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Solitary fibrous tumors (SFTs) are classified as fibroblastic/myofibroblastic tumors that originate from CD34-positive dendritic cells and usually occur in the pleura. In this paper, we describe a case of SFT within the joint cavity of the left knee. A 60-year-old man was admitted to hospital due to swelling in the left knee for the past 8 months without relevant trauma history. X-ray, computed tomography (CT), magnetic resonance imaging (MRI), and positron emission tomography–computed tomography (PET-CT) presented a large, ill-circumscribed, hypervascular, and highly enhanced mass with eccentric calcification and peripheral, intra-lesional vessels. Subsequently, the patient underwent surgical resection. Postoperative pathology confirmed the neoplastic cells to be positive for CD34, Bcl-2, and SATA6, therefore was finally diagnosed as malignant SFT. The patient developed bone metastases within 1 year after surgery. SFT in the joint cavity is rare, and it is difficult to make a preoperative diagnosis.

Introduction

Klemperer and Rabin reported the first case of a fibrous mesothelioma located in the pleura and showed a benign clinical course (1). The understanding of solitary fibrous tumors (SFTs) has evolved over time. According to the recent World Health Organization (WHO) classification of soft tissue tumors, SFTs are classified as fibroblastic/myofibroblastic tumors that originate from CD34-positive dendritic cells, the biological behavior of which is further subdivided into intermediate/locally aggressive (SFT-benign), intermediate/rarely metastasizing (SFT-NOS, including fat-forming SFT and giant cell-rich SFT), and malignant (SFT-malignant) (2). Hemangiopericytoma was once used as a synonym for SFT occurring extra-pleura. Subsequently, giant cell angiofibroma and hemangiopericytoma were removed from the WHO classification of soft tissue tumors in 2013, with the former being classified as SFT outside the pleura, which was described as one morphologic variant, while the latter was recommended to be disregarded as a lot of soft tissue tumors could display a hemangiopericytomatous vasculature (3, 4). In addition, the NAB2–STAT6 gene fusion was identified in SFTs in 2013 (5).

SFTs are relatively rarely seen in clinical practice, but occur in a wide variety of body sites (6–9). With regard to the imaging features of SFTs, there are no characteristic findings. Computed tomography (CT), magnetic resonance imaging (MRI), echocardiography, and positron emission tomography–computed tomography (PET-CT) imaging findings have been reported mainly in case series (10). Radiographs, echocardiography, and CT alone generally yield non-specific findings for SFTs, and the density of SFTs can be uniform or uneven. At ultrasound, SFTs are typically hypoechoic, but are occasionally heterogeneous. Calcification is rare, but could exist and can be observed using CT or by detecting the presence of bright echo-dense areas on echocardiography. Compared with those of CT, the imaging findings of MRI are more conducive to helping radiologists in analysis and assessment due to their high resolution of soft tissue and multi-parameter imaging. MRI can reflect the histological characteristics of SFTs. Low-signal-intensity foci on T1- and T2-weighted images, representing the collagen content, are a frequent feature of SFTs. As highly vascular tumors, SFTs are avidly enhanced on both CT and MRI. Whole-body imaging with PET-CT can be performed to determine multiple tumor sites and to detect metastatic lesions in the case of malignant SFTs. An increased uptake of fluorodeoxyglucose F18 (F-18 FDG), which corresponds to glucose metabolism, could indicate an aggressive or a malignant type of SFT (10, 11).

In this paper, we present a case of a malignant SFT within the joint cavity of the left knee, a rare site of occurrence, and the morphology of the mass led to this case being rare. We mainly focused on its radiological features, followed by a review of the literature. It is hoped that the study findings will be beneficial to radiologists in clinical work.

Case presentation

Ethics approval for this study was granted by the Institutional Review Board of our hospital. A 60-year-old man was admitted to the Department of Orthopedics of our hospital due to swelling in the left knee for the past 8 months. The patient had no history of relevant trauma and had no other complaints of discomfort. The skin and the temperature of the left knee were normal. X-ray of the left knee was first taken (Figure 1), which showed soft tissue swelling mainly on the anteromedial side of the knee, where the density was higher than that of normal soft tissue. Moreover, some strip calcification could be seen in the X-ray, but there was no destruction of the bone. To further define the lesion, the radiologist recommended CT or MRI. Enhanced CT and MRI were both performed, which confirmed the irregular soft tissue mass within the joint cavity of the left knee. CT of the left knee (Figure 2) showed that the calcification in the mass was mainly located at the edge. After injection of a contrast material, the enhancement was heterogeneous, and a nodular enhancement of the lesion was presented. Small enhanced vessels could be seen in and around the mass. There was no destruction to the adjacent bones. MRI presented the shape of the lesion better (Figure 3). The mass was irregular, and it appeared to grow along the synovial membrane of the joint and was partly nodular. On T2-weighted images (T2WI), the lesion was hyperintense. On T1-weighted images (T1WI), the lesion was mainly hypointense, but with some nodular hyperintensity. In the enhanced images, the lesion showed a markedly heterogeneous enhancement, especially the nodules. The margin of the lesion was unclear. The adjacent medial collateral ligament of the left knee was swollen and was partly invaded by the mass. Edema was evident in the soft tissue surrounding the mass. It was difficult to determine what the lesion was; therefore, a needle biopsy was performed, which suggested malignancy. F-18 FDG PET-CT, which was performed for tumor restaging, revealed active heterogeneous enhancement of only one tumor, with a maximum standardized uptake value (SUVmax) of 7.51 (Figure 4). No other metabolically active lesions were seen in the body; therefore, the patient underwent surgical resection. The soft tissue and muscle around the tumor were separated, and the distal femur and the proximal tibia and fibula were excised for complete resection of the tumor. Subsequently, knee prosthesis replacement was performed. The whole operation was successful. Postoperative pathology confirmed malignant SFT with focal necrosis. Immunohistochemical examination revealed that the neoplastic cells were positive for CD34, Bcl-2, and SATA6 (Supplementary Figure S1). The immunohistochemistry results were as follows: CD34 (partially ++), Bcl-2 (partially ++), STAT6 (weak +), EMA (focal +), desmin (−), S-100 (−), beta-catenin (focal +), SMA (focal +), CK(AE1/AE3) (scattered +), CAM5.2 (scattered +), and Ki-67 (30% +). Approximately 11 months after surgery, the patient came to our hospital again due to bone abnormalities, and it was confirmed by imaging examination and puncture pathology that the patient had multiple bone metastases