Page 53 - ARNM-3-2

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

360 × 0.25 × 0.5/6.05 = 1.23 Sv/week (XIV) The ceiling shielding calculations at point G were
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To reduce this value to the weekly dose constraints performed similarly to those followed in primary barriers
for the workers (120 µSv/week), the transmission factor B and B’. Although the occupancy factor of the roof is very
becomes B = 9.76 × 10 . Accordingly, the number of TVLs small, the final thickness is determined based on 20 µSv/h
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needed to achieve this value is 4.0; considering that the criteria at 10 MV (FFF).
TVL at 6 MV is 34.3 cm, the primary barrier should be Table 2 summarizes the shielding calculation
1.38 m thick. results obtained for the two workload cases. Notably,
The difference between the two thicknesses is 30 cm; the maximum barrier’s thickness is obtained when
this difference reduces the weekly dose at 6 MV to IDR criteria were applied at 10 MV (FFF), that is,
15.84 µSv/week, which made the total weekly dose ≤20 µSv/h, regardless of any thickness obtained at
6 MV or 10 MV (FF). Table 3 summarizes the expected
135.84 µSv/week. Therefore, one HVL of 10 MV must equivalent dose received by personnel behind the primary
be added to the total thickness to ensure the weekly dose barriers B and B’.
rate is <120 µSv/week. The final thickness of the primary
barrier will be 1.8 m. The total weekly dose from the two 3.3. Secondary barriers at points C, D, E, F, and G
energies at this final thickness is 67.21 µSv/week.
Points C, D, E, and F have the same scattering angle
Since the primary barrier thickness is 2.45 m based (28° – 29°) and the same slant radiation path inside the
on the IDR criteria mentioned above, there is a 36.45% concrete (approximately 1.13 cm). Accordingly, the
increase in the barrier thickness. Notably, for this primary following observations are made:
barrier (B’), the IDR of the FF decreases from 7.5 µSv/h at (i) The distances of points C and D from the isocenter are
2.38 m to 5 µSv/h at 2.45 m, which is the thickness of the longer than those of E and F
FFF case. (ii) The occupancy factor at C and E is greater than that at
The expected equivalent dose behind the primary points E and F
barrier at the final thickness, 2.45 m, equals 2.679 µSv/week (iii) The dose constraint at points C and E is less than at
or 134 µSv/year, indicating that the use of IDR criteria points E and F.
reduces the equivalent dose to just 2.23% of the annual Thus, the shielding requirements at points C and D
dose constrain, that is, 6 mSv/year. are greater than those needed for E and F. Similarly, the
Given that only 185 cm of concrete exists, the remaining shielding requirements at point G are less than those
required thickness of the primary barrier (60 cm) should required at points C and D since its distance from the
be compensated with a high-density material due to the isocenter is much longer (8.24 m) and its scattering angle
limited space available. This required 60 cm thickness is greater (65°). Therefore, if the existing thickness (1.0 m)
corresponds to 1.54 TVL at 10 MV. Considering that the of all secondary barriers is adequate at points C or D, it will
TVL at 10 MV for iron is 10.5 cm, a 16.2 cm layer of iron also be sufficient at E, F, and G. The shielding requirement
should be used. Hence, for practical reasons, 16.5 cm of at point C is presented by evaluating the leakage and
iron will be implemented. patient scatter at this barrier.
Figure 2 represents the primary barrier’s final design The use factor for leakage radiation is 1, given that it hits
with an additional iron thickness. As shown, there are two all barriers regardless of the linac’s head angle. Secondary
options: (i) use one iron layer weighing approximately barriers adjacent to the primary barriers receive significant
16.9 tons or (ii) use multilayers of iron with different patient scatter radiation only when the primary beam is
thicknesses and dimensions. In this case, calculations directed at the primary barriers. Thus, assuming that the
were conducted in 40 cm steps in horizontal and vertical use factor at points C, D, E, and F equals 0.25, that is, the
directions. This was done because the layers will be same use factor as primary barriers will be reasonable.
arranged and fixed on the wall using 40 cm × 40 cm tiles Using a use factor of 1 for an adjacent secondary barrier
of iron, which will be well welded together. The thickness exaggerates conservatism. This study uses this assumption
of each layer is determined based on its distance from and verifies its validity through an experimental radiation
the isocenter and the slant path of radiation inside the survey at a later stage. Notably, the use factor for secondary
concrete, as shown in Figure 2. Using the multi-layers of barriers adjacent to the primary barriers in NCRP 151, SRS
iron requires only 13 tons, saving about 3.6 tons. Notably, 47, and IPEM 75 equals unity.
the thickness of the existing (1.35 m) and new concrete Although the average field size used in treating patients
(0.5 m) is adequate if the IDR criteria were not applied. daily is 20 × 20 cm , the same approach mentioned in NCRP
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Volume 3 Issue 2 (2025) 45 doi: 10.36922/ARNM025070007