Solitary fibrous tumor is a mesenchymal tumor composed of spindle cells, whose origin is poorly understood. It exhibits fibroblastic and myofibroblastic differentiation, as well as intermediate malignant potential, known to present a broad histopathological spectrum [1]. It was first mentioned by Wagner in 1870 in the pleura; however, the first description was made by Klemperer in 1931, who named it fibrous mesothelioma [2]. Subsequently, in 1942, Stout and Murray were the first to discuss hemangiopericytoma [3].

Solitary fibrous tumors represent less than 2% of all soft tissue tumors, less than 1% of vascular tumors, and approximately 5% of all sarcomatous tumors [4]. The age of presentation varies, with an average age at diagnosis of 45, and the distribution is equal in both sexes [4]. Retroperitoneal location is extremely rare, with evidence of only 9 published cases in literature [5].

Lesions can present as painless, slow-growing masses or may cause symptoms due to mass effect and pressure on adjacent structures [6]. Since these tumors can arise in various locations, the differential diagnosis is broad and largely depends on the location [7].

Primary surgical resection is the treatment of choice whenever possible, and no benefit has been shown from systemic chemotherapy [3]. Radiation therapy can improve local control rates postoperatively but may not achieve complete disease remission [4].

Hemangiopericytomas are rare vascular neoplasms derived from Zimmerman pericytes, smooth muscle cells that line capillaries, post-capillary venules, and sinusoidal spaces [8]. Most neoplasms previously called hemangiopericytomas are now included in the category of solitary fibrous tumors, although some strictly defined categories of hemangiopericytoma are still recognized as distinct pathological entities [6]. The fourth edition of the World Health Organization's Classification of Tumors of Soft Tissue and Bone, published in February 2013, merged these two entities into a single tumor. However, some pathologists continue to maintain the old classification, despite the morphological and pathological characteristics of the lesions being related [9].

These tumors can be located in the lower extremities (34.9%) and the pelvic cavity (24.5%). The greater omentum is an uncommon site of occurrence, with approximately 20 cases described in the literature [4].

They rarely present with symptoms of hypoglycemia due to the secretion of insulin-like growth factors [6]. In particular, intra-abdominal and cerebromeningeal locations are associated with an aggressive clinical course. Conversely, those located in the pleuropulmonary region show a lower tendency for distant metastasis, which may be related to microenvironmental and epigenetic factors [1].

Macroscopically, solitary fibrous tumors appear as a well-circumscribed rubbery mass with clear evidence of fibrotic qualities. Occasionally, a fish flesh appearance is described [10]. Histologically, they exhibit a combination of hypercellular and collagenous areas. In cases where assessing malignancy based solely on morphology and immunohistochemical characteristics is difficult, markers such as CD34, CD99, Bcl-2, and STAT6 can be useful [9]. Occasionally, they may be positive for Epithelial Membrane Antigen (EMA) and Smooth Muscle Actin (SMA), and generally negative for cytokeratin, S100, and desmin.

The NAB2-STAT6 fusion genes are specific to solitary fibrous tumors, and their detection can be useful in challenging diagnostic cases. Immunohistochemistry for STAT6 has recently been introduced as a substitute for detecting the fusion gene. The intense and diffuse nuclear staining of STAT6 is an important characteristic of these tumors, observed in over 90% of cases. Less than 10% of other spindle cell tumors in different body locations have tested positive for STAT6, most of which do not exhibit the same diffuse and intense staining as seen in solitary fibrous tumors [2].

Imaging in these patients typically shows a mass with heterogeneous enhancement on computed tomography, appearing isointense on magnetic resonance imaging. In some cases, the feeding vessels of the lesion can be visualized on angiography. Some of these tumors have shown muscle and nearby vascular structure invasion, as well as destruction of ethmoid cells and nasal turbinates [7].

According to the literature, these tumors have favorable long-term outcomes following primary surgical resection, and systemic chemotherapy has not shown any benefit. However, when they are located in sites such as the greater omentum, they tend to have a higher recurrence rate in local or distant sites [3]. Due to their characteristics, anti-angiogenic inhibitors have emerged as a therapeutic alternative [3]. Diffuse expression of platelet-derived growth factor, as well as expression of basic fibroblast growth factor and transforming growth factor-alpha, has been observed, suggesting that various factors responsible for endothelial and vascular proliferation may contribute to angiogenesis in these patients [11].

Radiation therapy can improve local control rates postoperatively but may not achieve complete disease remission. It may be recommended for tumors that exhibit criteria for malignancy and as an alternative treatment for patients with criteria for irresectability or recurrence [4]. In a study conducted by Krengli et al., it was concluded that the addition of radiation therapy could improve both local control and disease-free survival rates in patients who underwent surgery for extracranial pathology. However, subgroup analysis showed that the advantage of adding radiation therapy was more significant for patients with extremity and superficial trunk locations than for intrathoracic or retroperitoneal tumors [12].