a standard MOSFET dosimeter. 4. Angular Dependence Figure 5 shows the angular dependence of standard and micro MOSFET dosimeter for a 20 MeV electron beam. The relative response is obtained from normalization of the dosimeter reading to the vertical at 0 de-

5 Journal of the Korean Physical Society, Vol. 55, No. 6, December 2009 grees. The average directional dependence of the standard MOSFET measurement is for 90 and 270 degrees and for 180 degrees. For the micro MOS- FET dosimeter, the directional dependence is for 90 and 270 degree and for 180 degrees. The error bars present the standard deviations of five readings for each dosimeter. The directional dependences of stand and micro MOSFET dosimeters were found to be within ±5 % and ±1 % between 0 and 270 degree gantry angles, respectively. It can be seen that the micro MOS- FET dosimeter has a smaller angular dependence than the standard MOSFET dosimeter. For lower energy electron beams, the angular dependence is greater than that of a 20 MeV energy electron beam. MOSFET dosimeters exhibit an angular dependence even though they are small in size because the Si layer of the MOSFET dosimeter has a higher atomic number than water (or epoxy), leading to increased electron backscatter in one direction. The angular dependence may be improved if precise alignment between the beam axis and the dosimeter axis is achieved during measurements. IV. CONCLUSION A phantom for calibration and for determing the characteristics of the standard and micro MOSFET dosimeters was developed in this study. Both dosimeters were evaluated and compared according to these dosimetric characteristics for clinical electron beams with the homemade phantom. The above results show that the two types of MOSFET dosimeters showed similar results for dose-linearity, reproducibility, and calibration factors whereas remarkable differences for angular dependence were found. As previously noted, the current study shows similar results as the characteristic of both MOSFET dosimeters for clinical photon beams [17]. The micro MOSFET dosimeter was superior to the standard MOSFET dosimeter for measurement of the angular dependence. Therefore, a micro MOSFET application of in vivo dosimetry has reduced the angular dependence. This allows for better dose verification for multidirectional clinical electron beam treatments. Also, MOSFET dosimeter may be used as an in vivo dosimeter for dose verification in patients undergoing electron beam radiotherapy by applying proper calibration and correction factors, ACKNOWLEDGMENTS This work was supported by a nuclear research & development program of the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korean government (MEST) (Grant code, ). REFERENCES [1] D. Huyskens, R. Bogarets, J. Verstraete, M. Loof, H. Nystrom, C. Fiorino, S. Broggi, N. Jornet, M. Ribas and D. I.Thwaites, Booklet no. 5. ESTRO (2001). [2] G. J. Kutcher, L. Coia, M. Gillin, W. F. Hanson, S. Leibel, R. J. Morton, J. R. Palta, J. A. Purdy, L. E. Reinstein, G. K. Svensson. M. Weller and L. Winfield, Med. Phys. 21, 581 (1994). [3] P. Franscenscom, S. Cora, C. Cavedon, P. Scalchi, R. Sonia and F. Colombo, Med. Phys. 25, 503 (1998). [4] R. Ramaseshan, S. Russel and P. O Brien, Int. J. Radiat. Oncol. Biol. Phys. 37, 959 (1997). [5] M. Butson, A. Rozenfeld, J. Mathur, M. Carolan, T. Wong and P. Metcalfe, Med. Phys. 23, 655 (1996). [6] P. Scalchi, P. Francescon and P. Rajaguru, Med. Phys. 32, 1571 (2005). [7] K. Y. Quach., J. Morales, M. J. Butson, A. B. Rosenfeld, P. E. Metcalfe, Med. Phys. 27, 1676 (2000). [8] C. F. Chuang, L. J. Verhey and X. Ping, Med. Phys. 29, 1109 (2002). [9] R. Ferrand, Les semi-conducturs MOSFET, Teaching course (2001). [10] J. J. Wood and W. P. Mayles, Phys. Med. Biol. 40, 309 (1995). [11] R. Ramaseshan, K. S. Kohli, T. J. Zhang, T. Lam, B. Norlinger, A. Hallil and M. Islam, Phys. Med. Biol. 49, 4031 (2004). [12] M. Soubra, J. Cygler and G. Mackay, Med. Phys. 21, 567 (1994). [13] P. Halvorsen, Med. Phys. 32, 110 (2005). [14] C. Benson, R. A. Price, J. Silvie, A. Jasksic and M. J. Joyce, Phys. Med. Biol. 21, 3145 (2004). [15] Thomson and Nielsen Electronics Ltd (Ottawa), Technical note no:4 (1996). [16] R. A. Kinhikar, P. K. Sharma, C. M. Tambe, U. M. Mahantshetty, R. Sarin, D. D. Deshpande and S. K. Shrivastava, Phys. Med. Biol. 51, N263 (2006). [17] J. B. Chung, J. W. Lee, Y. L. Kim, D. H. Lee, K. S. Choi, J. S. Kim, I. Kim, S. Hong and T. S. Suh, Korean J. Med. Phys. 18, 48 (2007). [18] S. Best, A. Ralston and N. Suchowerska, Phys. Med. Biol. 50, 5909 (2005).