Transforming ATTR cardiac amyloidosis into a chronic disease: The enormous potential of quantitative SPECT to improve diagnosis, prognosis, and monitoring of disease progression

Single-photon emission computed tomography/computed tomography quantification of Tc-99m pyrophosphate uptake to assess tafamidis treatment response in transthyretin cardiac amyloidosis

BACKGROUND

Cardiac imaging with bone-avid tracers for the diagnosis of transthyretin amyloid (ATTR) cardiac amyloidosis uses only limited quantification, but single-photon emission computed tomography/computed tomography (SPECT/CT) acquisition can provide volumetric assessment with quantification of tracer uptake. Tafamidis is routinely used in the treatment of cardiac amyloidosis, but there are scant data on changes in imaging results during therapy. The purpose of this study was to perform a longitudinal assessment of Tc-99m-pyrophosphate (PYP) imaging to determine if tafamidis therapy results in any change in quantitative measures of tracer uptake.

METHODS

The study incorporated a prospective, single-center study of ATTR patients being treated with tafamidis using Tc-99m-PYP SPECT/CT to quantify cardiac tracer uptake in the whole heart and left ventricle. Standardized uptake values (SUVs) were adjusted for blood pool activity. Comparison of baseline activity was made to values obtained approximately every 6 months during treatment.

RESULTS

Twenty-two patients (77.0±7.5 years old, 86.4% male) were on tafamidis for 15.3±4.0 months, with an average time between baseline and final follow-up study of 16.8±4.7 months. Thirteen patients (59.1%) had multiple follow-up amyloid studies. Statistically significant reductions in total SUVs, SUV volume, and percentage of injected dose were seen. Adjusted for the maximal aortic SUV, the total SUV in the left ventricle decreased by 36.9%, the SUV volume by 38.7%, and the percentage of injected dose decreased by 34.9% (all P values≤0.0001). Over the study duration, there was a decrease of 7.7%/month in the measured metrics.

CONCLUSION

The quantitative SUV measurements from Tc-99m-PYP SPECT/CT revealed an overall decrease in scintographic amyloid burden during the course of tafamidis therapy, but additional work is needed to determine the optimal metrics and improve the reproducibility of the quantification.

Performance of 99mTc-PYP scintigraphy in the diagnosis of hereditary transthyretin cardiac amyloidosis

OBJECTIVE

Most reported research has primarily investigated wild-type transthyretin cardiac amyloidosis (ATTRwt-CA). However, the application of bone scintigraphy for hereditary transthyretin cardiac amyloidosis (ATTRv-CA) has not been systematically investigated. Therefore, in this study, we aimed to evaluate the diagnostic value of 99mTc-PYP scintigraphy in ATTRv-CA.

METHODS

Fifty-four patients were enrolled in a highly suspected cardiac amyloidosis cohort. Transthyretin (TTR) gene characteristics were summarized in the ATTRv-CA group. In 99mTc-PYP scintigraphy, the diagnostic efficiency of the visual score (VGS) and heart-to-contralateral chest (H/CL) ratio were evaluated. Furthermore, the interobserver consistency among the diagnosticians was investigated.

RESULTS

Twenty-eight patients were diagnosed with ATTRv-CA with eight genotypes. The Ala97Ser genotype accounts for 46% (n = 13) with a mean age of disease onset, definite diagnosis, and interval of 61.6 ± 1.9, 66.5 ± 1.3, and 4.0 (3.0, 6.2) years, respectively. Their VGS is Grade 3, and their H/CL ratio is higher than that of the non-Ala97Ser group, but no statistical significance exists (mean H/CL: 1.95 ± 0.06 vs. 1.87 ± 0.02, p = 0.844). Additionally, ATTRv-CA patients showed VGS ≥ 2, and mean H/CL ratio of 2.09 ± 0.06. The sensitivity and specificity of VGS were 100% and 65%, respectively. And the interobserver consistency analysis of VGS showed the intraclass correlation coefficient is 0.522. The best cutoff value of H/CL ratio was 1.51 (AUC = 0.996), and the diagnostic consistency of H/CL (bias: 0.018) was high.

CONCLUSIONS

Ala97Ser is the most common genotype in ATTRv-CA in our cohort, with characteristics of later onset and rapid progression, but delayed diagnosis and extensive 99mTc-PYP uptake. Overall, ATTRv-CA patients showed moderate-to-extensive myocardial 99mTc-PYP uptake. Additionally, VGS carries subjectivity, low specialty and interobserver consistency. But H/CL exhibit high diagnostic efficacy and interobserver consistency. The H/CL ratio is more useful than VGS.