Thin-cap fibroatheroma (TCFA), a lipid-rich plaque overlaid with a thin fibrous cap, has been considered a major vulnerable or high-risk atherosclerotic plaque associated with future events [ , ]. Optical coherence tomography using infrared light has about 20 μm resolution and can evaluate TCFA in vivo. The fibrous cap thickness to define TCFA has been considered a surrogate of stabilization for high-risk plaque and was used for the recent randomized studies using proprotein convertase subtilisin/kexin type 9 (PCSK9) inhibitors [ , ]. However, substantial variability of diagnosis for TCFA or fibrous cap thickness has been reported [ ]. Inter- or intra-observer variability of TCFA (Cohen kappa) was 0.23–0.66, 0.50–0.64, respectively. Inter- or intra-observer variability of fibrous cap thickness (intraclass correlation coefficient [ICC]) was 0.52–0.71, 0.70–0.79, respectively.