COMMISSIONING AND QUALITY ASSURANCE PROTOCOL FOR DUAL ENERGY X-RAY ABSORPTIOMETRY (DEXA) SYSTEMS N. F. Sheahan, A. Dowling  , G. O’Reilly and J. F. Malone Department of Medical Physics and Bioengineering, St James’s Hospital, Dublin 8, UK This study reports on the development and evaluation of a protocol for testing DEXA systems, which can be incorporated into a routine medical physics/engineering service. Methodologies are reported for (1) scatter measurement, (2) estimation of reference dose and (3) enquiry into potential sources of overexposure. Results show that fan-beam and cone-beam systems require shielding if the walls or operator are within 1 m of the table. Patient reference dose was estimated using the dose–area product (DAP). This varied over an order of magnitude from 2 to 36 mGy cm 2 in the range of systems studied. The inquiry into potential sources of overexposure revealed some weaknesses in current practise including a design which prevents the inclusion of DAP metres; beam non-uniformity; incorrect specification of patient dose and the risk of overexposure when tube operates during patient positioning. INTRODUCTION Dual-energy X-ray absorptiometry (DEXA) is used to measure bone mineral density (BMD) (1) . The technique involves digital imaging to locate the skeletal regions of interest, followed by estimation of X-ray attenuation in these regions. Comparison of attenuation at high-energy and low-energy regions of the X-ray spectra yields an estimate of the BMD (2) . DEXA systems are particularly important in screening for, and management of, osteoporosis. The original DEXA systems had very low dose of radiation, and excited little interest from the medical physics/engineering profession. However, the dose from the ‘second-generation’ of DEXA systems is much higher (3–5) . In addition, the EU Medical Exposures Directive requires particular attention to be paid to radiation protection for screening systems. For these reasons, the medical physics/ engineering profession needs to become more closely involved in the management of DEXA systems, and this study reports on the development of a protocol for testing DEXA systems. This provides an import- ant step towards defining an internationally agreed standard for DEXA commissioning and quality control (QC). Commissioning and QC tests for DEXA systems must support the principal tenets of radiation protection:  Justification. DEXA justification requires an assessment of the information quality produced by DEXA systems as well as the patient dose. There is a large volume of ongoing research to determine the quality of BMD measurements from DEXA systems, but little effort has been paid to the measurement of patient dose.  Optimisation. DEXA optimisation requires min- imisation of unnecessary radiation exposure. An important tool for optimisation is the establish- ment of reference dose records.  Dose limitation. This requires an assessment of scatter from DEXA systems, so that it can be determined whether or not dose limits to the operator or general public might be breached. The aims of the study were to develop and assess a protocol for DEXA commissioning and QC. The particular requirements for the protocol are to estab- lish methodologies for the following:  estimation of DEXA reference dose;  enquiry into potential sources of overexposure;  estimation of scatter in the environment of the DEXA scanner. This protocol aims to be suitable for a routine medical physics/engineering service. MATERIALS AND METHODS Scatter dose estimation A 1800 cm 3 ionisation chamber which is certified to 2 nGy min 1 was used. This detector is suited to the measurement of very low dose rates which occur especially for pencil-beam scanners. A volume of 20 cm of water was used to simulate the patient. Since the scanned volume can be quite large, e.g. 5000 cm 3 , the inverse square law does not hold exactly, so three scatter estimates were made in the horizontal plane—at 30 cm, 1 m and 2 m from the centre of the water phantom. From these three meas- urements, an inverse square law correction was applied to estimate scatter to walls, doors and the operator, using the scatter estimate which most closely approximates to the distance in question, e.g. for distances >2 m we used the scatter estimate  Corresponding author: adowling@stjames.ie Radiation Protection Dosimetry (2005), 1 of 3 doi:10.1093/rpd/nci741 Ó The Author 2005. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org