Caren van Roekel

52 Chapter 2 baseline, measurement of the ‘hottest’ single tumor and background area is required. The background data are needed to establish the right threshold for the standardized uptake value (SUV) of a lesion at baseline. The SUV is corrected for lean body mass and abbreviated as ‘SUL’. A single target lesion is chosen at each time point during follow-up, because the most metabolically active tumor focus corresponds to the most aggressive portion of the tumor. The tumor portion with the highest average SUL value, along with the activity in the surrounding 1 cm 3 , is recorded as SULpeak. PERCIST 1.1. categories for response are a bit more elaborate than the RECIST response categories. Complete response is the complete resolution of FDG uptake of the target lesion, with the uptake of all other lesions returning to background values as well. Partial metabolic response is a decrease of ≥30% and of ≥0.8 SUL units between the most intense lesion at follow-up and the most intense lesion at baseline. This is not necessarily the same lesion. Furthermore, there should be no new lesions compared to baseline and no increase of ≥30% in size of a non- target lesion. Stable metabolic disease is defined as an increase or decrease in SULpeak of <30%. Progressive disease, finally, is characterized by an increase ≥30% and of ≥0.8 SUL units in a target lesion, or by the development of a new lesion (106). 7.3.4. Comparison of RECIST and PERCIST criteria In a recent review, Min et al. have analyzed six studies that compared RECIST and PERCIST response criteria. Overall, there was moderate agreement of tumor response between the two criteria (linear weighted kappa=0.59, 95%CI = 0.52- 0.66) and in almost 38% there was discordance in response category. Overall response rates were 35.1% by the RECIST and 54.1% by the PERCIST criteria. Thus, the PERCIST criteria significantly increased the overall tumor response rate. An example of a difference in response according to the RECIST and PERCIST criteria after radioembolization is depicted in Figure 9a-d. This figure shows images of an 80-year-old woman with metastases of an intrahepatic cholangiocarcinoma who was treated with radioembolization. According to the RECIST 1.1 criteria, the response to treatment would be characterized as SD (stable disease). According to the mRECIST and PERCIST criteria, however, the response to treatment would be characterized as CR (complete response) and CMR (complete metabolic response). Since functional imaging can detect metabolic changes when there are no morphological changes yet, response