**1. Introduction**

Approximately 14% of patients with a soft tissue sarcoma (STS) present with metastatic disease [1]. Additionally, up to 34% of high-grade STS patients develop distant metastases within 5 years after resection of localized STS [2,3]. While several studies show an improvement in the survival of patients with metastatic STS over the last decades, the two-year survival rate remains less than 50% [4–6]. Indeed, treatment of these patients is complex due to the heterogeneous and aggressive nature of these tumors. Generally, therapies can

**Citation:** Kalisvaart, G.M.; Grootjans,W.; Bovée, J.V.M.G.; Gelderblom, H.; van der Hage, J.A.; van de Sande, M.A.J.; van Velden, F.H.P.; Bloem, J.L.;de Geus-Oei, L.-F. Prognostic Value ofQuantitative [18F]FDG-PET Features in Patients with Metastases from Soft Tissue Sarcoma. *Diagnostics* **2021**, *11*, 2271. https://doi.org/10.3390/ diagnostics11122271

Academic Editor: Giorgio Treglia

Received: 30 October 2021 Accepted: 2 December 2021 Published: 4 December 2021

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consist of combinations of surgery, radiotherapy, and systemic treatment. Personalized decision making is important in designing treatment strategies, and a multitude of parameters is used for this purpose [7]. Prognostic factors play an important role among these parameters, and several studies have identified a group of characteristics that is associated with prognosis in these patients [4–6,8–10]. These studies strike the consensus that patient age, performance status, disease-free interval, and histological subtype are strong predictors for overall survival (OS). Nevertheless, stratification of patients on an individual level remains a difficult challenge and requires further insight in the link between tumor characteristics and prognosis.

The use of 18F-fluorodeoxyglucose ([18F]FDG) positron emission tomography (PET) for the characterization of malignant lesions is widely studied [11]. In STS patients, [18F]FDG-PET imaging is regularly performed for (re)staging and follow-up [12]. Furthermore, in metastatic STS, specifically, a recent study has shown value of [18F]FDG-PET in monitoring the response to systemic treatment [13]. The uptake of [18F]FDG, as expressed by the standardized uptake value (SUV), reflects the degree of glucose metabolism of a lesion. High [18F]FDG-uptake has shown to be connected to increased tumor aggressiveness in many STS subtypes. Especially in localized STS, several [18F]FDG-PET features, such as maximum SUV (SUVmax), peak SUV (SUVpeak), metabolic tumor volume (MTV), and total lesion glycolysis (TLG), are found to have significant prognostic value [14,15]. Moreover, in other tumor types, these parameters have shown to be predictive for survival in metastatic disease and demonstrated to be valuable for the personalization of treatment decisions [16]. While the metabolic properties of lesions, as indicated by quantitative [18F]FDG-PET features, might also provide valuable information for the prognosis of metastatic STS patients, no literature is readily available on the correlation between these features and survival. In the current study, we assessed the prognostic value of quantitative [18F]FDG-PET features in patients diagnosed with metastases from STS.

#### **2. Materials and Methods**
