34 Chapter 2 The interpretation of involved organ sites in all three modalities was challenging, especially considering the limitation of each modality. For example, spatial resolution is lower with PET/CT compared to CT, resulting in a partial volume effect affecting small (<2 cm), low-contrast lesions both visually and quantitatively24. CT can detect sub-centimeter or indeterminate pulmonary nodules and lymph nodes, although distinguishing nonspecific from small metastatic lesions with CT is notoriously difficult. Based on studies of pulmonary metastases in RCC and RECIST 1.1 criteria, we used a diameter cut-off of 10 mm and in lymph nodes 15 mm to prevent overestimating of the number of detected lesions25,26. This ultimately reduced the number of (small) lung and lymph node lesions detected by CT, thereby underestimating the overall lesion detection by CT. All lesions visible on either CT, [89Zr]Zr-DFO-girentuximab or [18F]FDG-PET/CT were defined as metastases, which introduces potential bias and a risk of possible false-positive. Despite the high specificity of [89Zr]Zr-DFO-girentuximab to visualize primary and metastatic ccRCC-lesions expressing CAIX5,13-15,27 and the careful assessment of [89Zr]Zr-DFO-girentuximab-PET/CT by three independent nuclear physicians with a fairly good agreement (kappa 0.71;95%CI:0.60–0.82), our results are limited by the lack of histological confirmation of the detected lesions. Lesions not visible on CT but only [89Zr]Zr-DFO-girentuximab and [18F]FDG PET/CT could be false-positive lesions. Alternatively, the tracer-uptake may resemble a new tumor lesion that is not yet visible on CT due to a dedifferentiated state with a different metabolic state and could become apparent in a period of follow-up. Finally, false negative lesions could be present as well; however, with the available data we cannot draw any conclusions on this. Interestingly, [18F]FDG and [89Zr]Zr-DFO-girentuximab-uptake strongly depend on the organ where the lesion is localized, which is also previously described for [89Zr]Zr-bevacizumab uptake in mccRCC28. Overall, highest [18F]FDG and [89Zr]Zr-DFO-girentuximab SUV max values were visualized in metastases of the adrenal gland and the kidney. In the six patients without previous nephrectomy, the highest SUVmax value was measured in the metastatic lesions and not in the primary tumor. Organ-specific characteristics influence [89Zr]Zr-DFO-girentuximab-uptake, e.g., presence of stromal and immune cells, stroma and/or vasculature affecting perfusion. This is illustrated by the notably high SUVmax values of [ 89Zr]Zr-DFO-girentuximab uptake in adrenal gland lesions as compared to other lesion sites, e.g., lung (median SUVmax 69.9 and 9.4, respectively). Depending on the clinical question, both [89Zr]Zr-DFO-girentuximab and [18F]FDG-PET/CT are valuable as additional imaging techniques by visualizing whole-body mccRCC lesions where the combination of [89Zr]Zr-DFO-girentuximab with CT increases the total number of detected lesions most and supports the role of [89Zr]Zr-DFO-girentuximab-PET/CT in the early detection of mccRCC lesions27. Furthermore, the quantification of tracer-uptake in both PET-imaging modalities offers a better understanding of the heterogenic study population4. Combining anatomical imaging techniques with functional imaging techniques targeting glucose metabolism and CAIX expression offers a better representation of the heterogeneity by visualizing whole body tumor nature and active metabolic processes (e.g., glycolysis, GLUT-1-expression)8.