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Background/Aims: Several tumor factors seem to be related to the hepatocellular carcinoma (HCC) prognosis, but they are not definitive, except for major vascular invasion. We analyzed the impact of serosal invasion, which is histologically classified into four groups according to the definition of the Liver Cancer Study Group of Japan - S0: no tumor invasion of serosa, S1: tumor invasion of serosa, S2: tumor invasion of adjacent organs, S3: tumor rupture with intraperitoneal bleeding. Methods: This retrospective study included 421 consecutive patients who underwent hepatectomy as an initial treatment for HCC between September 2002 and December 2014. Results: Kaplan-Meier analysis and log-rank tests revealed that both recurrence-free survival (RFS) and disease-specific survival (DSS) worsened as the serosal invasion status advanced. Multivariate analyses showed that S2/S3 was independently correlated with RFS (hazard ratio [HR] 3.52; p < 0.001) and DSS (HR 2.58; p = 0.039). Another multivariate analysis showed that serosal invasion (S1-S3) was independently correlated with RFS (HR 1.70; p = 0.015) but not with DSS (HR 1.38; p = 0.27). Conclusion: Invasion to adjacent organs and tumor rupture were crucial prognostic factors for both DSS and RFS. Serosal invasion was an independent prognostic factor only for RFS.