mRECIST

mRECIST, or modified RECIST for hepatocellular carcinoma, is a set of imaging response criteria developed to evaluate treatment effects in hepatocellular carcinoma (HCC). Unlike conventional RECIST, mRECIST assesses only viable tumor tissue, defined as areas showing arterial-phase enhancement on dynamic contrast-enhanced CT or MRI.

The sum of the diameters of viable (enhancing) portions of target lesions constitutes the measurable parameter.

Lesion selection and measurements: up to five target lesions are selected, with a maximum of two per organ. For each lesion, the longest diameter of the enhancing (viable) portion is measured, and the sums across all target lesions are used for response assessment.

Response categories: complete response means disappearance of any arterial enhancement in all target lesions. Partial response requires a reduction of at least 30% in the sum of the diameters of viable lesions. Stable disease denotes neither sufficient shrinkage to qualify for partial response nor sufficient increase to qualify for progressive disease. Progressive disease is defined as at least a 20% increase in the sum of the viable diameters, or the appearance of new enhancing lesions.

Imaging and timing: assessment relies on dynamic CT or MRI with arterial-phase imaging; baseline imaging is required, followed by regular scans at defined intervals (commonly every 4–8 weeks after therapy) to monitor response.

Applications: mRECIST is used primarily in HCC to evaluate responses after locoregional therapies such as transarterial chemoembolization (TACE), radioembolization, ablation, or tumor-directed therapies, as well as in systemic therapy trials. It is endorsed by major guidelines as an HCC-specific response metric.

Limitations: results depend on imaging quality and protocol, and may be confounded by necrosis without reduced enhancement; interobserver variability can occur; not applicable to non-enhancing tumors or non-HCC contexts.