*2.4. [18F]]FDG-PET Features*

Image analysis was performed using Philips Intellispace Portal software v10.1 (Philips Healthcare, Best, The Netherlands). Segmentation of all STS lesions was performed using an adaptive threshold algorithm. A segmentation threshold of 50% of the SUVpeak corrected for local background was used (Figure 1) [19]. After image segmentation, the resulting volumes of interest (VOIs) were used to calculate relevant uptake parameters in the PET images. For VOIs that covered normal tissue surrounding tumor lesions due to relatively high FDG uptake (e.g., heart tissue or urinary bladder), manual adjustment was performed to exclude normal tissue from the VOI. For every patient, the SUVmax and SUVpeak were calculated on the lesion with the highest SUVmax and with the highest SUVpeak, respectively. The SUVmax is defined as the voxel with the highest intensity within a tumor. The SUVpeak is defined as the largest mean value of a 1 cm<sup>3</sup> sphere positioned within a tumor. Furthermore, SUVmean, MTV, and TLG were calculated for all lesions combined per patient (whole body). The SUVmean is defined as the mean of all pixel values within all tumor lesions in a patient. The MTV is defined as the sum of the volume of all tumor lesions in a patient. The TLG is defined as the sum of the products of the SUVmean and its corresponding MTV of each lesion.

**Figure 1.** (**A**,**B**): [18F]FDG-PET of a man 8 months after resection of a primary undifferentiated soft tissue sarcoma (grade 3). ( **A**): Coronal view of a maximum-intensity projection (MIP) showing two metabolically active lesions in the right lung. (**B**): Axial plane showing two metastases in the right middle lobe and their VOIs outlined in red. ( **C**–**F**): [18F]FDG-PET of a man with three known tumor locations 6 months after resection of a primary myxofibrosarcoma (grade 2). ( **C**): Coronal projection of a MIP showing two FDG-avid lesions in the right upper leg. ( **D**): Axial plane showing a histologically proven metastasis in the left iliac bone and the corresponding VOI outlined in red. (**E**): Axial plane showing a tumor lesion and the corresponding VOI outlined in red in the adductor compartment of the right thigh just cranial of the primary tumor bed. (**F**): Axial plane showing local recurrence and corresponding VOI outlined in red in the adductor compartment of the right thigh.
