Caren van Roekel

51 Radioembolization The sum of diameters should be recorded and this parameter is used as a reference. At follow-up evaluation, the same target lesions must be evaluated. Complete response is defined as a disappearance of all target lesions. Partial response is classified as a ≥30% decrease in the sum of diameters of target lesions. Progressive disease is characterized by a ≥20% increase in the sum of diameters of target lesions. The sum must also demonstrate an absolute increase of ≥5 mm, and the appearance of a new lesion denotes progression as well. Stable disease, at last, is depicted by an increase in diameter of target lesions of <20% or a decrease of <30% (102). Limitations of the RECIST criteria are that the thresholds for response and progression are arbitrarily chosen (sometimes a small change in size can already have quite some influence on overall survival) and that there is a large inter- observer variability (103). Furthermore, necrosis (i.e. an effect that regularly occurs after loco-regional treatments) is included in the target lesion, leading to an overestimation of remaining tumor load (104). 7.3.2. mRECIST The modified RECIST (mRECIST) criteria have been developed for the evaluation of hepatocellular carcinoma (HCC). They take into account the tumor necrosis induced by treatment. Viable tumor shows an uptake of contrast agent in the arterial phase on contrast-enhanced CT or MRI. Just as the RECIST criteria, target lesions are defined at baseline. An HCC lesion should be ≥10 mm, measurable, well-defined, and should have intratumoral arterial enhancement on contrast-enhanced CT or MRI. Response categories are defined like the RECIST response categories with the alteration that only the viable, enhancing part of the lesions should be measured. Not all HCC lesions show arterial enhancement or are well-defined. For those lesions, conventional RECIST criteria must be used (103, 105). mRECIST could also be used for other hypervascular tumors, like neuro-endocrine neoplasms, however need to be validated in future studies. 7.3.3. PERCIST 1.0 Metabolic tumor response can be assessed using the Positron Emission Tomography (PET) Response Criteria in Solid Tumors (PERCIST 1.0) criteria. At 2