International Journal of Medical Sciences and Health Care Vol-1 Issue-7 (Ijmshc-701)
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Radiatio...
International Journal of Medical Sciences and Health Care Vol-1 Issue-7 (Ijmshc-701)
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Oncology...
International Journal of Medical Sciences and Health Care Vol-1 Issue-7 (Ijmshc-701)
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The over...
International Journal of Medical Sciences and Health Care Vol-1 Issue-7 (Ijmshc-701)
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Fig 1 Th...
International Journal of Medical Sciences and Health Care Vol-1 Issue-7 (Ijmshc-701)
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Fig 2 MO...
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Dose Rat...
Vol 1,issue 7 Radiation therapy treatment unit dose-rate effects on metal–oxide–semiconductor field-effect  transistor (MO...
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Vol 1,issue 7 Radiation therapy treatment unit dose-rate effects on metal–oxide–semiconductor field-effect transistor (MOSFET) detectors

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Metal oxide semiconductor field effect transistor (MOSFET) detectors have recently been introduced to radiation therapy.

However, the response of these detectors is known to vary with dose rate. Therefore, it is important to evaluate how much variation between the treatment prescribed dose and the dose that is actually delivered to the
patient using high-energy photon or electron beams under conditions of different dose rates can be attributed to the detector.

The aim of this study was to investigate MOSFET dependence on different dose-rate levels. The measurements were done by exposing the mobile MOSFET detectors to a dose of 100 cGy using a linear accelerator
with energy of 6 MV and different dose rates from 100 cGy/MUs to 600 cGy/MUs.

The results showed that the dose rate dependence of a MOSFET dosimeter was within ±1.0%. MOSFET detectors are suitable for dosimetry of photon
beams, since they showed excellent linearity with dose rate variation.

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Vol 1,issue 7 Radiation therapy treatment unit dose-rate effects on metal–oxide–semiconductor field-effect transistor (MOSFET) detectors

  1. 1. International Journal of Medical Sciences and Health Care Vol-1 Issue-7 (Ijmshc-701) http://www.ijmshc.com Page 1 Radiation therapy treatment unit dose-rate effects on metal–oxide–semiconductor field-effect transistor (MOSFET) detectors Tamader Y. AL-Rammah1 , H. I. Al-Mohammed2 , F. H. Mahyoub3 1 Division of Radiological Sciences, College of Applied Medical Sciences, King Saud University,Riyadh, Saudi Arabia 2 Correspondence to: Dr. H. I. Al-Mohammed, King Faisal Specialist Hospital &Research Centre Dept of Biomedical PhysicsMBC # 03, POB 3354 Riyadh 11211, Saudi Arabia. Abstract Metal oxide semiconductor field effect transistor (MOSFET) detectors have recently been introduced to radiation therapy. However, the response of these detectors is known to vary with dose rate. Therefore, it is important to evaluate how much variation between the treatment prescribed dose and the dose that is actually delivered to the patient using high-energy photon or electron beams under conditions of different dose rates can be attributed to the detector. The aim of this study was to investigate MOSFET dependence on different dose-rate levels. The measurements were done by exposing the mobile MOSFET detectors to a dose of 100 cGy using a linear accelerator with energy of 6 MV and different dose rates from 100 cGy/MUs to 600 cGy/MUs.The results showed that the dose rate dependence of a MOSFET dosimeter was within ±1.0%. MOSFET detectors are suitable for dosimetry of photon beams, since they showed excellent linearity with dose rate variation. Key Words: MOSFET, dose rate response, megavoltage photon beam (MV), monitor unit (MU) Introduction Monitoring the radiation dose delivered to a patient during a radiation therapy session has been accomplished recently by the use of metal oxide semiconductor field effect transistor (MOSFET) detectors. The system may be used to measure doses at specific patient sites such as skin dose, and for exit and entrance doses during a treatment with total body irradiation (TBI) (1). The detectors show good reproducibility and stability for measuring the skin dose during radiation therapy treatment (2). The MOSFET system allows immediate dose readout and is small and easy to use. The detection system is based on the measurement of threshold voltage shift(3,4). MOSFET detectors have dosimetric dependence characteristics of temperature, dose and dose rate, source-to-skin distance (SSD), angular dependence and energy dependence (5).The energy dependence varies not only with the silicon oxide layers but also depends on the detector construction as well as the materials used in the construction of the substrate and the detector housing (6 ,7). The system consisted of five high- sensitivity dosimeters attached to a reader. The five supporting MOSFET probes permit measurements of five different locations (8 ,9). The attached reader records a voltage difference in each of the dosimeters if exposed to radiation. MOSFET calibrations are performed under full buildup conditions, which then produce a very small sensing volume and less than 2% isotropy under full buildup through 360 degrees rotation. All five probes of the mobile MOSFET are made for multiple uses and can accumulate doses up to 7000 cGy before needing to be replaced (2). The system is controlled by remote dose-verification software running on a personal laptop computer. The aim of this study was to investigate the reproducibility of mobile MOSFET detectors with variable dose rates. Materials and Methods All mobile MOSFET detectors (TN-RD-16, Thomson-Nielson, Ottawa, Ontario, Canada) were calibrated in full buildup conditions prior to use. The calibration was performed to obtain maximum accuracy and repeatability of the system. The calibration was carried out using a Varian Clinac 2300 EX linear accelerator (Varian
  2. 2. International Journal of Medical Sciences and Health Care Vol-1 Issue-7 (Ijmshc-701) http://www.ijmshc.com Page 2 Oncology Systems, Palo Alto, CA, USA) using 6 MV beams and a field size of 10 × 10 cm2 at 100 cm SSD and 100 cGy. All measurements were performed by placing the mobile MOSFET detectors at a depth of 1.5 cm using a tissue- equivalent bolus to represent the Dmax of 6 MV. Five sequential measurements at each dose rate setting were recorded using the five detectors (Figure 1). The overall physical size of the sensors is 1.0 x 1.0 x 3.5 mm3 (Figure 2), and the actual sensitive volume is 0.2 mm x 0.2 mm x 0.5 µm. Statistical analysis Data from each sample were run in duplicate and expressed as means ± standard deviation (SD) (n = 5 sequential reading for each channel). The results were compared using one-way ANOVA analysis followed by Tukey’s test for multiple comparisons. Means were considered significant if P<.05. Results The dependence of mobile MOSFET detectors to variation of dose rate was determined. Figure 3 shows the average dose rate dependence of the MOSFET detectors at different dose rates ranging from 100 cGy/MUs to 600 cGy/MUs. The system shows acceptable reproducibility and stability at the delivered dose rates. The highest level of fluctuation was seen with dose rate of 100 cGy/MUs, within ± 0.72 %, with a standard deviation of 2.16. However, less fluctuation was observed with other dose rates, and no significant difference was seen between dose rates (P< 0.001). The MOSFET response was within ± 1.1 % and remained uniform with variable dose rates. Discussion Although the device shows sensitivity dependence to integrated dose, this sensitivity dependence and other MOSFET dosimetric dependences were outside the scope of this study, which examined the reproducibility of mobile MOSFET detectors with variable dose rates. A commercially available mobile MOSFET detector (TN-RD-16, Thomson- Nielson, Ottawa, Ontario, Canada) verification system was used for this study, which showed a linear response with the dose and no dependency with the dose rate was found. MOSFET calibration was performed in order to convert the radiation-induced dosimeter voltage shift to cGy. The calibration coefficient was defined as the ratio of the measured voltage shift of the dosimeter and the actual dose measured with the 0.6 cc Farmer- type ionization chamber (Model -2571) at the depth of maximum dose (Dmax). The MOSFET system has ports for five probes which can be used simultaneously. The MOSFET probes were placed at a source-to-dosimeter distance of 100 cm. The system included a wireless (Bluetooth) MOSFET reader (TN-RD-16, Thomson-Nielson) connection controlled with remote dose verification software running on a laptop computer. Although the system has two bias supply settings (high and standard), for this study and for the calibration of the MOSFET detectors we used the standard setting giving a normal sensitivity of ~1mV/cGy. All dose measurements were carried out with the flat side of the MOSFET detectors placed to face the beam. The detectors were inserted in grooves in the surface of a 1-cm thick polymethyl methacrylate (PMMA) slab of dimensions 30 × 30 × 1.0 cm3 (Figure 2). In addition, a bolus sheet of 1.5 cm was placed on the top of the MOSFET to minimize air gaps. The MOSFET detectors were then irradiated with 100 MUs using a 10 x 10 cm2 field size, and calibration factors in cGy/mV were obtained. Calibration factors for each MOSFET were determined by recording detector response in millivolts (mV) and normalizing by absorbed dose (cGy). In this study the calibration factor for the detectors was 1.12. MOSFET probes were connected to the bias for one hour prior to measurement as recommended by the manufacturer. Six dose rates were used in this investigation (100, 200, 300, 400, 500, and 600 cGy/MUs. The recording results were normalized to the MOSFET response at 300 cGy/MUs. The responses from each channel were recorded at the end of each exposure. The mean MOSFET responses were calculated and standard deviation was obtained to evaluate the variation of MOSFET with different dose rates.
  3. 3. International Journal of Medical Sciences and Health Care Vol-1 Issue-7 (Ijmshc-701) http://www.ijmshc.com Page 3 The overall uncertainty with different dose rates using calibrated MOSFET detectors in this study was about 1.1 %. The percentage dose difference was calculated for every channel in MOSFET after taking the mean and the standard deviation at different dose rates at a fixed delivered dose of 100 cGy. Mobile MOSEFT detectors are easy to use and give immediate dose readouts. This study demonstrated that mobile MOSFET are reliable detectors that have limited fluctuation with variations of dose rate. Conclusion MOSFET detectors, with their properties of small size, accuracy, reproducibility and immediate readout make good detectors for radiation therapy treatment. MOSFET detectors showed good responses at all dose rates in comparison to the delivered dose. These detectors were fast, reliable, small, and user-friendly. MOSFET detectors offer outstanding potential as a dose monitor for treatment and quality assurance in medical radiation therapy departments. Acknowledgements The authors would like to express their gratitude to the Biomedical Physics Department and the Radiation Therapy Department at King Faisal Specialist Hospital and Research Center, Riyadh, Saudi Arabia, and to Radiological Sciences Department; King Saud University, Riyadh, Saudi Arabia for continuous support. The authors would like to acknowledge the professional editing assistance of Dr. Belinda Peace. References 1. Al-Mohammed HI, Mahyoub FH, Moftah BA. Comparative study on skin dose measurement using MOSFET and TLD for pediatric patients with acute lymphatic leukemia. Med Sci Monit. 2010; 16: CR325-9. 2. Essam H. Mattar, LinaF.Hammad, Huda I. Al-Mohammed.Measurement and comparison of skin dose using OneDose MOSFET and Mobile MOSFET for patients with acute lymphoblastic leukemia. Med Sci Monit. 2011;17(6):MT1-MT5. 3. Bulinski K, Kukolowicz P. Characteristics of the metal oxide semiconductor field effect transistor for application in radiation therapy. Pol J Med Phys Eng. 2004; 10: 13-24. 4. Rosenfeld AB. MOSFET dosimetry on modern radiation oncology modalities. Radiat Prot Dosimetry. 2002; 101: 393-8. 5. Qi ZY, Deng XW, Huang SM, et al. Real- Time in vivo Dosimetry with MOSFET Detectors in Serial Tomotherapy for Head and Neck Cancer Patients. Int J Radiat Oncol Biol Phys. 2011. 6. Ehringfeld C, Schmid S, Poljanc K, et al. Application of commercial MOSFET detectors for in vivo dosimetry in the therapeutic x-ray range from 80 kV to 250 kV. Phys Med Biol. 2005; 50: 289-303. 7. Manigandan D, Bharanidharan G, Aruna P, et al. Dosimetric characteristics of a MOSFET dosimeter for clinical electron beams. Physica Medica. 2009; 25: 141-47. 8. Ramaseshan R, Kohli KS, Zhang TJ, et al. Performance characteristics of a microMOSFET as an in vivo dosimeter in radiation therapy. Phys Med Biol. 2004; 49: 4031-48. 9. Glennie D, Connolly B, Gordon C. Entrance skin dose measured with MOSFETs in children undergoing interventional radiology procedures. Pediatric Radiology. 2008; 38: 1180-87. 10. Lavallee MC, Gingras L, Beaulieu L. Energy and integrated dose dependence of MOSFET dosimeter sensitivity for irradiation energies between 30 kV and 60Co. Med Phys. 2006; 33: 3683-9.
  4. 4. International Journal of Medical Sciences and Health Care Vol-1 Issue-7 (Ijmshc-701) http://www.ijmshc.com Page 4 Fig 1 The experimental setup for metal oxide semiconductor field effect transistor ( MOSFET). The setup consists of the reader, the bias box, and the MOSFET dosimeter with phantom. In addition, it shows the setup for the measurement where is the detectors are placed in the top of water slab phantom and covered with 1.5 cm bolus.
  5. 5. International Journal of Medical Sciences and Health Care Vol-1 Issue-7 (Ijmshc-701) http://www.ijmshc.com Page 5 Fig 2 MOSFETs probes dosimeters placed with the flat side facing the photon beam.
  6. 6. International Journal of Medical Sciences and Health Care Vol-1 Issue-7 (Ijmshc-701) http://www.ijmshc.com Page 6 Dose Rate (cGy/MU) Dose(100cGy) 92 96 100 104 100 200 300 400 500 600 . Fig 3 Dose-rate dependence of the MOSFET dosimeter for different dose rates from 100 to 600 cGy/MUs.

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