[18F]Florbetapir Positron Emission Tomography: Identification of Muscle Amyloid in Inclusion Body Myositis and Differentiation from Polymyositis

Objectives
With the tools available currently, confirming the diagnosis of inclusion body myositis can be difficult. Many patients are initially misdiagnosed with polymyositis. In this observational study at a UK adult neuromuscular centre we investigated whether amyloid positron emission tomography could differentiate between inclusion body myositis and polymyositis.

Methods
Ten patients with inclusion body myositis and six with polymyositis underwent clinical review, [18F]florbetapir positron emission tomography and magnetic resonance imaging of skeletal musculature. Differences in [18F]florbetapir standardised uptake value ratios in skeletal muscle regions of interest were evaluated. Relationships between [18F]florbetapir standardised uptake value ratios and measures of disease severity (clinical and by magnetic resonance imaging of skeletal muscle) were assessed.

Results
[18F]florbetapir standardised uptake value ratios were significantly higher in those with inclusion body myositis compared to polymyositis for all assessed regions (total-[18F]florbetapir standardised uptake value ratio 1.45 [1.28-2.05] versus 1.01 [0.80-1.22], p=0.005). For total-[18F]florbetapir standardised uptake value ratios ≥1.28, sensitivity and specificity for inclusion body myositis was 80% and 100% respectively.

Conclusions
[18F]florbetapir amyloid positron emission tomography differentiates IBM from PM. Successful development could facilitate accurate diagnosis, inclusion in clinical trials and help avoid unnecessary exposure to potentially harmful treatments.