Conversion surgery means surgery for initially unresectable tumors which are converted to resectable tumors by multidisciplinary treatment. Since the introduction of highly effective drug therapies including immune checkpoint inhibitors for hepatocellular carcinoma(HCC), conversion surgery for HCC has become a common experience. Recently, the Japanese Expert Consensus 2023 have reported on the oncological criteria of resectability for HCC, allowing us to use common terms to describe unresectable HCC, which had been ambiguously defined in the past. HCC cases are classified into 3 criteria, based on the tumor number and size, the degree of vascular invasion, and the degree of extrahepatic involvement. The 3 criteria are R(resectable
oncological status for which surgery alone may offer clearly better survival outcomes compared to the other treatment), BR1(borderline resectable 1
oncological status for which surgical intervention as a part of multidisciplinary treatment may offer survival benefit), and BR2(borderline resectable 2
initially unsuitable for resection: oncological status for which efficacy of surgery is indeterminate and surgical indication should be carefully determined under the standard multidisciplinary management of HCC). Whether conversion surgery improves prognosis under the assumption that nonsurgical treatment such as drug therapy has been successful is a question for further investigation.