Page 56 - ARNM-3-2

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

value is 3.64; considering that the TVL leakage for 10 MV (ii) Although applying the IDR criterion significantly
is 27.5 cm, the secondary barrier should be 1 m thick. reduces the personal equivalent doses behind
The weekly unshielded patient scatter dose at 6 MV the barrier, does it truly satisfy the core radiation
(20 patients) is presented in Equation XXII. protection principles, namely optimization; in other
words, is the thickness justified and well optimized?
360 × 1 × 0.25 × 2.77 × 10 ×1,600/400 ×5.12 = 38 mSv/ (iii) Do the IDR criteria reflect the same parameters and
−3
2
week (XXII) conditions used in evaluating the barrier thickness by
The transmission factor becomes B = 4.28 × 10 to the equations mentioned in NCRP 151 or SRS 47?
−4
reduce this value to the weekly dose constraints. Hence, Certainly, applying the IDR criteria does not
the number of TVLs needed to achieve this value is 3.28; comply with the optimization principle. Applying the
considering that the TVL leakage for 6 MV is 26.1 cm, dose constraint principle provides a significant safety
the secondary barrier should be 0.86 m thick. A simple factor to protect personnel in radiotherapy facilities. In
calculation shows that the total scatter radiation from 6 and addition, the analytical equations listed in NCRP 151
10 MV at the existing slant thickness equals 7.87 µSv/week. and SRS 47 for primary barriers, in particular, are highly
Accordingly, the total weekly dose from leakage and conservative as they are formulated without accounting
scatter radiation at point C is 22.44 µSv. Given that the for the presence of patients or phantoms in the path of
total dose of 22.44 µSv is only slightly greater than the the radiation. Moreover, the IDR criteria do not consider
weekly dose constraint, 20 µSv, the existing thickness use or occupancy factors, which are fundamental in
is left unaltered. Since the existing secondary barrier at traditional shielding design. This omission leads to overly
point C, with a thickness of 1 m, is considered adequate, conservative estimates, resulting in exaggerated barrier
the secondary barriers at points D, E, F, and G are also thicknesses. Therefore, the current application of the IDR
regarded as adequate. criteria in determining the final barrier thickness should
be reconsidered, or its intended role should be more
4. Discussion clearly defined.

The shielding calculations performed, particularly for In this context, it is important to present the expected
primary barriers B and B’, showed that the final thickness IDR behind the primary barriers based on the barrier
of the barriers is determined solely by the IDR criteria at thicknesses determined using the analytical equations
10 MV in FFF mode. Specifically, the barrier thickness provided in NCRP 151 and SRS 47. Table 4 shows the
must ensure that the IDR does not exceed 20 µSv/week expected dose rate for both FF and FFF modes at the
at the maximum dose rate at the isocenter. In practical actual barrier thicknesses obtained at the workloads of 40
terms, neither the standard workload (40 patients/day) and 60 patients per day. The expected weekly equivalent
nor the heavy workload (60 patients/day) influences the dose and its percentage to the weekly dose constraint are
determination of barrier thickness. Furthermore, the IDR also included. Based on the table, all the IDRs for FF listed
criteria for 6 MV, whether in FF or FFF mode, are irrelevant significantly exceed 7.5 µSv/h, and all the IDR values of FFF
in this context, as their IDRs are inherently lower than exceed 20 µSv/h. Despite these exceedances, the expected
those for 10 MV. This leads to several important questions: equivalent doses behind the barrier remain well below the
(i) If the thickness of the primary barrier can be weekly dose constraint. Therefore, the current approach
determined in a single calculation step, what is the to applying IDR (FF/FFF) criteria should be reconsidered.
practical value of performing extensive and time- The following recommendations are proposed to optimize
consuming calculations shown above? their usage:

Table 4. IDR of FF and FFF behind the primary barriers B and B’ at the thicknesses obtained using NCRP 151 analytical
equations
Barrier ID Thickness (m) IDR (FF) IDR (FFF) Pw, weekly dose Pcw, weekly Pcw/Pw (%)
(patients/day) 600 MU/min 2,400 MU/min constraint equivalent dose
B (60) 2.12 20 µSv/h 80 µSv/h 20 µSv 10.88 µSv 54.4
B (40) 2.07 40 µSv/h 160 µSv/h 20 µSv 11.94 µSv 59.7
B' (60) 1.95 240 µSv/h 960 µSv/h 120 µSv 67.22 µSv 56.02
B' (40) 1.68 480 µSv/h 1,920 µSv/h 120 µSv 75.85 µSv 63.21
Abbreviations: FF: Flattening filter; FFF: Flattening-free filter; IDR: Instantaneous dose rate; NCRP 151: National Council on Radiation Protection and
Measurements Report No. 151.

Volume 3 Issue 2 (2025) 48 doi: 10.36922/ARNM025070007