99m Technetium-Hydroxy-Diphosphate Tracer (99mTcHDP) Bone Scintigraphy: An Easily Accessible, Rapid, Non-Invasive Diagnostic Tool for Cardiac Amyloidosis

Aim:

Cardiac amyloidosis is a protein deposition disease that can be difficult to diagnose and has a poor prognosis if diagnosis or treatment are delayed. The two major subtypes are AL and transthyretin (ATTR). Both have vastly different treatments so confirming the correct amyloid subtype is crucial.

A tissue biopsy is usually required for the diagnosis of amyloidosis and to distinguish between the subtypes. No blood test, echocardiography or cardiac MRI can reliably distinguish between AL and ATTR. With a cardiac biopsy, distinguishing AL from ATTR can be challenging with immunohistochemistry, and time consuming with mass spectrometry.

Recently, Gillmore et al demonstrated bone scintigraphy with 99mTc-DPD tracer can reliably diagnose ATTR, avoiding endomyocardial biopsies to confirm the subtype in most cases. ^[1] 99mTc-HDP is a tracer similar to 99mTc-DPD and is more readily available in Australian and the USA.

We sought to examine the use of 99mTcHDP bone scintigraphy in Australia and determine the accuracy of this tracer to diagnose cardiac amyloidosis and distinguish between the AL and ATTR subtypes.

Methods:

All patients with confirmed ATTR or AL who had 99mTcHDP bone scintigraphy were analysed. Results were correlated with histology, NT ProBNP, Troponin-T, free light chains, cardiac MRI and echocardiography. Grading was conducted with Perugini scoring.^[1]

Results:

25 patients with amyloidosis diagnosed by cardiac MRI and/or biopsy, had 99mTcHDP bone scintigraphy. 18 were confirmed ATTR, 7 AL. Two ATTR patients had hereditary disease (Thy60Ala and Gly109Lys), the remainder were wildtype.

17 (94%) patients with ATTR and 2 (29%) AL had positive scans. The negative ATTR patient had localized bladder disease only with a normal echocardiogram and cardiac biomarkers. All ATTR patients with positive scans had Perugini scores of 2 or 3, including the 2 patients with hereditary mutations, while the two positive AL only had scores of 1. All 11 patients with amyloid features on cardiac MRI had positive scans.

Mean NTproBNP values for ATTR and AL were 530pmol/L and 1396pmol/L respectively. The two AL amyloidosis patients with positive bone scans had higher NTproBNP values. No ATTR patient had a detectable plasma cell dyscrasia.

Conclusion:

Bone scintigraphy with 99mTcHDP tracer is an easily accessible, rapid and non-invasive method of diagnosing cardiac amyloidosis. In our small series, all patients with Perugini scores 2 or 3 had ATTR, while those with negative or Perugini score 1 scan either had AL or no cardiac amyloidosis. There was a suggestion that AL patients with higher NTproBNP scores were more likely to have positive scans.

This suggests that the 99mTcHDP tracer can be used like 99mTc-DPD to aid in the diagnosis of cardiac amyloidosis, and, as suggested by Gillmore et al, can confirm the ATTR subtype in those with no detectable plasma cell dyscrasia and Perugini score 2 or 3 scans, thus hastening accurate diagnosis and avoiding cardiac biopsies.

Reference:

1. Gillmore JD, Maurer MS, Falk RH, Merlini M, Damy T, Dispenzieri A, et al. Non-biopsy diagnosis of cardiac transthyretin amyloidosis. Circulation. 2016 Jun 14;133(24):2404-12