Page 52 - ARNM-3-2

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

720 × 1 × 0.25/6.65 = 4.07 Sv/week (IX) needed to achieve this value is 4.0; considering that the
2
To reduce this value to the weekly dose constraints for TVL at 10 MV is 38.9 cm, the primary barrier should be
the public (20 µSv/week), the transmission factor becomes 1.56 m thick.
−6
B = 4.91 × 10 . Accordingly, the number of TVLs needed to The weekly unshielded dose at 6 MV (20 patients) is
achieve this value is 5.31; considering that the TVL at 10 MV presented in Equation XII.
is 38.9 cm, the primary barrier should be 2.07 m thick. 360 × 0.25 × 0.5/6.05 = 1.23 Sv/week (XII)
2
The weekly unshielded dose at 6 MV (20 patients) is To reduce this value to the weekly dose constraints
presented in Equation X. for the workers (120 µSv/week), the transmission factor

360 × 1 × 0.25/6.65 = 2.5 Sv/week (X) becomes B = 9.76 × 10 . Accordingly, the number of TVLs
2
5
To reduce this value to the weekly dose constraints for needed to achieve this value is 4.0; considering that the
the public (20 µSv/week), the transmission factor becomes TVL at 6 MV is 34.3 cm, the primary barrier should be
−6
B = 9.83 × 10 . Accordingly, the number of TVLs needed 1.38 m thick.
to achieve this value is 5.0; considering that the TVL at 6 MV The difference between the two thicknesses is
is 34.3 cm, the primary barrier should be 1.72 m thick. 18.4 cm; this difference reduces the weekly dose at 6 MV
The difference between the two thicknesses is to 34.78 µSv/week, which made the total weekly dose
34.7 cm; this difference reduces the weekly dose at 154.78 µSv/week. Therefore, one HVL of 10 MV will be
6MV to 1.94 µSv/week, which makes the total weekly dose added to the total thickness to ensure the weekly dose
21.94 µSv/week. Therefore, one HVL of 10 MV will be rate is < 120 µSv/week. Hence, the final thickness of the
added to the total thickness to ensure that the weekly dose primary barrier will be 1.68 m. The total weekly dose from
rate is <20 µSv/week and the final thickness of the primary the two energies at this final thickness is 75.85 µSv/week.
barrier will be 2.18 m. The total weekly dose from the two In the case of 10 MV FF beam modality, to fulfill the
energies at this final thickness is 10.88 µSv/week. IDR criteria of ≤7.5 µSv/h at a dose rate of 600 MU/min at
Since the primary barrier thickness is 2.42 m based the linac’s isocenter, the primary barrier thickness should
on the IDR criteria mentioned above, the primary barrier be 2.38 m. In the case of 10 MV FFF beam modality, to
thickness for the workload of 60 patients/week must be fulfill the IDR criteria to be ≤20 µSv/h at a dose rate of
increased by 10.73%. 2,400 MU/min at the linac’s isocenter, the primary barrier
thickness should be 2.45 m. Accordingly, the highest
The expected equivalent dose behind the primary value will be considered. Consequently, using IDR criteria
barrier at the final thickness, 2.42 m, equals 2.684 µSv/week necessitates that the primary barrier thickness for the
or 134.2 µSv/year, which means that the use of IDR criteria workload of 40 patients/week must be increased by 46%.
reduces the equivalent dose to just 13.4% of the annual
dose constrain, that is, 1 mSv/year. For this primary barrier The expected equivalent dose behind the primary
(B), it is worth noting that the IDR in the FF case decreases barrier at the final thickness, 2.45 m, equals 1.429 µSv/week
from 7.5 µSv/h at 2.35 m to 5 µSv/h at 2.42 m, the thickness or 71.45 µSv/year, which means that the use of IDR criteria
required in the FFF case. reduces the equivalent dose to just 1.91% of the annual
dose constrain, that is, 6 mSv/year.
3.2. Primary barrier at point B’
3.2.2. Case 2: Working load of 60 patients/day
This barrier is made partially of ordinary concrete, and
any additional thickness is made of iron. At this point, the The weekly unshielded dose at 10 MV (40 patients) is
occupancy factor T = 0.5, the use factor U = 0.25, and the presented in Equation XIII.
dose constraint = 120 µSv/week. 720 × 0.25 × 0.5/6.05 =2.46 Sv/week (XIII)
2
3.2.1. Case 1: Working load of 40 patients/day To reduce this value to the weekly dose constraints
for the workers (120 µSv/week), the transmission factor
The weekly unshielded dose at 10 MV (20 patients) is becomes B = 4.88 × 10 . Accordingly, the number of TVLs
−5
presented in Equation XI. needed to achieve this value is 4.31; considering that the
360 × 0.25 × 0.5/6.05 = 1.23 Sv/week (XI) TVL at 10 MV is 38.9 cm, the primary barrier should be
2
To reduce this value to the weekly dose constraints 1.68 m thick.
for the workers (120 µSv/week), the transmission factor The weekly unshielded dose at 6 MV (20 patients) is
becomes B = 9.76 × 10 . Accordingly, the number of TVLs presented in Equation XIV.
5

Volume 3 Issue 2 (2025) 44 doi: 10.36922/ARNM025070007