Journal of the Korean Physical Society, Vol. 67, No. 3, August 2015, pp. 599∼607
Some Folded Issues Related to Over-shielded and Unplanned Rooms for
Medical Linear Accelerators - A Case Study
Wazir Muhammad and Asad Ullah
Health Physics Division, Pakisntan Institute of Nuclear Science & Technology (PINSTECH), Islamabad 45650, Pakistan
Amjad Hussain
Department of Oncology, Aga Khan University Hospital, Karachi 74800, Pakistan
Nawab Ali
Physics Division, Pakisntan Institute of Nuclear Science & Technology (PINSTECH), Islamabad 45650, Pakistan
Khan Alam
Department of Physics, University of Peshawar 25000, Khyber Pakhtunkhwa, Pakistan
Gulzar Khan
Department of Physics, Abul Wali Khan University, Mardan 23200, Khyber Pakhtunkhwa, Pakistan
Matiullah
Directorate of System & Services, Pakisntan Institute of Nuclear
Science & Technology (PINSTECH), Islamabad 45650, Pakistan
Seongjin Maeng and Sang Hoon Lee∗
School of Architectural, Civil, Environmental and Energy Engineering,
Kyungpook National University, Daegu 702-701, Korea
(Received 7 April 2015, in final form 27 May 2015)
A medical linear accelerator (LINAC) room must be properly shielded to limit the outside radiation exposure to an acceptable safe level defined by individual state and international regulations.
However, along with this prime objective, some additional issues are also important. The current case-study was designed to unfold the issues related to over-shielded and unplanned treatment
rooms for LINACs. In this connection, an apparently unplanned and over-shielded treatment room
of 610 × 610 cm2 in size was compared with a properly designed treatment room of 762 × 762 cm2
in size (i.e., by following the procedures and recommendations of the IAEA Safety Reports Series
No. 47 and NCRP 151). Evaluation of the unplanned room indicated that it was over-shielded
and that its size was not suitable for total body irradiation (TBI), although the license for such a
treatment facility had been acquired for the installed machine. An overall 14.96% reduction in the
total shielding volume (i.e., concrete) for an optimally planned room as compared to a non-planned
room was estimated. Furthermore, the inner room’s dimensions were increased by 25%, in order to
accommodate TBI patients. These results show that planning and design of the treatment rooms
are imperative to avoid extra financial burden to the hospitals and to provide enough space for easy
and safe handling of the patients. A spacious room is ideal for storing treatment accessories and
facilitates TBI treatment.
PACS numbers: 87.52.-g, 87.52.Ga, 87.65.-v
Keywords: Radiation shielding, Medical linear accelerator (LINAC), Room design
DOI: 10.3938/jkps.67.599
∗ E-mail:
lee@knu.ac.kr
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Journal of the Korean Physical Society, Vol. 67, No. 3, August 2015
I. INTRODUCTION
Time, distance and shielding are the three important
parameters that influence external radiation exposure.
In other words, the radiation dose received by an individual is directly proportional to the spent time in the
radiation field and inversely proportional to the square
of distance from the source. The dose is also reduced if a
shielding material is introduced [1,2]. In special circumstances, when the first two parameters (i.e., time and distance) cannot be compromised, radiation shielding plays
an important role in reducing the individual exposure.
The primary purpose of radiation shielding is to limit
radiation exposure to the general public and employees of the radiation facilities to an acceptable level recommended by the International Atomic Energy Agency
(IAEA) [3] or by an individual state regulatory body.
Two types of radiation barriers or shielding, known as
primary and secondary barriers, are considered to reduce
radiation exposures from high-energy medical linear accelerators (LINACs) to within safe limits [1,2,4]. The primary barrier of the treatment room is irradiated directly
by the radiation produced by the LINAC while the secondary barrier receives radiation scattered by the patient
and by the different surfaces of the room, as well as leakage radiation from the machine’s head. The maximum
beam size of the LINAC is used to determine the characteristics of the primary barrier. On the other hand,
secondary radiations are, emitted in all directions and
are therefore, considered for the entire treatment room
[1,2].
Neutrons are produced at high-energy X-rays and electron beams (energies > 10 MV) through X-ray-neutron
(x, n) and electron-neutron (e, n) reactions [1, 2, 4–12,
12, 14]. The X-ray target, primary collimators, beam
flattening filters, collimator jaws, and X-ray beam accessories are the main sources of neutron contamination [1,
2,10]. The cross section for the (x, n) reaction is at least
an order of magnitude larger than for the (e, n) reaction
at the high energies of LINACs [2, 15]. The neutrons
produced during the X-ray mode are of primary concern. Some elements, such as 15 O (t1/2 : 2 minutes) and
13
N (t1/2 : 10 minutes), inside the LINAC’s treatment
room may also become radioactive with short half-lives
(i.e., t1/2 ∼ seconds to a few minutes) by direct activation due to (x, n) reactions or by activation of secondary
neutrons. The radiation produced by these radioactive
elements contributes to the radiation exposure to the radiotherapy staff, who enter the treatment room after a
high-energy photon-beam treatment [2,15].
Both the primary and the secondary barriers are designed to protect against expected photon and contamination neutron doses. The primary barrier of the
treatment room is responsible for providing protection
against primary/direct X-ray beams. On the other hand,
secondary barriers are designed to provide protection
against scattered and leakage photons and neutrons.
These also attenuate the neutron-capture γ-rays [1].
A specially designed maze and door are essential to reduce the scattered and the leakage radiation fields along
the maze [2]. For the design of a maze or duct, a strong
knowledge of the scattering characteristics of X-rays is
required [4]. Efficient room ventilation is an effective
method for removing the radioactivity and will reduce
the exposure to the radiotherapy staff who enter the
treatment room after a high-energy photon-beam treatment [2]. The treatment room should be designed on the
lowest floor of the building to avoid huge shielding loads
on the floor [1,2].
The area around the treatment room is usually classified into two subtypes, controlled and uncontrolled, on
the basis of the access, occupancy and working conditions. The former has limited-access immediate areas
where radiation is used and where the occupational exposure to the radiation workers is under the supervision of
an individual in charge of radiation protection [2,3]. Examples are treatment rooms, control consoles, and other
areas that require control of access, occupancy and working conditions for radiation protection purposes. The
latter are those occupied by the general public (i.e., patients, visitors) and workers not involved in any radiation
related task [1,2].
The factors used in shielding calculations are as follows:
• The occupancy factor (T) is the fraction
of time that a particular area (i.e., controlled/uncontrolled) is occupied by radiation/nonradiation workers, patients or the general public.
The choice of an appropriate T ensures the protection of the occupationally exposed (i.e., radiation
workers), non-radiation workers, and members of
the general public who may be exposed to radiation in these areas. The typical values of T are
1, 0.25 and 0.125 for offices, corridors and waiting
rooms, respectively [1,2]. The first and main feature in the design of a LINAC’s treatment room
is to surround it with areas having a zero or low
value of T while keeping the heavily-occupied areas
as far away as possible. The other feature of the
LINAC’s treatment room is that it should be large
enough that the movements of equipment during
installation/service and of gurnies containing the
patients for treatments are easy.
• The workload (W) is expressed in Gy/week or
Gy/year and is used to provide an indication of
the amount of radiation produced in a week by Xray or γ-ray sources. It depends upon the dose per
fraction, the expected number of patients treated
per day, the number of work days per year and
other uses such as physics quality assurance, blood
irradiation and maintenance services [4]. A workload of 1000 Gy/week based on a dose of 4 Gy at 1
m per patient and a 5 day week for photon energies
up to 10 MV is suggested in NCRP Reports # 49
Some Folded Issues Related to Over-shielded and Unplanned Rooms · · · – Wazir Muhammad et al.
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& 151 [1,16]. For high-energy beams in dual energy
LINACs, a workload of 500 Gy/week is estimated
in NCRP Report 51 [4,17].
• The use factor (U) is the fraction of the beam’s on
time for the primary beam in a particular direction.
For primary beams, typical values of U are 1 for the
floor and 0.25 each for the walls and ceiling. For
secondary radiation, U is always equal to 1 because
scatter is present always and everywhere [1, 2, 16,
18].
The main goal of the shielding is to reduce the
dose equivalent (H) to minimum acceptable levels for
radiation/non-radiation workers and members of the
public. The dose levels used for the shielding calculation and for the evaluation of the radiation barriers are
normally termed as the shielding design goals (P). There
are different recommendations on the values of P for controlled and uncontrolled areas [1, 2, 16, 18]. At present,
the IAEA recommended dose limits for controlled and
uncontrolled areas are 20 mSv/year (i.e., averaged over
5 consecutive years and 50 mSv in any single year) and
1 mSv/year, respectively [3]. In the USA, for controlled
and uncontrolled areas, doses are limited to 10 mSv/year
(cumulative dose of age × 10 mSv, and 50 mSv in any
single year) and 1 mSv/year (infrequently, 5 mSv annually), respectively [4,19]. According to NCRP Rep. 151,
the recommended P values for controlled areas and uncontrolled areas are 0.1 mSv/week (5 mSv/year) and 0.02
mSv/week (1 mSv/year), respectively [1].
Normally, the consequences of under-shielded area are
well known. However, an over-shielded unplanned area
may have other associated financial and physical issues.
A thorough literature survey was conducted to properly
address optimal structural shielding & design requirements [1,2,4,17,20–22]. In light of the recommendations
and technical information, issues associated with overshielded and unplanned LINAC exposure areas are identified to provide the reader with a better understanding
of the factors involved in designing exposure areas for
medical linear accelerators. The study is intended to be
useful for medical/health physicists and other radiation
protection professionals in the planning and designing
of new exposure areas and in the assessment of existing
facilities.
II. MATERIALS AND METHODS
1. Calculational Methods
A new room was designed for a LINAC producing
multiple-energy photons (i.e., low & high with a maximum energy of 20 MV) with a maximum field size of
40 × 40 cm2 for intensity modulated radiation therapy (IMRT) and total-body irradiation by adopting the
Fig. 1. (Color online) (a) Simplified schematic diagram
and (b) side view of the treatment room for a 20-MV LINAC.
guidelines recommended in the IAEA Safety Reports Series No. 47 and NCRP 151 [1, 4]. A workload of 1000
Gy/week based on a 4 Gy per treatment fraction at 1 meter for 5 days a week, as suggested in the NCRP Reports
# 49 & 151, was used [1,16,17]. Regarding the permissible dose limits, the guidelines of the IAEA and ICRP
Publication 33 were followed (i.e., 20 mSv/year and 1
mSv/year for controlled and uncontrolled populations,
respectively) [2,3,23]. In accordance with the recommendations of the local regulatory authority, lower P values
of 0.04 mSv/week and 0.002 mSv/week for controlled (radiation workers) and uncontrolled (non-radiation workers & member of the general public) populations, respectively, were used. All the distances, areas and values of T
for the calculations were estimated from a scaled diagram
of the new optimally-designed facility (Fig. 1). Concrete
with a of density 2350 kg/m3 is assumed for shielding
purposes. For concrete of this density, the documented
values of the first tenth-value layer (TVL1) and the equilibrium tenth-value layer (TVLe ) are 0.46 m and 0.44 m,
respectively, for the primary beam. For the scattered
beam, a single value of 0.343 m is adopted for 20-MV
photons [4,24].
The width of the primary barrier (WP B ) in meters is
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Journal of the Korean Physical Society, Vol. 67, No. 3, August 2015
estimated as follows [1,2,4,16]:
WP B = (F × X + 0.60)m,
(1)
where F is the diagonal of the maximum radiation field
size (i.e., 0.56 m for a field size of 0.40 m × 0.40 m), X is
the distance from the target to the wall-ceiling junction
in meters, and 0.60 is an additional margin (i.e., 0.30 m
on each side of the primary barrier) [1,16].
The barrier transmission factor (B) is the fraction of
the incident beam’s air kerma in air transmitted through
a given thickness of shielding material [1, 4, 16]. The
Bpri corresponds to the primary radiation barrier and is
calculated as below [1,4,16]:
Bpri = P × (dpri )2 /(W × U × T ),
(2)
where dpri (m) is the distance from the source to the
point protected outside the primary barrier.
The number of TVLs based on the maximum energy of
the beam (i.e., 20 MV) and the type of shielding material
was calculated by using the following equation [1,4]:
n = −T V L × log10 (B).
(3)
Hence, the required thickness of barrier in meters (m) is
given by [1]
S = T V L1 + (n − 1)T V Le .
(4)
The first (i.e., TVL1 ) and the equilibrium (i.e., TVLe),
TVLs are used to account for the spectral changes as the
radiation penetrates the barrier [1].
For designing secondary barriers, scattered (from the
patient and surfaces) and leakage radiations were computed separately and compared in order to arrive at the
final recommended thickness [1]. The required barrier
transmission (Bs ) to shield against radiation scattered
by the patient was computed as follows [1,4]:
Bs =
P d2sec d2sca 400
,
aW T
F
(5)
where dsca is the distance from the radiation source to
the patient, (i.e., 1 m for LINACs), dsec is the distance
from the patient to the point of interest, and ‘a’ is the
scatter fraction defined at dsca. The primary scatter
ratio ‘a’ depends on the energy of the X-ray beam and the
scattering angle, and F is the area of the field incident
on the patient (i.e., for LINACs with a maximum field
size of 40 × 40 cm2 , F = 1600 cm2 ) [4].
Similarly, the barrier transmission factor (Bw ) that is
needed to shield against scattered radiation when the
primary beam strikes a wall is given by the following
equation [1,4]:
Bw =
P d2w d2r
.
aAW T U
(6)
Here, dw and dr are the distances from the radiation
source to the scattering surface (wall) and the distance
from the scattering surface (wall) to the point of interest,
respectively, α is the reflection coefficient for the barrier
material, which depends on the wall’s material, the scattering angle, and the beam’s energy, and A is the beam’s
area at first reflection. Finally, the required attenuation
(Bl ) to shield against leakage radiation was calculated as
follows [1,4]:
Bl =
P d2l
10−s W T
(7)
where dl is the distance from iso-center to the point of
interest.
Equations (3) and (4) were used to calculate the number of TVLs for the barrier thickness necessary to shield
against the head leakage radiation and the radiation
scattered from the patient and the surfaces. If the differences between the shielding thicknesses necessary to
shield against head leakage and scatter from patient and
from surfaces are less than three half-value layers, then
to be on safe side, one half-value layer (HVL) of concrete is added to the larger thickness; otherwise, the
larger shielding is enough for the required attenuation
[4]. These techniques were used to design the primary
and the secondary barriers of the walls and the roof of
the room. For a gantry rotation axis perpendicular to
the maze axis, the total dose at the maze entrance Dd
(Gy/week) can be calculated by using the following relation [1]:
Dd = ΣG DP + ΣG f × DwT + ΣG DL + ΣG DT , (8)
where Dd is the dose due to radiation scattered from the
patient, f is the primary radiation transmitted through
the patient, DwT is the primary radiation scattered by
the wall into the maze, DL is the leakage radiation scattered down the maze, and DT is the leakage radiation
transmitted through the maze wall.
In the NCRP-151 formalism, the first scatterer is taken
to be the wall, but a better approximation is obtained
if the patient is taken to be the first scatterer [1,16,17].
All the factors in Eq. (8) were calculated individually according to the guidelines provided in NCRP-151, and Dd
(Gy/week) was finally calculated by using Eq. (8) [1,4,16,
17]. The calculated Dd was 1.14 × 10−4 Gy/week. This
dose was added to the capture gamma and neutron doses
at the maze entrance, which was calculated as below:
The capture gamma dose in the maze Dϕ depends on
the maze length d2 and the total neutron fluence ϕA =
ϕd + ϕsc + ϕth at the inner maze point ‘A’, as shown in
Fig. 1(a) [1, 4]. Here, ϕd , ϕsc and ϕth are the fluencies
due to direct neutrons from the head of the accelerator
and due to scattered and thermal neutrons, respectively.
‘A’ indicates the point of intersection of the center line
of the maze and the line joining the iso-center and the
end of the maze wall. The total neutron fluence was
calculated as follows [4,11,12,25]:
ϕA = (QN /4πd21 )+(5.4QN /2πS)+(1.26QN /2πS), (9)
Some Folded Issues Related to Over-shielded and Unplanned Rooms · · · – Wazir Muhammad et al.
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Table 1. The widths of the primary barriers, the distances from the source to the point protected outside the primary barrier
(i.e., dpri ), the occupancy factor (T) and the shielding design goal (P) for each barrier, the calculated transmission factor (Bpri ),
the number of TVLs (n), the thickness of the primary barriers (S), and the final recommended shielding thickness against the
primary radiation of a LINAC having a maximum photon energy of 20 MV.
Wall
B
D
Width (cm)
450
450
dpri (cm)
748
748
Ceiling
450
590
T
0.063
0.063
0.025
0.063
P (Sv/w)
2E-6
2E-6
2E-6
4E-5
Bpri
7.16E-6
7.16E-6
1.11E-5
8.91E-5
n
5.15
5.15
4.95
4.05
Calculated S (cm)
237
237
228
186
Final Thickness (cm)
240
240
230
190∗
∗
If a chiller room is designed above the treatment room, the chiller room will have limited occupancy and access may be
restricted, thus allowing a greater design dose limit.
where d1 is the distance from the source to the point of
interest, and QN is the apparent neutron source strength
in number of neutrons emitted from the shielded accelerator head per unit dose of photon delivered to the isocenter. QN is related to the neutron source strength Q
[4]. The values of the Q are available in the published
literature for LINACs with energies in the range of 10
− 25 MV [4, 21, 26]. S is the surface area of the treatment room, excluding the maze area, in m2 . The surface
area S is the sum of all wall areas visible from the isocenter. Once the total neutron fluence ϕA at the inner
maze point is determined, the capture gamma dose Dϕ
is determined for the maze length d2 as follows [4,26]:
d2
Dϕ = 5.7 × 10−16 ϕA × 10− 6.2 .
(10)
The weekly dose equivalent due to the capture gamma
dose (in Sv/week) is calculated as follows:
Dc = W × Dϕ .
(11)
The neutron dose at any point in the maze is calculated
on basis of the distance from the inner maze point A to
the iso-center (d1 ), the surface area S of the treatment
room, the cross-sectional area of the inner entrance to the
maze (Ar ) and the cross-sectional area of the maze (S1 ).
These quantities are calculated from a scaled diagram
of the room, as shown in Fig. 1(a). The neutron dose
is also a function of the energy, the gantry angle and
the field size of the photon beam. With the Kersey’s
method to measure the neutron dose equivalent at the
maze entrance, Dn in Sv per X-ray·Gy at the iso-center
is given as follows [4,27,28]:
Dn = 2.4 × 10−15 × ϕA × Ar /S1
d2
−(
d2
)
× [1.64 × 10−( 1.9 ) + 10 TN ]
(12)
√
where TN = 2.06 × S1 , Ar and S1 are the crosssectional areas in m2 of the inner entrance to the maze
and the maze, respectively, d1 and d2 are the distances
in m from the iso-center to the inner maze point A as defined above and from the inner maze point A to the outer
entrance of the maze, respectively. The weekly dose due
to neutrons is calculated as follows [1,4,16]:
DE = W × Dn
(13)
By using the above formalism (i.e., Eqs. 1 − 13) and professional guidelines, we performed optimized shielding
calculations to design the treatment room for a LINAC
with a maximum photon energy of 20 MV.
2. Evaluation and Comparison
The newly-designed room was compared with an already constructed LINAC treatment room (610 × 610
cm2 ). A Varian’s CLINAC 2100C LINAC with dual
mode (photons & electrons) and dual photon energy (i.e.,
6 MV and 15 MV) for IMRT and total body irradiation (TBI) was installed in the room [29–33]. The total
shielding requirements (i.e., concrete volume in m3 ) and
the inner treatment room sizes were calculated and compared with the values for an optimally-planned and an
already-constructed room.
III. RESULTS AND DISCUSSION
The widths of the primary barriers, the distances from
the source to the point protected outside the primary
barrier (dpri ), the occupancy (T) factor for each barrier,
the calculated transmission factor of the primary barrier (Bpri ), the number of TVLs, the calculated thicknesses of the primary barriers (S) on the basis of Bpri ,
and the final recommended shielding thickness against
the primary radiation (20 MV energy) are listed in Table 1. The calculated thicknesses were increased 2 −
4 cm to compensate for any human errors in the final
construction of the walls. Similarly, the calculated values of the transmission factors and the corresponding
number of TVLs and thicknesses of the barriers against
radiation scattered by the patient, head leakage radiation and radiation scattered from surfaces, respectively,
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Journal of the Korean Physical Society, Vol. 67, No. 3, August 2015
Table 2. The occupancy factor (T) and the shielding design goal (P) for each barrier, the distances from the patient to the
point of interest (i.e., dsec ), the scatter fraction defined at dsca (a), the calculated transmission factor (Bp ), the number of
TVLs (n) and the thickness of barriers (Sp ) against radiation scattered by the patient.
Wall
A
B
C
D
Ceiling
P (Sv/w)
2E-6
2E-6
4E-5
2E-6
2E-6
T
1.000
0.063
1.000
0.063
0.025
a
0.000386
0.006320
0.000386
0.006320
0.006320
dsec (m)
5.70
6.48
8.50
6.48
4.50
Bp
4.21E-5
5.32E-5
1.87E-3
5.32E-5
6.41E-5
n
4.38
4.27
2.73
4.27
4.19
Sp (cm)
150
146
94
146
144
Table 3. The occupancy factor (T) and the shielding design goal (P) for each barrier, the distance from the iso-center
to the point of interest (i.e., dsec ), the calculated transmission factor (Bl ), the number of TVLs (n) and the thickness
of barriers (Sl ) against head leakage.
Wall
A
B
C
D
Ceiling
P
(Sv/w)
2E-6
2E-6
4E-5
2E-6
2E-6
T
1.000
0.063
1.000
0.063
0.025
dsec
(m)
5.70
6.48
8.50
6.48
4.50
Bl
n
6.50E-5
1.34E-3
2.89E-3
1.34E-3
1.62E-3
4.19
2.87
2.54
2.87
2.79
Sl
(cm)
144
98
87
98
96
Table 4. The occupancy factor (T) and the shielding design goal (P) for each barrier, the distances from the radiation source to the scattering surface (wall) (i.e., dsca ) and
from the scattering surface (wall) to the point of interest (i.e.,
dsec ), the calculated transmission factor (Bs ), the number of
TVLs (n) and the thickness of barriers (Ss ) against surface
scatterer.
Wall
A
B
C
D
Ceiling
P
(Sv/w)
2E-6
2E-6
4E-5
2E-6
2E-6
T
1.000
0.063
1.000
0.063
0.025
dsca
(m)
3.81
4.81
3.81
4.81
3.35
dsec
(m)
5.70
6.48
8.50
6.48
4.50
Bs
n
4.64E-5
1.53E-3
2.06E-3
1.53E-3
8.94E-3
4.33
2.82
2.69
2.82
3.05
Ss
(cm)
149
97
92
97
105
are summarized in Tables 2 − 4. The secondary barrier
thicknesses against radiation scattered by the patient,
head leakage radiation and radiation scattered from surfaces were computed separately due to the fact that each
type of radiation has a different energy. The computed
value for each secondary barrier thickness against radiation scattered by the patient, head leakage radiation and
radiation scattered from surfaces were compared for the
Fig. 2. (a) Neutron- and photon-shielded door for the outer
maze entrance (20 MV) and (b) the ducts wrap technique for
HVAC openings.
final recommended secondary barrier thicknesses. Based
on the above shielding calculation, we presented the final schematic diagram of the new radiation facility in
Fig. 1(a); Fig. 1(b) shows a side view. Figure 1(a) and
(b) display complete information on the occupancy of
the surroundings areas and on the various distances and
areas used in the above calculations, as well as the thicknesses and the lengths of both the primary and the secondary barriers.
The calculated values of DP , Dw , DL , DT and Dd were
4.01 × 10−5 Gy/week, 4.02 × 10−6 Gy/week, 1.67 ×
10−6 Gy/week, 8.67 × 10−8 Gy/week and 1.14 × 10−4
Gy/week, respectively, for a gantry rotation axis perpendicular to the maze axis. These doses were added
Some Folded Issues Related to Over-shielded and Unplanned Rooms · · · – Wazir Muhammad et al.
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Table 5. Comparison of the optimized shielded LINAC treatment room to the non-optimized shielded LINAC treatment
room. Here, PB represents the primary barrier while SB represents the secondary barrier.
Walls
A
PB
SB
B
C
PB
SB
D
E
Ceiling
Total
PB
SB
Optimized shielded LINAC room
Size: 762 × 762 cm2
Length
Width
Height )
Volume
(cm)
(cm)
(cm)
(m3 )
1242
165
365
74.8
450
240
365
39.4
312
150
365
17.1
1242
110
365
49.9
450
240
365
39.4
1099
150
365
60.2
817
122
365
36.4
1242
450
190
106.2
1242
924
150
172.1
595.5
to the capture gamma and neutron doses, especially for
the maze door. The calculated value of Dϕ was 3.71 ×
10−7 Gy per iso-centre X-ray·Gy based on the computed
value of ϕA at the inner maze point (i.e., 1.53 × 1010
n·m−2 per X-ray·Gy at 1 m). Therefore, the value of
P due to capture gammas was 4 × 10−5 Sv/week. To
reduce this dose to 2 × 10−5 Sv/week (P = 4 × 10−5
Sv/week by considering that half of the dose at the door
is due to photons plus capture γ-rays and half is due to
neutrons), we take the number of required TVLs to be
1.38 and the total calculated thickness of lead to be 8.3
mm (i.e., 1 TVL of lead = 6 mm). Similarly, the calculated Dn at the maze entrance and the weekly dose due
to neutrons (DE ) based on A were 2.01 × 10−6 Sv per
X-ray·Gy at the iso-center and 2.01 × 10−3 in Sv/week,
respectively. The required number of TVLs to reduce
the neutron dose of 2.01 × 10−3 Sv/week to 2 × 10−5
(P/2) Sv/week was 2.00. By using a TVL of 45 mm of
BPE (borated (5%) polyethylene) [4] for neutrons, we
found the required thickness to be 90 mm. The HVAC
duct opening was approximately 4 × 12 cm2 in cross
section above the room and 1.22 meter down the maze.
One cm of lead and 2.54 cm of polyethylene were used
to enclose the duct for proper shielding. The final design
of the neutron- and photon-shielded door for the outer
entrance to the maze and of the HVAC duct for a highenergy LINAC (i.e., 20 MV) are shown in Figs. 2(a) and
(b), respectively.
Figure 3(a) shows the final approved sketch of the nonplanned LINAC facility while Fig. 3(b) presents the final approved design of a properly planned and optimized
shielded room based on the above calculations. The term
unplanned and non-optimized was used due to the fact
that all the walls and the ceiling, except the maze and
the wall next to it, were built as primary barriers. Figure
Non-optimized shielded LINAC room
Size: 610 × 610 cm2
Length
Width
Height
Volume
(cm)
(cm)
(cm)
(m3 )
663
244
366
98.1
610
244
366
54.5
442
122
366
49.0
915
244
366
81.7
377
122
366
30.0
1281
1098
264
371.3
684.6
Fig. 3. (Color online) Final approved designs for (a) a nonplanned and non-optimized (over) shielded LINAC treatment
room and (b) a pproperly planned and optimized shielded
LINAC treatment room based on Fig. 1.
3(a) indicates that the primary and the secondary barriers are of the same thickness. The maze and the outer
parallel wall of the maze are about equal to the secondary
barriers of the planned LINAC facility and are, therefore,
regarded as secondary barriers. The inner dimensions of
the unplanned room, excluding the maze area, is 610 ×
610 cm2 . The volume of required shielding for each portion of the wall, the total shielding volume for both the
planned and the existing volumes of each portion of the
wall, and finally the total shielding volume for the unplanned LINAC facility were calculated and are listed in
Table 5. A total of 595.5 m3 of concrete were required
for the planned room whereas 684.6 m3 were estimated
for the existing unplanned room (see Table 5).
Figure 4(a) shows a comparison of the volumes of the
walls and the ceiling, as well as total volumes for the
planned and the unplanned facilities, while Fig. 4(b)
shows the percent reduction in the required volume of
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Journal of the Korean Physical Society, Vol. 67, No. 3, August 2015
and shielding calculation.
IV. CONCLUSION
A careful plan for a radiation room is imperative to
avoid an extra financial burden on the hospital. This
will also ensure optimal utilization of the available space
for providing special radiotherapy procedures (i.e., TBI)
and storage of various treatment accessories and/or immobilization devices. In doing so, a balance between
safety & cost-effectiveness must be achieved.
ACKNOWLEDGMENTS
This research was financially supported by the Ministry of Education, Science Technology (MEST) and the
National Research Foundation of Korea (NRF) through
the Human Resource Training Project for Regional Innovation (2012H1B8A2026280) and by the Kyungpook
National University Research Fund (2014).
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Fig. 4. (a) Comparison of an optimized shielded LINAC
treatment room having a size of 762 × 762 cm2 with a nonoptimized shielded LINAC treatment room having a size of
610 × 610 cm2 . (b) Percent reduction in the volume of
the shielding requirements for the planned LINAC treatment
room as compared to that for the non-planned LINAC treatment room.
shielding. The results show a 14.96% reduction in the required total shielding volume for the optimally-planned
facility, but the inner dimensions were increased from 610
× 610 cm2 to 762 × 762 cm2 . The reduction in required
shielding means that the total construction cost for the
facility may reduce by up to 14.96%. A 14.96% decrease
in construction cost is a significant figure for a developing
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facility for patients, even though the machine installed
inside could not only be used for TBI but also had a
license. The facility, to be cost-effective, must be optimally designed by consulting experts in radiation safety
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