Extraskeletal localization of 99mTc-labeled bone-seeking tracers in bone scintigraphy

In this article clinical examples of extraskeletal localization of technetium-99m (99mTc)-labeled diphosphonate are described. In several of these cases this finding was relevant for clinical decision-making. 99mTc-labeled diphosphonates have been used for skeletal imaging since 1971, and from 1973 on many reports of extraskeletal localizations of these agents have been published. This visualization can be due to (1) an abnormality of the excretory route of 99mTc-labeled diphosphonate or (2) increased uptake in pathologically altered tissues and organs. The latter has been recorded in a multitude of disease processes, such as ischaemic injury, inflammatory processes, benign or malignant neoplasms, non-skeletal metastases, malignant pleuritis, scars, radiation injury and amyloidosis. Although the precise mechanism of extraskeletal uptake of 99mTc-labeled diphosphonate remains uncertain, binding of this radiopharmaceutical to calcium-containing compounds is thought to play a critical role.