Page 51 - ARNM-3-2

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

Many of the radiotherapy facilities in Egypt are operating B = 9.83 × 10 . Accordingly, the number of TVLs needed to
−6
at an overload, with an average of 60 patients per day. achieve this value is 5.0; considering that the TVL at 6 MV
Table 1 summarizes the two cases of workload distribution is 34.3 cm, the primary barrier should be 1.72 m thick.
during the working week. The difference between the two thicknesses is 23 cm.
3. Results This difference reduces the weekly dose at 6 MV to
4.26 µSv/week, making the total weekly dose 24.6 µSv.
3.1. Primary barrier at point B (public waiting area) Therefore, one half-value layer (HVL) of 10 MV will be
This barrier is made entirely of ordinary concrete. The added to the total thickness to ensure that the weekly dose
unshielded weekly dose behind this primary barrier is rate is <20 µSv/week; the final thickness of the primary
determined as in Equation VI, barrier will be 2.07 m. The total weekly dose from the two
energies at this final thickness is 11.94 µSv/week.
W × T × U/((d+0.3) +SAD) 2 (VI)
However, in the case of 10 MV FF beam modality, to
Where, the occupancy factor T = 1, the use factor fulfill the IDR of ≤7.5 µSv/h at a dose rate of 600 MU/min
U = 0.25, dose constraint = 20 µSv/week, d = 5.35, and at the linac’s isocenter, the primary barrier thickness
SAD = 1 m.
should be 2.35 m. In addition, in the case of 10 MV FFF
3.1.1. Case 1: Working load of 40 patients/day beam modality, to fulfill the IDR criteria of ≤20 µSv/h at
a dose rate of 2,400 MU/min at the linac’s isocenter, the
The weekly unshielded dose at 10 MV (20 patients) is primary barrier thickness should be 2.42 m. Accordingly,
presented in Equation VII. the highest value will be considered. Consequently, using
360 × 1 × 0.25/6.65 = 2.04 Sv/week (VII) IDR criteria necessitates that the primary barrier thickness
2
To reduce this value to the weekly dose constraints for for the workload of 40 patients/week should be increased
the public (20 µSv/week), the transmission factor becomes by 17.0%.
B = 9.83 × 10 . Accordingly, the number of TVLs needed to The expected equivalent dose behind the primary
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achieve this value is 5.0; considering that the TVL at 10 MV barrier at the final thickness, 2.42 m, equals 1.434 µSv/week
is 38.9 cm, the primary barrier should be 1.95 m thick. or 71.7 µSv/year, which means that the use of IDR criteria
The weekly unshielded dose at 6 MV (20 patients) is reduces the equivalent dose to just 7.17% of the annual
presented in Equation VIII. dose constrain, that is, 1 mSv/year.
360 × 1 × 0.25/6.65 = 2.04 Sv/week (VIII) 3.1.2. Case 2: Working load of 60 patients/day
2
To reduce this value to the weekly dose constraints for The weekly unshielded dose at 10 MV (40 patients) is
the public (20 µSv/week), the transmission factor becomes presented in Equation IX.

Table 1. Details of weekly workload distribution for a 6 and 10‑MV linear accelerator
Planned weekly Energy Patients Dose fraction Weekly workload Total dose IMRT Total dose produced
workload using 6 and 10 (treatment per day (Gy/patient) (Gy/week) at isocenter ratio by linear accelerator
MV linear accelerator modality) Gy/week (Gy/week)
Case 1: 40 patients/day 10 (3D) 10 3 180 360 1 855
10 (IMRT) 5 3 90 5
10 (VMAT) 5 3 90 2.5
6 (3D) 10 3 180 360 1 855
6 (IMRT) 5 3 90 5
6 (VMAT) 5 3 90 2.5
Case 2: 60 patients/day 10 (3D) 20 3 360 720 1 1,710
10 (IMRT) 10 3 180 5
10 (VMAT) 10 3 180 2.5
6 (3D) 10 3 180 360 1 855
6 (IMRT) 5 3 90 5
6 (VMAT) 5 3 90 2.5
Abbreviations: 3D: Three-dimensional; IMRT: Intensity-modulated radiotherapy; VMAT: Volumetric-modulated arc therapy.

Volume 3 Issue 2 (2025) 43 doi: 10.36922/ARNM025070007