A shielding plan is a document which details the level of shielding that is, or will be, installed in order to protect workers and members of the public. This could include dental, veterinary, chiropractic, or other medical premises where ionising radiation apparatus is used. The plan will typically include floor plans, estimated dose calculations and other details related to radiation protection.
Where a radiation source is used it is a requirement that radiation shielding be documented as part of commissioning and where shielding modifications are made subsequent to commissioning.
Any source that will be installed in new or renovated premises must have a radiation shielding plan prepared and submitted with the registration application. Where a replacement source is installed into existing premises which have not been renovated submitting a copy of the existing shielding plan will generally be sufficient.
To ensure that the shielding plan meets all relevant requirements, it should be submitted as early as possible and prior to construction. The radiation shielding needs to be inspected by HPS during the construction phase before it is covered by paint, plaster, floor coverings or other fittings – or must be tested by accredited testers.
Any penetrations to the shielding, such as for cables, power points, water pipes or light switches, need to be addressed with material equal to or greater than the lead equivalence of the material which was removed.
The additional material needs to extend beyond the edges of the penetration by at least twice the offset distance*. If the location of a stud or nogging makes this impossible the additional material must return along the stud or nogging to meet the original layer of material.
*The offset distance is the distance between the original material and the additional material.
Third party verification of installed shielding may be requested by HPS when assessing a source registration application or re-application (renewal).
For further information contact the Health Protection Service, and consult the following documents: National Directory for Radiation Protection, June 2017 (RPS 6), Radiation Protection in Planned Exposure Situations (2016) (RPS C-1), Fundamentals for Protection Against Ionising Radiation (2014) (RPS F-1), and any relevant RPS publication, e.g. Code of Practice for Radiation Protection in the Medical Applications of Ionizing Radiation (2008) (RPS14). The National Council on Radiation Protection and Measurements (NCRP) Report No. 147, Structural Shielding Design for Medical X-Ray Imaging Facilities may also be useful.
Design Constraints
The following design constraints should be used in radiation shielding calculations:
- Controlled areas 2mSv/yr (40µSv/wk)
- Other areas 0.5mSv/yr (10µSv/wk)
A controlled area, in relation to a radiation source, is a limited access area:
- in which the exposure of persons to radiation is under the supervision of an individual in charge of radiation protection. This implies that access, occupancy and working conditions are controlled for radiation protection purposes; OR
- to which access is subject to control and in which employees are required to follow specific procedures aimed at controlling or monitoring exposure to radiation.
If a blanket design constraint is used for an entire practice or department then this should be 0.5mSv/yr (10µSv/wk).
Shielding design height requirements from the finished floor level are:
- no less than 2.1m for general radiography, fluoroscopy, mammography, BMD/DEXA, OPG, intraoral, dental CBCT and nuclear medicine; and
- no less than 2.7m or to the upper slab, whichever is the lower for CT, including SPECT/CT, PET/CT and all other non-dental CBCT unless a lower height has been authorised by HPS
However, for high dose areas including CT and interventional radiology, shielding design depends on a number of site-specific factors so shielding designers are asked to refer to authoritative texts which deal with these cases such as [1] and to specify height requirements greater than those indicated above, where applicable.
[1] Sutton, D.G., Martin, C.J., Williams, J.R., Peet, D.J., Radiation Shielding for Diagnostic Radiology, 2nd Edition, British Institute of Radiology, London (2012).
Dental shielding design
The British Institute of Radiology design methodology as indicated in [1] below must be used in radiation shielding calculations for intraoral, OPG and CBCT units in the ACT, along with the design constraints of 40µSv/wk for controlled areas and 10µSv/wk for other areas.
For OPG the required incident air kerma at 1m from the patient is 1µGy per dental image [2]. Variations from this requirement may be considered if sufficient evidence is provided to justify the variation.
For CBCT the incident air kerma at 1m varies with the type of unit and the operating kV. Similarly, the amount of shielding is critically dependant on the kV used. Shielding experts are required to comply with the BIR [1]. The incident air kerma at 1m may be provided by the manufacturer or may need to be determined by measurement.
[1] Sutton, D.G., Martin, C.J., Williams, J.R., Peet, D.J., Radiation Shielding for Diagnostic Radiology, 2nd Edition, British Institute of Radiology, London (2012).
[2] HOLROYD, J., Measurement of scattered and transmitted x-rays from intra-oral and panoramic dental x-ray equipment, J Radiol Prot 38 2 (2018) 793-806.
Plasterboard densities
For radiation shielding purposes the density of gypsum plasterboard is assumed to be 0.705g/cm3, as indicated in [1] above. Since plasterboard products with a density lower than this are available on the Australian market, calculation of a density scaling factor is required for any shielding design which specifies a thickness of plasterboard to achieve a particular level of X-ray attenuation.
If the specific plasterboard product which will be used is not known, then the lowest density plasterboard should be assumed for calculation of the scaling factor (currently approximately 0.57g/cm3 which gives a scaling factor of 0.81).
The calculated plasterboard thickness should be divided by the scaling factor to produce an adjusted thickness which takes account of a lower density plasterboard material.
In the absence of a published and peer-reviewed scientific paper, or data from a NATA accredited measurement facility, indicating the X-ray attenuation properties of the particular material at clinically relevant kVp values, plasterboard with a higher density will be assumed to provide only the X‑ray attenuation properties of the ‘standard’ density (0.705g/cm3) material.
Alternative shielding materials
Fyrchek plasterboard
Submission of a shielding plan which includes the specification of this material may result in HPS requesting supporting information in relation to the X-ray attenuation properties of the material. The supporting information must be in the form of a published and peer-reviewed scientific paper that details the use of relevant thicknesses of this material at various kVp values.