Upload
ijmshc
View
216
Download
0
Embed Size (px)
Citation preview
7/27/2019 Vol-1,Issue 7 Paper (1) Page -1-6
1/7
7/27/2019 Vol-1,Issue 7 Paper (1) Page -1-6
2/7
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 metaloxidesemiconductor field-effect
transistor (MOSFET) detectors
Tamader Y. AL-Rammah1, H. I. Al-Mohammed
2, F. H. Mahyoub
3
1
Division of Radiological Sciences, College of Applied Medical Sciences, King Saud University,Riyadh,Saudi Arabia2Correspondence 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. Themeasurements 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 apatient 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 touse. 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 thesystem. The calibration was carried out using a
Varian Clinac 2300 EX linear accelerator (Varian
7/27/2019 Vol-1,Issue 7 Paper (1) Page -1-6
3/7
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 m.
Statistical analysis
Data from each sample were run in duplicate and
expressed as means standard deviation (SD) (n
= 5 sequential reading for each channel). Theresults were compared using one-way ANOVA
analysis followed by Tukeys test for multiple
comparisons. Means were considered significant if
P
7/27/2019 Vol-1,Issue 7 Paper (1) Page -1-6
4/7
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 of100 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 therapytreatment. 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 gratitudeto 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, MoftahBA. 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 forpatients with acute lymphoblastic
leukemia. Med Sci Monit.
2011;17(6):MT1-MT5.
3. Bulinski K, Kukolowicz P. Characteristicsof 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 onmodern 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 250kV. Phys Med Biol. 2005; 50: 289-303.
7. Manigandan D, Bharanidharan G, ArunaP, 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.
7/27/2019 Vol-1,Issue 7 Paper (1) Page -1-6
5/7
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.
7/27/2019 Vol-1,Issue 7 Paper (1) Page -1-6
6/7
7/27/2019 Vol-1,Issue 7 Paper (1) Page -1-6
7/7
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(100cG
y)
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.