Clinical Significance of CD90(+) Circulating Tumor Cells as Dynamic Biomarkers in Unresectable Hepatocellular Carcinoma Treated with Atezolizumab/Bevacizumab and Lenvatinib

<i>Background:</i> Atezolizumab plus bevacizumab and lenvatinib are standard treatments for advanced hepatocellular carcinoma, but conventional tumor markers such as alpha-fetoprotein and des-gamma-carboxy prothrombin have a limited ability to reflect treatment responses. Circulating tumor cells with cancer stem cell characteristics have emerged as promising biomarkers. We examined the dynamics of cancer stem cell-related circulating tumor cell subsets and tumor markers at early and maximal response phases in patients with unresectable hepatocellular carcinoma undergoing systemic therapy. <i>Methods:</i> Sixty-two patients treated with either atezolizumab plus bevacizumab or lenvatinib were retrospectively analyzed. Peripheral blood was collected at baseline, during the early phase (during one to three months), and at maximal response. Circulating tumor cell subsets expressing cancer stem cell markers (CD90, epithelial cell adhesion molecule; CD133, vimentin) were assessed using multiparametric flow cytometry and compared with alpha-fetoprotein and des-gamma-carboxy prothrombin. <i>Results:</i> Early decreases in CD90-positive circulating tumor cells after therapy were associated with tumor shrinkage, longer periods of progression-free survival in both groups, and prolonged overall survival in the atezolizumab plus bevacizumab group. By contrast, early changes in alpha-fetoprotein and des-gamma-carboxy prothrombin were not consistently related to tumor size, progression-free survival, or overall survival. At maximal response, changes in CD90-positive circulating tumor cells reflected tumor burden more accurately than alpha-fetoprotein or des-gamma-carboxy prothrombin. <i>Conclusions:</i> These findings indicate that cancer stem cell-related circulating tumor cell subsets, particularly CD90-positive cells, may serve as valuable biomarkers for monitoring treatment response and predicting prognosis in unresectable hepatocellular carcinoma. CD90-positive circulating tumor cells perform dynamic monitoring superior to conventional markers such as alpha-fetoprotein and des-gamma-carboxy prothrombin.