Vol-1,Issue 7 Paper (1) Page -1-6

7/27/2019 Vol-1,Issue 7 Paper (1) Page -1-6

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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


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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


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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.

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7. Manigandan D, Bharanidharan G, ArunaP, et al. Dosimetric characteristics of a

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8. Ramaseshan R, Kohli KS, Zhang TJ, et al.Performance characteristics of a

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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.


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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.

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