Achilles Tendon Xanthoma Inflammation Revealed by FDG-PET/CT in Familial Hypercholesterolemia: Insights From a 2-Generation Case Series.

Achilles tendon xanthomas are hallmark lesions of familial hypercholesterolemia (FH), but their metabolic activity remains poorly characterized. We report a 2-generation case series illustrating phenotypic heterogeneity in tendon xanthomas despite shared FH-associated variants. In case 1, a 35-year-old man with bilateral Achilles tendon thickening was diagnosed with FH. After 12 months of lipid-lowering therapy, intense 18F-fluorodeoxyglucose (FDG) uptake on positron emission tomography/computed tomography (PET/CT) indicated metabolically active inflammation. In case 2, the man's 71-year-old father, who had been receiving long-term statins (>10 years), ezetimibe (>1 year), and recent proprotein convertase subtilisin/kexin type 9 inhibitor (1 month), had Achilles tendon thickening but no FDG uptake despite age and coronary artery disease history. These findings show that xanthomas exhibit persistent metabolic activity despite short-term lipid lowering, whereas prolonged therapy may attenuate inflammation. Phenotypic differences underscore roles of disease stage, cumulative lipid exposure, and treatment duration. FDG-PET/CT provides metabolic information for risk assessment and therapeutic monitoring.