Abstract
241747
Introduction: Idiopathic inflammatory myopathies (IIMs) are systemic autoimmune disorders typically characterized by muscle inflammation, often accompanied by extramuscular manifestations. Common presenting features include muscle weakness, fatigue and myalgia, with or without extramuscular symptoms. Although muscle biopsy remains the gold standard, its invasive nature has prompted the exploration of imaging techniques as an alternative tool for assessing muscle pathology.
Magnetic resonance imaging (MRI) is considered the reference radiological modality for IIM, aimed at visualizing edema and fatty replacement. However, its use is constrained by a limited imaging field-of-view and difficulty in distinguishing between myopathic etiologies. Recently, 18F-fluorodeoxyglucose positron emission tomography with computed tomography (FDG-PET/CT) has demonstrated the ability to detect myositis, offering the additional benefit of rapid assessment of whole-body muscular involvement and detection of extramuscular manifestations. However, there is limited data on the current use of FDG-PET/CT in the work-up of patients with suspected IIM and its correlation with MRI, histopathology, and other clinical features.
Our study aimed to assess the current use of FDG-PET/CT in evaluating patients with clinical suspicion of IIM, focusing on comparing muscle histopathology with FDG-PET/CT, MRI, and serological markers.
Methods: Patients with suspected IIM who underwent both FDG-PET/CT and MRI within a 28-day period between January 2018 and November 2023 were retrospectively reviewed. FDG-PET/CT and MRI images were each viewed by two blinded radiologists or nuclear medicine specialists of at least 4 years’ experience, to confirm the clinical relevance of imaging findings, and to assess significance of ancillary findings on FDG-PET/CT. FDG-PET/CT muscle abnormalities were based on qualitative and semi-quantitative assessment. Patient demographics, serological markers, and imaging findings were analyzed and correlated with the muscle histopathological diagnosis.
Results: 17 patients met inclusion criteria, which included clinical suspicion of IIM, and received both MRI and FDG-PET/CT (mean age 67.8 ± 9.5 years, 47% female), 2 were lost to follow-up and did not have a final clinical diagnosis. 12 patients underwent a muscle biopsy with 6 of 12 (50%) positive for IIM, whilst the remainder demonstrated either normal muscle histology or alternative etiology (Table 1). Biopsy positivity was not correlated with statistically significant differences in age, gender, timing of muscle biopsy relative to imaging, FDG-PET/CT or MRI positivity, immunosuppressant use, or hematological and biochemical markers (p-value and α-value > 0.05). FDG-PET/CT positivity for myositis correlated with higher mean serum creatinine kinase (773 vs 171 U/L, Z = -2.2, p < 0.05) and lower lymphocyte count (1.1 vs 2.0 109 cells/L, Z = -2.03, p < 0.05), while MRI positivity did not.
7 of 15 patients had concordant imaging results (positive or negative for myositis) on both FDG-PET/CT and MRI (Table 2). However, no significant differences between the concordant and discordant groups. Of the 8 patients with discordant findings, only 3 had a final clinical diagnosis of IIM of which 2 were identified on MRI but not FDG-PET/CT. MRI identified 4 false positives patients, compared to 1 false positive by FDG-PET/CT. Additional ancillary findings were identified on all FDG-PET/CT studies, including inflammatory arthritis, pulmonary abnormalities or lymph node abnormalities.
Conclusions: In a diverse patient group with suspected IIM, both MRI and FDG-PET/CT proved useful for identifying IIM and guiding the selection of appropriate muscle biopsy sites. Although not statistically significant, MRI tended to yield more false positives for myositis compared to FDG-PET/CT, suggesting increased specificity of the latter for IIM. Additionally, FDG-PET/CT was valuable in detecting ancillary findings not identified within the MRI’s field-of-view.
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