52 Chapter 3 Figure 2. Heterogeneity of tracer-uptake Representative images of [18F]FDG PET/CT (left) and [89Zr]Zr-DFO-girentuximab PET/CT (right) are shown of two patients. In patient 1, lymph node metastases as visualized on axial sections of [18F]FDG PET/CT (A) and [89Zr]Zr-DFO-girentuximab PET/CT (B). In patient 2, Lung lesions as visualized on transverse sections of [18F]FDG PET/CT (C) and [89Zr]Zr-DFO-girentuximab PET/CT (D). Follow-up of patients during and after WW After a median follow-up of 48.0 months (95%CI 44.8-56.5), 36 patients (90%) required systemic treatment. As of December 2021, four patients are still on WW (Figure 1C). Two patients were censored prematurely during WW due to drop-out or surgical resection of all lesions without disease progression. Four other patients underwent local therapy during WW but continued their WW period. This included two patients who underwent stereotactic radiotherapy for a newly detected asymptomatic solitary brain metastasis at baseline. One patient underwent radiotherapy for a growing pancreatic lesion. One other patient underwent radiotherapy for a symptomatic bone lesion. There was no clinical indication for systemic treatment in these patients In total 36 patients (90%) had RECIST PD; three patients had clinical disease progression without RECIST confirmation. The median time to RECIST PD was 7.8 months (95%CI 6.1-12.1); the overall median WW-period was 16.1 months (95%CI: 9.0-31.7) (Figure 3A) including 4 patients with rapid progression and 10 with indolent disease. The median OS was 54.8 months (95%CI 34.2-NA) (Figure 3B). RECIST PD involved growth of known metastases (89%) and/or development of new lesions (31%), with four patients showing only new lesions (11%). In 11 (31%) patients, RECIST PD was the trigger to immediately start systemic treatment, while 24 other patients continued surveillance for an additional median time of 11.1 months (95%CI 5.7-31.9).