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We have conducted a survey to determine the whole body radiation dose received specifically through operation of the dual energy X-ray absorptiometry (DXA) system installed in our unit. A clinical bone mineral densitometry (BMD) service is delivered using a single Hologic QDR-4500 (Hologic Inc., Bedford, MA) fan beam system. This is operated by departmental technologist staff working in rotation. At least three members are generally trained in its use at any given time. Approximately 4000 patients are investigated annually. Each patient routinely receives a standard DXA scan of the lumbar spine and right hip, with each scan performed in "array" mode (scan duration approximately 60 s) following a scout scan performed in "turbo" mode.
The Ionising Radiations Regulations 1999 [1] follow the recent recommendations of the International Commission on Radiological Protection [2] in requiring that the whole body radiation dose to a radiation worker is kept below 20 mSv per year, and below three-tenths of this limit (i.e. 6 mSv year-1) for workers who are not classified. The radiation dose received by a member of the public must be kept below the significantly lower limit of 1 mSv year-1. This limit applies to the fetus of a pregnant radiation worker over the course of her "declared term of pregnancy" in recognition of the status of the fetus as a member of the public. Thus, we are obliged to ensure that the fetal dose is unlikely to exceed 1 mSv over the declared term of pregnancy. In the light of this change to statutory dose limits, a survey was performed to determine the integrated radiation dose resulting from normal clinical use of the facility.
12 film badge dosemeters were fixed in selected locations around the room in which the DXA system is located (Figure 1
), covering sites defining the boundary of the operator's workstations as well as at the room's periphery. Dosemeters were sited at the end of the working day on 6 March 2001 and were removed at the end of the day on 10 April 2001. A total of 354 clinical BMD studies were performed during this period (comprising 20 working days). A total of 3846 studies were performed during the previous calendar year 2000. Recorded radiation doses were multiplied by a factor of [3846/354] to determine an annualized radiation dose estimate consistent with that likely to be received by a single operator working in the facility full-time and continuously over the course of a full year (Table 1).
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A specific assessment of work practices should always be conducted for all workers upon declaring their pregnancy. However, it is clear from the results of this survey that existing radiation safety arrangements in our facility are sufficient to ensure regulatory compliance for both pregnant and non-pregnant radiation workers. Adequate protection is offered to staff provided that, as per current practice, they are positioned within the region indicated by the boundary of dosemeter locations 2–8 (Figure 1
) when a DXA study is in progress, where this region includes both the DXA acquisition console and the patient database terminal. A relationship between recorded dose and dosemeter location with respect to orientation of the fan beam is also clear, with locations 7 and 8 at the foot of the patient couch recording significantly lower doses than locations 2, 3 and 6 despite an approximate 2 m separation for each.
Whilst this is obviously a site-specific finding, our BMD suite is not untypical in its workload, layout and overall dimensions, with both workstations positioned such that the operator is seated approximately 2 m distant from the patient while scanning. However, smaller suites where the operator is required to be sited closer to the scanner couch may result in potentially higher doses depending upon the precise layout. Routine scanning of the left hip may result in an approximately three-fold higher integrated radiation dose to the operator compared with that for the right hip when combined with a scan of the lumbar spine [3] owing to the reduced attenuation offered by the patient and the shorter distance from the operator. Other clinical units may wish to demonstrate compliance by the same simple technique, thereby reducing any concern felt by staff in this matter and validating directly any requirement for extra shielding, where this is provided by an often obstructive lead glass screen.
Yours etc.,
1Institute of Nuclear Medicine and 2Department of Medical Physics and Bioengineering, University College London Hospitals NHS Trust, Middlesex Hospital, Mortimer Street, London W1T 3AA, UK
Received for publication August 14, 2001. Revision received October 1, 2001. Accepted for publication October 9, 2001.
References
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  M BOYANOV Estimation of lumbar spine bone mineral density by dual-energy X-ray absorptiometry: standard anteroposterior scans vs sub-regional analyses of whole-body scans Br. J. Radiol., August 1, 2008; 81(968): 637 - 642. [Abstract] [Full Text] [PDF]
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