Serum Procalcitonin: A Novel Tumor Biomarker for Diagnosis and Follow-Up in Fibrolamellar Hepatocellular Carcinoma.

Introduction: Fibrolamellar carcinoma (FLC) is a rare primary liver cancer that predominantly affects young patients with normal known serum tumor biomarkers (alpha-fetoprotein (AFP) and CA19-9). An observation of a markedly elevated procalcitonin (PCT) in one patient prompted us to investigate the potential role of PCT as a biomarker in a larger cohort of FLC. Methods: We measured serum PCT levels in 34 samples from 18 patients with metastatic FLC and in 64 patients with hepatocellular carcinoma (HCC), 24 with cholangiocarcinoma (CCA), and 20 with cirrhosis. Using RNA sequencing, we analyzed CALCA expression, the gene encoding PCT, in 27 FLC tumors, 331 HCC tumors, 39 CCA tumors, 71 hepatoblastomas, 34 hepatocellular adenomas, and 55 non-tumor liver samples. Spatial transcriptomics was performed on three FLC and PCT immunohistochemistry was conducted on 13 FLC and 34 other primary or secondary liver cancers. Results: In 8 FLC from the European cohort, median serum PCT was significantly elevated (55.2 μg/l) compared to patients with HCC (0.14 μg/l), CCA (0.16 μg/l), and cirrhosis (0.11 μg/l; P=0.0005). These findings were independently validated in a U.S. cohort of 10 FLC patients compared to HCC and CCA (P=0.0002). Across these cohorts, elevated serum PCT was observed in 83% of FLC cases versus 3% of HCC and CCA cases (P<0.0001). In four patients with longitudinal measurements, changes in PCT levels correlated with radiologic response according to RECIST 1.1. RNA sequencing demonstrated significant overexpression of CALCA in FLC compared to other primary liver tumors (P<0.0001), and spatial transcriptomics localized CALCA expression specifically to tumor cells. Immunohistochemistry confirmed PCT overexpression in 77% of FLC but not in other liver cancers. Conclusion: Procalcitonin is a sensitive and specific biomarker for FLC at both the serum and tumor levels among primary liver cancers, with potential utility in diagnosis and monitoring of treatment response.