Utility and limitations of 3,3-diphosphono-1,2-propanodicarboxylic acid scintigraphy in systemic amyloidosis

Aims Technetium-99m-labelled 3,3-diphosphono-1,2-propanodicarboxylic acid (Tc-99m-DPD) is a sensitive method for imaging cardiac transthyretin (ATTR) amyloid. We report utility and limitations of Tc-99m-DPD scintigraphy in 321 patients with suspected cardiac amyloidosis. Methods and results The cohort included wild-type ATTR (ATTR(wt)) amyloidosis in 94 (29%), ATTR-Val122Ile amyloidosis in 38 (12%), hereditary ATTR (ATTR(mt)) amyloidosis in 46 (14%), primary light-chain (AL) amyloidosis in 44 (14%), secondary (AA) amyloidosis in three (1%), other hereditary amyloidosis types in nine (3%), undetermined types in two (0.5%), and 85 (26.5%) patients in whom systemic amyloidosis was ultimately excluded. All 158 patients with ATTR amyloidosis with clinical cardiac involvement had cardiac Tc-99m-DPD uptake, with median Grade 2 intensity. Thirteen further ATTR amyloidosis patients without clinical evidence of cardiac involvement also demonstrated 99mTc-DPD cardiac uptake. Eighteen of 35 (51%) AL patients with cardiac involvement had Tc-99m-DPD cardiac uptake (median Grade 1 intensity). SPECT imaging indicates that the apparent reciprocal reduction in bone uptake is due to masking of bone uptake by extensive soft-tissue uptake in ATTR amyloidosis, especially ATTR(wt), and ATTR-Val122Ile types. Conclusion Tc-99m-DPD scintigraphy is a highly sensitive technique for imaging cardiac ATTR amyloidosis and is an important investigation in the diagnostic pathway of patients with cardiac amyloidosis. It is not specific for ATTR in isolation but must be interpreted in a broad clinical context to avoid dangerous diagnostic errors. Diffuse skeletal muscle uptake identifies muscle as a hitherto unrecognized site that merits investigation as a target organ in ATTR amyloidosis.