False-negative bone imaging due to etidronate disodium therapy.

A 77-year-old man with prostate cancer was serially evaluated for bone metastases using Tc-99m methylene disphosphonate (Tc-99m MDP) both on and off treatment with etidronate disodium (EHDP). While the patient was receiving the medication only minimal bony uptake of the tracer was seen with the majority remaining in the soft tissues. The similarly structured EHDP probably saturated the binding sites that the radioactive MDP usually adheres to. Physicians should be aware of this interaction and may have to wait until the EHDP has been discontinued for several months before performing bone imaging on these patients.