The Effect of 99mTc on Dual-Energy X-Ray Absorptiometry Measurement of Body Composition and Bone Mineral Density

Whether the gamma-emission by radioisotopes influences the outcome of dual-energy X-ray absorptiometry (DXA) measurements is not fully elucidated. The aim of this study was to evaluate the effect of antecedent administration of Tc-99m on DXA measurements regarding body composition and bone mineral density (BMD) using a K-edge filter scanner. The phantom measurements were performed by placing a urinary bladder phantom containing 40 mL of radioisotope solution on the pelvic region of a whole-body phantom. Twenty-seven patients attending our department for a routine examination involving the administration of a tracer marked with Tc-99m were included. The patients underwent a whole-body DXA scan before and within 2 h after tracer injection using a GE/Lunar Prodigy scanner. Control scans were performed on 40 volunteers, who had not received any radioactive tracer. In both phantom and patient measurements, we found a significant dose-related decrease in fat mass and BMD and a corresponding increase in fat-free mass (p < 0.001). Based on the linear regression analysis, we suggest upper dose limits for the measurement of BMD at 0.77 mu Sv/h and body composition at 0.21 mu Sv/h (dose rate measured at a distance of 1 m from the patient). Caution should be taken when interpreting the results of DXA scans performed in close temporal proximity to procedures involving the administration of Tc-99m.