The modified RECIST (mRECIST) assessment is a revised method for evaluating tumor response and progression in hepatocellular carcinoma (HCC). It was developed to address limitations of the original RECIST criteria, which primarily focused on tumor shrinkage and were not suitable for molecular-targeted therapies or locoregional treatments in HCC. The mRECIST incorporates the concept of viable tumor, defined as tumor tissue that shows contrast uptake in the arterial phase of contrast-enhanced imaging. This approach better reflects the actual tumor response and is more accurate in assessing the effectiveness of treatments in HCC. The mRECIST guidelines were developed by a panel of experts convened by the American Association for the Study of Liver Diseases (AASLD) and adapted from the Journal of the National Cancer Institute (JNCI) guidelines. These guidelines provide a common framework for designing clinical trials in HCC and include specific criteria for assessing tumor response and progression. Key aspects of the mRECIST include the use of contrast-enhanced imaging to evaluate viable tumor tissue, the selection of target lesions based on their size and suitability for accurate measurement, and the definition of response and progression based on changes in viable tumor diameter. The mRECIST assessment emphasizes the importance of standardized imaging techniques and independent blinded assessments to ensure consistency and reliability in evaluating tumor response. It also addresses the challenges of assessing response in patients with cirrhosis and other complex cases. The guidelines recommend using a centralized radiologic review for image interpretation and emphasize the need for pathologic correlation and validation of the mRECIST criteria in future studies. The mRECIST is expected to provide a more accurate and reliable method for assessing tumor response in HCC clinical trials, particularly for molecular-targeted therapies. Further research is needed to confirm the accuracy of the mRECIST assessment compared to conventional methods such as pathologic studies of explanted livers.The modified RECIST (mRECIST) assessment is a revised method for evaluating tumor response and progression in hepatocellular carcinoma (HCC). It was developed to address limitations of the original RECIST criteria, which primarily focused on tumor shrinkage and were not suitable for molecular-targeted therapies or locoregional treatments in HCC. The mRECIST incorporates the concept of viable tumor, defined as tumor tissue that shows contrast uptake in the arterial phase of contrast-enhanced imaging. This approach better reflects the actual tumor response and is more accurate in assessing the effectiveness of treatments in HCC. The mRECIST guidelines were developed by a panel of experts convened by the American Association for the Study of Liver Diseases (AASLD) and adapted from the Journal of the National Cancer Institute (JNCI) guidelines. These guidelines provide a common framework for designing clinical trials in HCC and include specific criteria for assessing tumor response and progression. Key aspects of the mRECIST include the use of contrast-enhanced imaging to evaluate viable tumor tissue, the selection of target lesions based on their size and suitability for accurate measurement, and the definition of response and progression based on changes in viable tumor diameter. The mRECIST assessment emphasizes the importance of standardized imaging techniques and independent blinded assessments to ensure consistency and reliability in evaluating tumor response. It also addresses the challenges of assessing response in patients with cirrhosis and other complex cases. The guidelines recommend using a centralized radiologic review for image interpretation and emphasize the need for pathologic correlation and validation of the mRECIST criteria in future studies. The mRECIST is expected to provide a more accurate and reliable method for assessing tumor response in HCC clinical trials, particularly for molecular-targeted therapies. Further research is needed to confirm the accuracy of the mRECIST assessment compared to conventional methods such as pathologic studies of explanted livers.