Assessment of response to liver directed radiation-based therapies: Current guidelines, challenges, and future directions

Radiation-based local-regional therapies for hepatocellular carcinoma (HCC) have gained wide acceptance due to promising rates of tumor response, survival, and safety profiles. After treatment, it is important to assess tumor response to determine further management, patient prognosis, and endpoint outcomes for clinical trials. To standardize imaging interpretation and reporting of HCC response to local-regional treatment, a few imaging-based response assessment systems were developed. Two of them have emerged as the most used: the Liver Imaging Reporting and Data System (LI-RADS) Treatment Response Algorithm (LR-TRA) and the modified Response Evaluation Criteria in Solid Tumors (mRECIST). While these systems have been validated for the assessment of response to ablative locoregional therapies, assessment of response to radiation-based therapies can be challenged by persistent or evolving imaging features and is still an area of active research. Following the advances in technology and a better understanding of tumor biology that allowed for the increased application of radiation-based local-regional therapies for the treatment of HCC, research is still needed to address the limitations of current imaging criteria for assessing tumor response to these novel techniques. In this review, we describe radiation-based liver-directed treatment options, examine imaging criteria for assessing treatment response, discuss practical limitations and gaps in knowledge when applying these response criteria, and address future directions that may help to improve accuracy and outcomes when assessing response to radiation-based HCC treatment.