The Design of Diagnostic Medical Facilities where ... - ResearchGate

The two published methodologies differ in respect of the shielding design goal (NCRP) or dose constraint (BIR)
to be employed. These terms refer to the level of air kerma (mGy) or effective dose (mSv) that is employed in a
shielding calculation to ensure that the annual dose limit to the surrounding area is not exceeded. Dose limits
are typically given in terms of effective dose, however both reports take the view that it is not practical to use
effective dose when calculating shielding requirements and therefore use air kerma (mGy).
For the design of new facilities, the NCRP recommends that the shielding design goal for radiation workers
should be based on a fraction of the annual dose limit, and a value of 5 mSv per year (which corresponds
with an annual air kerma value of 5 mGy) is recommended for controlled/supervised areas. The BIR states
that areas in which exposure can be greater than 6 mSv per year should be controlled.
Based on the ICRP recommendations for the annual limit of effective dose to a member of the public, the
NCRP recommend a shielding design goal of 1 mSv per year for uncontrolled areas. However, the BIR note
that a design limit based on the annual dose limit for members of the public (1 mSv) does not represent a
solution which is As Low As Reasonably Achievable (ALARA). It also refers to guidance on optimisation from
the NRPB (NRPB, 1993). It concludes that a dose constraint would have to be based on the dose limit of 1
mSv per year and, taking account of the principle of optimisation, decided on a value of 0.3 mSv per year.
The Irish dose constraint is also set at 0.3 mSv per year. Although there is a difference in the design goals set,
it is important to be aware that both approaches may still be used for calculations provided the appropriate
value of dose constraint is substituted.
5.2 Variables affecting shielding design calculations
Performing shielding calculations at or near a boundary requires knowledge of the distance from the source
of radiation to the nearest occupants; the maximum expected workload; the levels of occupancy in nearby
areas and the type of radiation involved (energy, primary beam, or secondary/scattered radiation, or both); and
the particular approaches used in nuclear medicine. All of these are discussed in more detail below.
5.2.1 Distance
Radiation intensity will be reduced with distance, therefore it is important to maximise the distance from the
source where possible. This concept should influence equipment layout. It is important to weigh up whether
it is more appropriate in a particular project to reduce radiation levels by maximizing distances and hence
space allocated, or by installing more structural shielding.
Realistic distances to the occupants in adjoining areas beyond the room walls must be used when performing
shielding assessments. The distance to occupants in the rooms above and below must take into account the
heights involved. The measurements should be taken as the distance from the source of radiation to the
organs of interest of the occupant nearest to the boundary in question. In radiology, the source of primary
radiation is the X‐ray tube and the main source of scatter is the patient. In nuclear medicine, the patient or
the radiopharmaceutical can be the source. The following minimum distances from the boundaries (Table 5.2
and Fig. 5.1) are recommended; however realistic measurements of the room involved are worthwhile (NCRP,
2004, AAPM, 2006).
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