O. KARAMAN, A. GÜNEº TANÝR: INVESTIGATION OF PHOTONEUTRON CONTAMINATION ...
699–704
INVESTIGATION OF PHOTONEUTRON CONTAMINATION FROM
THE 18-MV PHOTON BEAM IN A MEDICAL LINEAR
ACCELERATOR
RAZISKAVA FOTONEVTRONSKIH PO[KODB ZARADI
OBSEVANJA Z 18 MV CURKOM FOTONSKIH @ARKOV V
MEDICINSKEM LINEARNEM POSPE[EVALNIKU
Onur Karaman
1*
,A y ºe Güneº Tanýr
2
, Ceren Karaman
3
1
Medical Imaging Program, Vocational School of Health Services, Akdeniz University, 07070 Antalya, Turkey
2
Department of Physics, Faculty of Sciences, Gazi University, 06500 Ankara, Turkey
3
Department of Chemical Engineering, Faculty of Engineering, Pamukkale University, Denzili 20070, Turkey
Prejem rokopisa – received: 2019-01-10; sprejem za objavo – accepted for publication: 2019-04-11
doi:10.17222/mit.2019.011
In medical applications, high-energy linear accelerators are increasingly used. However, during a cancer treatment, unwanted
photoneutrons caused by high-energy photon beams (>10 MV) increase the secondary cancer risk. This study aimed to
investigate the effects of the square field size and distance to the isocenter on the neutron contamination emitted by an Elekta
Versa HD medical linear accelerator using a Thermo Scientific RadEye neutron detector. The measurements were carried out for
18-MV photons with different square field sizes and different distances from the isocenter. It was clearly seen that the neutron
dose equivalent decreases with both the increasing distance from the isocenter and square field size. Also, in order to analyze
the neutron dose equivalent variation with field sizes and distances, Monte Carlo code Fluka was used. It was shown that
MC-Fluka modeling and experimental results were consistent with each other. It was concluded that it is crucial to take into
consideration the unwanted neutron dose in the radiation treatment.
Keywords: neutron contamination, linear accelerator (LINAC), Monte Carlo simulation, Fluka
V medicini se vse bolj uporabljajo visoko-energijski linearni pospe{evalniki. Vendar pa med obsevanjem bolnikov z rakom
prihaja do ne`elenih sekundarnih u~inkov zaradi delovanja fotonevtronov, ki jih seva visoko-energijski fotonski curek (>10
MV). Avtorji raziskovali vpliv velikosti in razdalje kvadratnega polja z enakim sredi{~em na po{kodbe povzro~ene z nevtroni, ki
jih emitira Elekta Versa HD medicinski linearni pospe{evalnik. Le-te so zaznavali z nevtronskim detektorjem Thermo Scientific
RadEye. Meritve so izvajali pri razli~nih velikostih in razdaljah od sredi{~a polja obsevanja, povzro~enega z 18 MV fotoni.
Popolnoma nedvoumno so ugotovili, da se jakost nevtronske doze zmanj{uje ekvivalentno s pove~evanjem razdalje od sredi{~a
obsevanja, kot tudi velikosti kvadratnega polja. Prav tako so avtorji, da bi analizirali variiranje doze nevtronov z velikostjo in
razdaljo, uporabili ra~unalni{ko kodo Fluka, ki temelji na metodi Monte Carlo simulacije. Pokazali so, da se simulacije, izde-
lane z MC-Fluka kodo, dobro ujemajo z eksperimentalnimi rezultati. Avtorji v zaklju~ku poudarjajo, da je za uspe{no obsevanje
klju~no upo{tevanje ne`elene nevtronske doze.
Klju~ne besede: nevtronske po{kodbe, linearni pospe{evalnik (LINAC), Monte Carlo simulacija, Fluka
1 INTRODUCTION
In order to treat cancer or deep body tumors in
patients, in recent years, the medical linac has attracted
tremendous attention as a promising high-energy
photon-beam source. Despite the fact that when com-
pared with low-energy accelerators, high-energy accele-
rators exhibit some advantages (i.e., a lower skin dose, a
larger depth dose and a lower scattered-radiation dose
outside the field), the production of unwanted neutrons is
the key issue in the use of high-energy accelerators.
1–3
The unwanted neutrons are produced due to the interac-
tion of high-energy photons with high-Z metals,
especially W and Pb used in the head of an accelerator.
The threshold-energy values of a photoneutron pro-
duction for Pb and W are 6.19 MeV and 6.17 MeV,
respectively, while the respective values for Fe and Cu
are 7.65 MeV and 9.91 MeV.
4–8
The probability of photo-
nuclear reactions increases with the photon energy. Due
to the fact that its maximum value in the radiotherapy
was found to be 13–18 MeV, it can easily interact with a
linac head including Pb, W, Fe and Cu. The target and all
the collimators are the main parts of a linac head con-
tributing to the photonuclear reaction. Most of the
studies reported that these unwanted neutrons emitted
from the linac head are not negligible.
3,9–11
They can
cause secondary harmful malignancies as well as contri-
buting an additional dose to the total neutron dose.
12,13
Neutrons with a radiation weighting factor of 20 also
affect the shielding requirements in radiotherapy. So, the
neutron contamination should be controlled and mini-
mized.
3,14–17
Most of the studies were carried out to deter-
mine the effects of different parameters such as the field
size and the distance from isocenter on the photoneutron
production of different medical linear accelerators. How-
Materiali in tehnologije / Materials and technology 53 (2019) 5, 699–704 699
UDK 620.1:620.179.155.5:621.039.55 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 53(5)699(2019)
*Corresponding author's e-mail:
onurkaraman@akdeniz.edu.tr

ever, dosimetric measurements are not sufficient to
explain the contribution of the photoneutron dose. The
majority of studies involve the Monte Carlo (MC)
simulations of the linacs. In several studies, different MC
codes are applied to show the contribution of the parts of
a linac to the neutron contamination and also the effects
of the linac model, field size and distance from iso-
center.
13,18,19
In one of these studies, Meshabi et al. simu-
lated an 18-MV photon-beam Elekta SL75/25 accele-
rator for open and wedged beams and its variation with
the field size by means of the MCNPX MC code. It was
shown that for open beams, the neutron dose equivalent
(NDE) decreased with the increasing field size. In
contrast, for wedged beams, it increased with the field
size.
3
M. Jahangiri et al.
20
investigated the effect of the
field size on the neutron dose equivalent in the center
and away from it. The results revealed that the NDEs
increased with the increasing field sizes, especially for
the5cm×5cmf ield size.
20
The undesirable neutrons originating from different
parts of a high-energy linear accelerator have different
energies. As a result, it is proper to suppose that if the
neutrons produced by different parts of the linac head
reach the measuring area in the patient plane, there will
be a neutron energy shift.
12,21–23
In order to model Varian
Clinac 2100C with a 18-MV photon beam, J. Ma et al.
23
used Monte Carlo code MCNPX. They investigated the
neutron contamination and determined the numbers of
photoneutrons at different field sizes. The authors noted
that most of the photoneutrons were produced by the pri-
mary collimator (55–60 %) and jaws (15–30 %). J. Pena
et al.
19
figured out the contributions of different compo-
nents of an 18-MeV Primus linac for the 10 cm × 10 cm
field size. It was reported that the contribution of the
primary collimator was 52 % and the contribution of the
secondary collimator was 21 %.
19
In the same manner, J.
Becker et al.
6
reported similar results, consistent with the
previous study by J. Pena et al.
19
It is important to determine which parameter affects
the neutron dose equivalent to better evaluate the in-
crease in the total dose given to a patient. The inconsis-
tency of the obtained results in MC modelling can be
explained with the differences between the MC codes
used for the simulation. According to the authors’knowl-
edge, the simulation of the 18-MV photon beams for an
Elekta Versa HD linac made by Fluka has not yet been
studied comprehensively. Thus, this study aimed to
simulate the head of an Elekta Versa HD linac using the
Fluka MC code. Since the Fluka code can process proper
electron-nucleus, electron-electron bremsstrahlung and
photonuclear interactions over the whole energy range, it
is one of the best choices for a simulation
24
. In order to
estimate the neutron dose equivalent for different points
as well as different field sizes, a Thermo Scientific
RadEye neutron detector was used. It is difficult to
perform experimental measurements of this kind of
doses from neutrons with the standard nuclear instru-
mentation. Therefore, a simulation code is useful for
simulating the photoneutron production and transpor-
tation across the accelerator head. With the help of these
simulation tools, the neutron contamination can be
evaluated. Monte Carlo code Fluka was used to achieve
the goal of this study. Using the Fluka code, it was
shown if the experimental measurements were in agree-
ment with the simulation or not.
2 EXPERIMENTAL PART
In this work, an Elekta Versa HD medical linac was
used to determine the neutron contamination. It is able to
generate 6-MV and 18-MV photon beams and electron
beams with different energies (i.e., 6–18 MeV) in
accordance with the desired treatment. In line with
International Atomic Energy Agency (IAEA) TRS-398,
the reference dose calibration protocol, the outputs of the
linac were calibrated as 1 cGy =1 monitor unit (MU).
The gantry and collimator angle were positioned at 0°
and vertically oriented, the isocenter was at position
(0,0,0) and the linac was operated in the 18-MV photon
mode. A water-equivalent solid slab phantom (PTW
RW3) was positioned at the isocenter. Figure 1 intro-
duces the layout of the radiotherapy treatment room. The
isocenter is located in the middle of the treatment room.
Dosimetric measurements for the neutron contamina-
tion were read by a Thermo Scientific RadEye NL
neutron detector, a
3
He gas detector. It was calibrated by
Thermo Scientific Inc. using
252
Cf sources. Since
neutrons are neutral particles, their behavior is different
from that of charged particles or gamma rays. Therefore,
their detection takes place by means of secondary
ionized particles/rays produced in their interaction with
matter. The incoming neutrons are captured by the trans-
O. KARAMAN, A. GÜNEª TANÝR: INVESTIGATION OF PHOTONEUTRON CONTAMINATION ...
700 Materiali in tehnologije / Materials and technology 53 (2019) 5, 699–704
Figure 1: Layout of the radiotherapy treatment room

ducer material and converted into ionic particles that can
be detected by nuclear reactions.
The protons and
3
H particles are detected and
measured using a typical gas-flow proportional counter
filled with the
3
He gas. As a result of capturing neutrons
by
3
He, the kinetic energy of 765 keV occurs. This
energy is separated as 0.191 MeV for tritium particles
and 0.574 MeV for protons. By means of measuring the
energy of the products produced in the following
reaction, detecting of thermal neutrons is performed.
Thermal-neutron (n) tagging in gaseous proportional
counters is achieved with the following reaction:
n+
3
He p (573 keV) +
3
H (191 keV); Q=764 keV(1)
where p and
3
H ionize the gas molecules and create a
track of ion-electron pairs, whose number is propor-
tional to the energy deposited in it. During the detection
of thermal neutrons,
3
He acts as the stopping medium
for them with reaction (1); meanwhile, with a higher
stopping power than that of
3
He, tritium and protons are
stopped by the filling gas.
25,26
The dosimetric measure-
ments taken by the Thermo Scientific RadEye NL
neutron detector were analyzed by an automatic reader.
To eliminate device-based errors, each measurement
was repeated three times.
The effect of the field size on the neutron contami-
nation was analyzed at a source-to-surface distance
(SSD) of 100 cm for the field sizes of (5 × 5), (10 × 10),
(20 × 20), (30 × 30) and (40 × 40) cm
2
. In order to
analyze the effect of the distance from the isocenter on
the neutron contamination, the RadEye NL detector was
located at the isocenter and various distances of 10–50
cm from the central axis (Figure 2).
In the next step of the study, Monte Carlo code Fluka,
version 2011.2c.6, was used to simulate the photo-
neutron production in the Elekta Versa HD linac. MC
methods were extensively used to evaluate the charac-
teristics of the neutron contamination.
Figure 3 illustrates the geometry of an accelerator
head simulated by the MC Fluka code. The primary
electron beam hitting the tungsten target was simulated
as a mono-energetic electron beam with an energy of 18
MeV. Induced X-rays pass through the primary
collimator. In order to activate the photoneutron reaction,
the simulation parameter in Fluka called the Photonuc
card was turned on. At the SSD of 100 cm, (30 × 30 ×
30) cm, in order to calculate the NDEs, the water-equiva-
lent solid slab phantom was divided into voxels and
simulated. To investigate the effect of the field size on
the neutron contamination, the jaws and the multi-leaf
collimators (MLCs) were set for different field sizes.
Each simulation was run on a personal computer (Intel 6
Core, 3.6 GHz CPU, 32 GB RAM). To reduce the sta-
tistical uncertainty of each set-up to an acceptable level,
the number of histories was set to 2×10
9
primary
electrons. During the final step of the study, the simula-
tion was evaluated by comparing the data obtained with
dosimetric measurements and MC simulations.
3 RESULTS
The photoneutron measurements were done with both
dosimetric measurements performed by the Thermo
Scientific RadEye NL dosimeter and the MC code Fluka
simulation. To evaluate the influence of the distance
from the central axis, a statistical analysis was performed
separately for each field size. The measurement results
were normalized for a 1-Gy photon dose. The measured
and MC-simulated NDEs at the patient plane and at
different distances from the isocenter are represented in
Figure 4. It is clear that the MC-Fluka model of the
accelerator complies well with the measured NDEs. It is
concluded that the simulation data can be used to
analyze the neutron contamination in further studies.
O. KARAMAN, A. GÜNEª TANÝR: INVESTIGATION OF PHOTONEUTRON CONTAMINATION ...
Materiali in tehnologije / Materials and technology 53 (2019) 5, 699–704 701
Figure 3: Geometry of the Electa Versa HD linac head simulated by
the Fluka code
Figure 2: Schematic presentation of the locations of detectors for the
measurements around the Electa Versa HD linac

The NDEs measured and MC-simulated at the
isocenter for different sizes are presented in Table 1. The
results show that there is acceptable agreement between
the measured and MC-simulated NDE values for all the
sizes considered. The best agreement is achieved for the
5cm×5cmand10cm×10cmf ield sizes where the
average local dose differences are 4.0–5.0 %.
Table 1: Measured and MC-simulated NDEs for different field sizes
at the isocenter
Field size
(cm
2
)
Measured
NDEs
(mSv Gy
–1
)
MC-simulated
NDEs
(mSv Gy
–1
)
Difference
%
5×5 1.17 1.13 4.0
10×10 1.10 1.05 5.0
20×20 1.01 0.95 6.3
30×30 0.87 0.83 6.5
40×40 0.58 0.55 6.2
The dependence of the MC-simulated NDEs on the
off-axis is shown in Figure 5. Attention is drawn to the
fact that the maximum dose equivalents are observed at
the isocenter. By changing the field size from (5 × 5) cm
to (40 × 40) cm, the NDEs decrease from 1.13 mSv Gy
–1
to 0.55 mSv Gy
–1
X-ray at the isocenter. It can be clearly
seen in Figure 5 that the NDE decrease with the
increasing simulation point takes place far from the
isocenter and at a constant field size. This is consistent
with the measured NDEs (Figure 4). The results also
indicate that the MC-simulated NDE values at the
isocenter and at 50 cm are significantly different.
MC-simulated NDEs are given in Figure 6 to show
the effect of the field size on the photoneutron
contamination. The neutron dose differentiation remains
constant for smaller field sizes and starts to decrease for
the field sizes larger than 20 cm × 20 cm. Figure 6
clearly shows that the NDEs decrease with the field size.
Since the attenuation effect of the secondary collimator
jaws decreases with the increasing field size, the
variation in the NDEs with the field size is considerable.
According to the NCRP, for 18-MV linac neutrons,
measurements can be done between distances of
0.8 mSv Gy
–1
and 2.5 mSv Gy
–1
from the isocenter and at
50 cm.
27
When comparing the results obtained at 50 cm
in this study, we find lower NDEs between 0.41 mSv Gy
–1
and 0.18 mSv Gy
–1
.
O. KARAMAN, A. GÜNEª TANÝR: INVESTIGATION OF PHOTONEUTRON CONTAMINATION ...
702 Materiali in tehnologije / Materials and technology 53 (2019) 5, 699–704
Figure 5: MC-simulated neutron-dose-equivalent variation with the
distance from the isocenter for different field sizes
Figure 4: Comparison between experimental measurements and
simulations with the MC-Fluka code for different field sizes: a) 5 cm
× 5 cm, b) 10 cm × 10 cm, c) 20 cm × 20 cm, d) 30 cm× 30 cm, e) 40
cm×40cm
Figure 6: MC-simulated neutron-dose-equivalent variation with the
field size for different distances from the isocenter

4 DISCUSSION
The measured and MC-simulated NDEs for varying
field sizes at different points from the isocenter are
presented in this paper. There are many studies about the
effect of the field size on the neutron contamination of
high-energy linear accelerators. While some of them
agree with each other, others show controversial results.
The results of this study show that the photoneutron dose
equivalents are maximum at the center of the field and
they decrease with the increasing distance from the
isocenter. Also, it is proved that with the increasing field
size, the NDEs decrease at all points. Using the Monte
Carlo Fluka code, the neutron spectrum of the 18-MV
photon beam is investigated. The NDEs are found bet-
ween 1.12 mSv Gy
–1
and 0.55 mSv Gy
–1
at the isocenter
of the field.
The results obtained with experimental measure-
ments partially disagree with the study by M. Jahangiri
et al.,
20
in which neutron dosimetry was carried out with
CR-39 films using the chemical etching technique. In the
study by A. Meshabi et al.,
18
it was demonstrated that a
linac with a flattening filter had more neutron fluence
than a flattening filter free linac with the same set-up and
collimator aperture.
18
In another study, a Varian 2300C
linac was simulated with the Monte Carlo code and it
was noted that the NDEs increased with the field size. It
was recorded that the secondary collimator was the main
neutron-contamination source.
28
The results of this study are in agreement with the
experimental and MC-simulation study by Mesbahi et al.
(2010). The authors aimed to investigate the effect of an
open beam and wedged beam beside the effect of field
size. While for the open beams, the NDEs decreased
with increasing field sizes, for the wedged beams, in-
creasing field sizes caused an increase in the NDEs.
Closing the jaws caused more neutron attenuation, so the
number of photoneutron interactions in the secondary
collimator jaws caused an increase in the neutron yield.
3
Comparing their results with the current results, it can be
seen that in this work, the measured and MC-simulated
NDEs are too close to each other for open beams.
Another Monte Carlo calculation based on the MCNP
code version 5 was used to study several components of
a 15-MV linac and the NDEs were measured with opti-
cally stimulated luminescence (OSL).
29
They reported
that for different field sizes, the NDEs increased due to
the closing of the MCL.
In the current study, the comparison between the
measured and MC-simulated NDEs is performed at
various field sizes opened by the MLC while the
collimator jaws are fixed. The NDEs increase due to the
closing of the MLC. When the MLCs are closed, more
neutrons can be produced in the MLCs made of tungsten.
The MC-simulated and measured NDEs are in agreement
in every position. However, the NDEs MC-simulated by
Fluka are slightly lower than the values measured with
the Thermo Scientific RadEye neutron detector, but still
within acceptable agreement. Furthermore, the measured
NDE values are lower than the NDEs reported by
Hashemi et al. (2007). In their study, polycarbonate films
were used as neutron detectors, but they were not sen-
sitive to the neutrons produced in the treatment-room
walls
2
. There are several important areas such as the
floor, wall and ceiling of the treatment room, from which
neutrons can scatter.
In this study, during the simulation, the effect of
walls on the neutron contamination is also ignored.
However, the results are still in contrast to the study by
Hashemi et al. (2007). It can be said that these diffe-
rences may be due to large dosimetric uncertainties
associated with the dosimeters. In the study of an
MCNPX simulation of a Varian linac by Chibani et al.
(2003), the NDE is reported to be 13.3 mSv/Gy for the
(10 × 10) cm field.
30
This is 10 times higher than the
NDEs MC-simulated in this work. The reason can be
explained with the fact that Electa Versa used in this
work is a flattening filter free (FFF) model. The
flattening filter is the major source of the head scatter.
The studies about the effect of the flattening filter on the
neutron contamination verify that by using the FFF
mode, the neutron contamination can be reduced.
According to the obtained results, it is clearly seen
that the Fluka code can be used to determine the photo-
neutron contamination inside a radiotheraphy treatment
room. As a result, it helps to predict the unwanted
neutron dose and provides crucial information about the
radiation-protection policy for the staff working at
clinics as well as patients.
5 CONCLUSIONS
The neutron contamination generated by a high-ener-
gy medical linac primarily depends on the treatment-
field size and off-axis distance. This work was mainly
motivated by the need to validate and develop accurate
computational tools to detect the unwanted neutron dose
in radiotherapy rooms. The effects of dosimetric pro-
perties of an 18-MV Elekta Versa HD linac on the
neutron contamination were determined using both a
Thermo Scientific RadEye neutron detector and the
Monte Carlo Fluka code simulation. This work proved
that the NDEs were a function of the field size, and they
increased with the field size. It was also shown that the
NDEs recorded at different distances from the isocenter
were reduced with the increasing off-axis distance.
When comparing the results obtained with dosimetric
measurement and simulation, it is concluded that the
Monte Carlo Fluka code is a reliable tool for determining
the neutron contamination, which can cause a secondary
cancer risk in the treatment room. Unwanted neutron
doses should not be neglected in a typical radiation-
therapy room. Consequently, it is necessary to take them
into account. Furthermore, it may also be necessary to
O. KARAMAN, A. GÜNEª TANÝR: INVESTIGATION OF PHOTONEUTRON CONTAMINATION ...
Materiali in tehnologije / Materials and technology 53 (2019) 5, 699–704 703

establish more conservative procedures and guidelines to
protect both the staff and the patients.
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O. KARAMAN, A. GÜNEª TANÝR: INVESTIGATION OF PHOTONEUTRON CONTAMINATION ...
704 Materiali in tehnologije / Materials and technology 53 (2019) 5, 699–704