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

(IMRT), and volumetric-modulated arc therapy (VMAT). of Physics in Medicine and Engineering Report No. 75
In recent years, IMRT and VMAT have increasingly (IPEM 75). The shielding evaluation and determination
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replaced three-dimensional conformal radiotherapy. were performed for the existing vault using the analytical
Conventionally, it was believed that an ideal photon equations listed in the International Atomic Energy
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beam used in treatment planning should be uniform across Agency Safety Reports Series No. 47 (IAEA SRS-47)
the treatment field. This uniformity is obtained using a and the National Council on Radiation Protection and
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flattening filter (FF) positioned in the linear accelerator Measurements Report No. 151 (NCRP 151). Both primary
(linac) head. However, the widespread adoption of and secondary barriers were assessed, with a particular
non-uniform beam distribution has challenged this focus on primary barriers that are directly exposed to
assumption. Removing the FF results in flattening filter- the beam. The impact of applying the IDR criteria on
free (FFF) beams, now widely used for stereotactic and shielding design has not been extensively studied. In this
non-stereotactic radiotherapy treatments. As a result, work, the thicknesses of the primary barriers obtained
advanced treatment techniques prefer FFF beams over FF with and without adhering to the above-mentioned IDR
beams. 1-6 criteria were analyzed in detail and evaluated in the
context of fundamental radiation protection principles,
Numerous studies 3,4,7-12 have thoroughly examined the with particular emphasis on the “optimization” principle.
main dosimetric properties of FFF beams, the precision The necessary recommendations were proposed regarding
of dose calculations, and the quality of treatment plans using the IDR criteria in its current form or modifying its
for IMRT with unflattened beams. The FFF beams exhibit use to suit the working conditions in radiation therapy
distinct characteristics compared to standard FF beams, units without overestimating the barriers’ thicknesses. This
specifically in terms of high dose rate and a profile shape study represents a step-by-step, comprehensive, practical
that is peaked along the beam’s central axis. Moreover, guide for determining shielding requirements for a real
removing the FF significantly lowers the leakage radiation linac bunker, with detailed explanations for each step.
from the treatment head and softens the photons’ energy
spectrum. 13-16 Furthermore, since the FF is a source of 2. Methods
photoneutrons, its removal reduces neutron production
from the linac’s head. 2.1. Bunker description and shielding upgrade
approach
The main objective of the radiation protection program
in medical linear accelerator facilities is to protect workers, Figure 1 represents the linac’s bunker before and after the
the general public, and the environment from the harmful shielding upgrade. The existing primary barrier thickness
effects of ionizing radiation (gamma rays and photo- (1.35 m) is inadequate for 6 or 10 MV linacs. One of the
neutrons) transmitted outside the linac’s vault. Thus, primary barriers is adjacent to the public waiting area, that
shielding requirements are defined by the dosimetric is, fully occupied, and fortunately, there is enough space to
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characteristics of the clinically applied MV photon beams extend it using ordinary concrete (density = 2.35 g/cm ).
– more precisely, by the beam modality used during The same situation applied to the primary ceiling, and it
treatment (FF or FFF). In all cases, shielding design should is also possible to extend the ceiling’s shielding since the
avoid unnecessary overestimation. A balanced approach bunker’s roof is dedicated to the linac’s chillers and the
must be taken, considering both the actual shielding hospital’s heating, ventilation, and air conditioning system.
needs under normal operating working conditions and the The other primary barrier is adjacent to the maze of
associated construction cost. another bunker, that is, partially occupied, and the space
is very limited for implementing any concrete shielding
This study aims to present a case analysis evaluating
the shielding requirements for an existing Co-60 beyond this barrier. Therefore, the necessary shielding
extension will be partially implemented using ordinary
radiotherapy bunker intended to adequately accommodate concrete inside the bunker; the largest thickness available
an Elekta Versa HD medical linear accelerator operating
at 6 and 10 MV, with and without an FF. The shielding for this internal extension is 0.5 m × 3.12 m. This internal
shielding extension is very advantageous to support the
evaluation considers both a standard operating workload ceiling’s shielding extension from a structural engineering
(40 patients/day) and an overload (60 patients/day) of 50% point of view. The remaining shielding extension will be
for each accelerator energy. The final thicknesses of the implemented using high-density material, such as iron,
primary barriers were determined such that the expected outside the bunker.
instantaneous dose rate (IDR) behind the barriers does not
exceed 7.5 µSv/h for FF mode and 20 µSv/h for FFF mode, The left secondary barrier of the bunker’s maze is
in accordance with the recommendations of the Institute comparatively short and thin (0.95 m). This results in a

Volume 3 Issue 2 (2025) 40 doi: 10.36922/ARNM025070007