ABSORBED DOSE MEASUREMENTS IN DUAL-ENERGY X-RAY ABSORPTIOMETRY (DXA)

In this study a predominantly film dosimetric method was used to measu re the effective dose from posteroanterior (PA) lumbar spine and proxi mal femur scans performed on a Lunar DPX-L machine. Because of the ver y low dose rate in scanning mode, the depth dose data were determined using a stationary detector configuration. The characteristic curve fo r the film (Kodak TMAT-H) was obtained and depth dose measurements wer e made using slabs of ''solid water'' The film was calibrated using a superficial X-ray unit (calibrated against a standard traceable to a n ational standard). To assess the change in film response with beam har dening at depth, the film was exposed to calibration beams of differen t half value layer (HVL). The HVL of the DXA beam was determined for s urface and depth doses using aluminium filters and a diamond detector (an energy independent device). All measurements were performed three times. Beam size was measured using film, and the scan areas and times were determined by scanning phantoms. The dose from a scan was calcul ated using D-sc=DT(sc)A(b)/A(sc), where D=dose rate (stationary), T-sc = scan time, A(b)=beam area, and A(sc)=scan area. Organ doses were det ermined using an anatomical atlas and ICRP 23 female reference. All fi lm measurements had good precision (coefficient of variation <4%). The re was little variation in film sensitivity with change in HVL (<1% ch ange for the first three HVLs) and consequently no corrections were ap plied to the depth dose data. Skin entrance dose was 11.5 mu Gy. Effec tive dose in females was 0.19 mu Sv for the PA lumbar spine. For the p roximal femur scan, the effective dose was 0.14 mu Sv (ovaries include d) and 0.023 mu Sv (ovaries excluded) for pre-menopausal and post-meno pausal women, respectively.