Extraosseous Tc-99m MDP uptake: a pathophysiologic approach

Scintigraphy with technetium-99m methylene diphosphonate (MDP) delineates a wide spectrum of nonosseous disorders. Neoplastic, hormonal, inflammatory, ischemic, traumatic, excretory, and artifactual entities demonstrate abnormal soft-tissue uptake of Tc-99m MDP. Mechanisms leading to increased extraosseous Tc-99m MDP uptake include extracellular fluid expansion, enhanced regional vascularity and permeability, and elevated tissue calcium concentration. The composition of the calcium deposition and the presence of other metallic ions (eg, iron and magnesium) are important. Soft-tissue Tc-99m MDP uptake is seen in benign (tumoral calcinosis, myositis ossificans) and malignant (sarcomas, adenocarcinomas, metastases) neoplastic entities. Hormonal disturbances in calcium metabolism, especially in hyperparathyroidism, can lead to metastatic calcification, visualized with Tc-99m MDP scintigraphy. Tissue damage from inflammation, infection, or physical trauma results in localized hyperemia, edema, or calcium (and hemosiderin) deposition based on their pathophysiologic characteristics. Urinary tract obstruction, anomalies, or dysfunction are demonstrated by Tc-99m MDP imaging. Common artifacts are related to faulty radiopharmaceutical preparation, Tc-99m MDP administration, and imaging technique. Recognition of these modes of extraskeletal Tc-99m MDP uptake can enhance the diagnostic value of bone scintigraphy.