Cross-calibration, precision and patient dose measurements in preparation for clinical trials using dual energy X-ray absorptiometry of the lumbar spine

Large, multicentre clinical trials using DXA to monitor bone density follow ing intervention are now common. At the same time, several different bone d ensitometers and calibration phantoms are currently in use. The aim of this study was to document the technical information required on cross-calibrat ion of equipment, reproducibility and patient dose before commencing a mult icentre clinical trial. To this end, we obtained an in vitro and in vivo cr oss-calibration of two machines (a Hologic QDR 2000 and a Lunar DPX-L) that were not significantly different. Interobserver and intraobserver precisio n, and radiation dose were also measured and three commonly used phantoms a ssessed for their usefulness in cross-calibration and quality assurance. Me asured in vitro precision on the two machines (0.3-0.7%) was better than th at specified by the manufacturers. In vivo precision was worse (1.4-2.1%), as might be expected in patients with reduced bone mass. Mean entrance skin radiation doses on each machine were 280 mu Sv for the QDR 2000 and 38 mu SV for the DPX-L. No one phantom is ideal, but the European Spine Phantom o r Lunar Aluminium Spine Phantom will provide an adequate cross-calibration for a clinical trial. This study demonstrates that an adequate cross-calibr ation can be obtained for use in groups of patients and that the equipment used is reproducible with a low radiation output.