Page 50 - ARNM-3-2

Advances in Radiotherapy
& Nuclear Medicine Shielding exaggeration in medical linac bunkers

In addition, the inner maze entrance was modified by scatter radiation, and d is the distance from the patient to
sec
adding a shielding extension of ordinary concrete, with the calculation point.
dimensions of 1.75 m × 0.65 m. This alteration effectively The designed dose constraint (P ) behind a particular
converted the original single-leg maze into a two-leg primary barrier is achieved at a specific value of the
w
configuration, significantly reducing the radiation (gamma transmission factor B of that barrier. This transmission
and neutrons) within the maze. As a result, radiation dose factor is achieved using a particular thickness of the barrier
0
behind the right-side barrier of the maze where two rooms of specific density that can attenuate the radiation dose to
are fully occupied, namely, the treatment planning system
and control rooms, are also expected to decrease. the designed dose constraint. The required transmission
factor B could be determined using Equation IV,
0
The internal shielding extensions were designed based
on two criteria, as illustrated in Figure 1: first, to allow TVLs log=  1  (IV)
free movement of the linac treatment couch without any 10   B  
0
obstacles while maintaining sufficient space for the linac
operator to position patients comfortably, and second, Where TVLs are the number of the tenth value
to allow for the safe and free maneuvering of the largest layers (TVLs) required to achieve the designed B . The
0
component of the Elekta Versa HD linac through the inner transmission factor B of that barrier is then given in
0
maze entrance. Equation V,
Given that the minimum slant path through the  P ×( SAD d) 2 
+
shielding is approximately 114 cm, the thicknesses of the B =  0 D  (V)
0
other secondary barriers (1.0 m) were left unaltered. This   0  
design assumption will be validated through calculations Where D is the dose rate at the isocenter, and P is the
0
0
and radiation measurements. equivalent dose rate constraint behind the barrier. IPEM
17
2.2. The national council on radiation protection and 75 proposed P = 7.5 µSv/h when testing the transmission
0
measurements report no. 151 and safety reports factor at the FF modality and 20 µSv/h in the case of the FFF
series no. 47 shielding calculation approach beam modality. In this study, all parameters, as mentioned
earlier, were retrieved from the NCRP 151 except the TVLs,
The weekly dose constraint (P ) received by a person which were extracted from the IAEA SRS-47 because the
w
behind a primary barrier is determined by Equation I. TVL values reported therein are more consistent with the
B × W UT× × experimental data results in recent studies. 20,21
P = 0 dSAD+ (µ Sv week) (I) Exposure to the radiation from the linac vault represents
/
w
planned exposure controlled within the framework of dose
The dose constraint (P ) received by a person behind constraints to ensure that the radiation exposure is as
w
a secondary barrier is determined by the sum of the doses low as reasonably achievable. Accordingly, the shielding
22
received due to secondary radiation, namely, head leakage design calculations are carried out using the following
[P ] and patient scatter [P ], as in Equations II and III, assumptions: (i) annual dose limit [P ] = 20 (1) mSv/year
L
p
o
P = P + P (μSv/week) (II) for occupational (public) and (ii) annual dose constraints
w
L
P
[P] = 6 (1) mSv/year for occupational (public). This means
*
a WT××( / 400
B × W × T B ×× F ) the weekly dose constraints [P ] = 120 (20) µSv/week.
w
P = L 1000 × d + P d × d (µ Sv week) The thicknesses of the primary and secondary barriers
/
w
sca
sec
(III) are determined so that the weekly dose constraints are not
Where W is the weekly workload (Gy/week) at the exceeded.
isocenter, W* is the weekly workload (Gy/week) produced 2.3. Workload considerations
by the linac, T is the occupancy factor, a is the scatter
fraction from the patient, U is the wall use factor, F is Two workload cases have been considered in this study;
field size incident on the patient, and SAD is the source the first one is 40 patients/day, representing the standard
to isocenter distance. B is the barrier transmission against workload in all literature. The number 40 patients/day
0
primary radiation, d is the distance from the isocenter to means five patients are treated in 1 h along an 8-h working
the calculation point, B is the barrier transmission against day. Another case represents an overloaded facility with
L
leakage radiation, d is the distance from the source to 60 patients/day, which means that seven to eight patients
sca
the patient, B is the barrier transmission against patient are treated in an hour or the working day is longer than 8 h.
P
Volume 3 Issue 2 (2025) 42 doi: 10.36922/ARNM025070007