Page 57 - ARNM-3-2

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

(i) The IDR (FF/FFF) criteria could only be used after behind the primary barriers in FF and FFF modes do not
incorporating the patient transmission factor and exceed 7.5 µSv/h and 20 µSv/h, respectively.
verified experimentally using appropriate phantoms Results showed that adhering to the IDR criteria yields an
(ii) Since no patient or phantoms are used while applying increment in shielding demands ranging from 11% to 46%
the IDR (FF/FFF) criteria, these criteria should be depending on the dose constraints (public or occupational)
used exclusively for shielding effectiveness verification as well as on the occupancy factor of the adjacent areas.
and validation
(iii) The IDR (FF/FFF) values should not be used to This will be significantly reflected in the cost of the linac
vault. Moreover, complying with IDR criteria reduces the
determine the equivalent dose behind the barriers; expected equivalent dose behind barriers ranging from
instead, they should verify the barriers’ transmission 1.2% to 13.4% of the dose constraints. The results proved
factor and refine the expected equivalent dose that applying the IDR criteria yields a violation of the
only when barrier densities differ from that used in radiation protection principles, in particular, optimization.
calculations
(iv) During shielding verification, it is sufficient to use On the other hand, non-complying with the IDR criteria,
the expected weekly equivalent dose behind barriers still
IDR only in FF mode, as these dose rates are lower shows conservative values ranging from 54% to 63% of the
than those in FFF mode
(v) High dose rate measurements using IDR (FF/FFF) weekly dose constraints.
should be acknowledged only in the context of Thus, this study presents an alternative approach to
shielding effectiveness. use the IDR criteria optimally. This study concludes that

Therefore, it is necessary to enhance the safety the current values of IDR, that is, ≤7.5 µSv/h at FF and
culture and professional awareness among workers in ≤20 µSv/h at FFF, can be used provided that the patient’s
radiotherapy units and regulatory bodies. There must transmission factor is included, or higher values of IDR are
be a clear understanding that the operational context of used only when testing the shielding efficiency; the higher
medical linacs in radiotherapy fundamentally differs from the IDR adopted, the lower the cost of the linac vault.
that of nuclear medicine. Radiation measurements using Some workers may not accept the newly adopted high
the IDR (FF/FFF) reflect testing conditions, not working IDR due to their lack of understanding of how shielding
conditions. High radiation levels observed during these requirements of a linac bunker are obtained. The workers,
tests are not indicative of typical staff exposure but rather a staff, and regulatory bodies should recognize that such
tool to validate the adequacy of structural shielding. high IDRs are expected only at extreme conditions used
Certainly, regulatory authorities can set a reasonable to verify the shielding effectiveness measurements. Such
maximum level for IDR (FF/FFF) during shielding measurements are made occasionally, just after shielding
efficiency tests based on the occupancy factor and the implementations, and never be experienced during daily
personnel behind the primary barriers. However, the working normal conditions. Such high dose rates could
higher it is, the lower the degree of overestimation in the be measured safely remotely under the supervision of a
required wall thickness. In any case, shielding efficiency professional radiation protection expert, considering all
tests and the expected high radiation readings can be protective measures.
performed safely under a strict radiation protection Therefore, the safety culture and professional awareness
program and the supervision of a qualified radiation regarding the working, inspecting or authorizing linac’s
protection expert. bunker should be raised and flourished among working
staff and regulatory bodies, particularly in developing
5. Conclusion countries.
This study represents a case study for upgrading the Further studies should focus on evaluating the actual
shielding of an existing Co-60 radiotherapy bunker to personal equivalent doses received by individuals behind
accommodate a medical linac operating at 6 MV and 10 shielding barriers in different radiotherapy units and
MV in FF and FFF modes. Shielding calculations were comparing them with the thickness of those barriers.
performed systematically using analytical equations listed These studies should focus on linac-based radiotherapy
in NCRP 151, SRS 47, and IPEM 75. units that use the FFF technology with different workloads

The shielding evaluation took into account a standard that cover different treatment protocols and modalities.
operating workload (40 patients/day) and an overload Relevant international institutions, especially the
(60 patients/day) of 50% for each accelerator energy, with International Commission on Radiological Protection,
the final wall thickness restricted so that the expected IDR could launch a task group to reconsider the concept of IDR

Volume 3 Issue 2 (2025) 49 doi: 10.36922/ARNM025070007