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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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

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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 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.

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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 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.
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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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Fig 2 MOSFETs probes dosimeters placed with the flat side facing the photon beam.

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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.