1. Introduction

Solitary fibrous tumor (SFT) is an uncommon spindle cell neoplasm with fibroblastic differentiation [1]. Originally identified as a tumor of the pleura, SFT is now understood to arise in a wide range of extrapleural sites, though its occurrence outside the thoracic cavity remains relatively rare [1, 2]. Histopathologically, SFT is characterized by spindle to ovoid cells arranged in a disorganized, “patternless” growth pattern with varying degrees of cellularity [1]. A distinctive feature is the presence of dilated, branching blood vessels forming a “staghorn-like” configuration, reminiscent of hemangiopericytoma. The majority of SFTs harbor a NGFI-A binding protein 2 (NAB2)-STAT6 gene fusion on chromosome 12, leading to nuclear overexpression of STAT6, which serves as a highly specific and sensitive immunohistochemical marker for SFT [1]. However, the clinical behavior of SFT remains unpredictable, as these tumors can range from indolent to highly aggressive. While SFTs of the female reproductive tract are exceptionally rare, ovarian SFTs have been documented only sporadically in the literature [1, 2, 3, 4]. Herein, we report a case of ovarian SFT in a postmenopausal patient, which initially exhibited a benign course but later demonstrated aggressive behavior upon recurrence.

2. Case report

A 72-year-old woman (gravida 1, para 1) presented to our institution with progressive abdominal distension and right lower limb edema. On physical examination, a firm, elastic abdominal mass was palpated. Serum tumor markers, including Cancer Antigen 125 (CA125), Carcinoembryonic Antigen (CEA) and Carbohydrate Antigen 19-9 (CA19-9), were within normal ranges. Contrast-enhanced Magnetic Resonance Imaging (MRI) demonstrated a large heterogeneous mass extending from the pouch of Douglas to the upper abdomen, suggestive of a degenerating fibroma or fibrosarcoma (Fig. 1). Exploratory laparotomy revealed a well-encapsulated, smooth-surfaced tumor without adhesion to surrounding structures (Fig. 2). The mass appeared contiguous with an atrophic uterus and was suspected to originate from the left ovary. The right ovary was grossly unremarkable. No ascites or peritoneal dissemination was noted. The tumor exerted significant pressure on the right external iliac vein. Intraoperative frozen section analysis suggested a fibroma. Given this diagnosis, bilateral salpingo-oophorectomy and total hysterectomy were performed, ensuring no capsular rupture. The excised tumor weighed approximately 7000 grams. The resected tumor measured 28.5 × 24 × 14.5 cm. Macroscopically, there was no evidence of extracapsular tumor extension. The tumor exhibited a lobulated appearance and was predominantly edematous. Although no hemorrhage was observed, some areas showed necrotic changes. No apparent abnormalities were noted in the left adnexa or the uterus. A total of 36 tissue sections were prepared for histopathological examination.

Fig. 1.Contrast-enhanced MRI. A large heterogeneous mass extending from the pouch of Douglas to the upper abdomen (arrows) suggesting a fibroma with degeneration or a possible fibrosarcoma.

Fig. 2.A diagnostic laparotomy revealed a smooth-surfaced tumor without a capsular rupture or adhesions to surrounding organs.

Histopathological analysis revealed a spindle-to-ovoid fibroblastic neoplasm within a variably collagenous stroma, displaying mild nuclear atypia, indistinct cell borders and inconspicuous nucleoli. The tumor exhibited prominent branching, staghorn-like (hemangiopericytoma-like) or hyalinized vasculature (Fig. 3A). The mitotic index was 5 per 10 high-power fields (HPFs), with tumor necrosis comprising less than 10% of the area. Immunohistochemistry demonstrated strong, diffuse cytoplasmic CD34 expression, with additional CD34 positivity in the endothelial cells of the staghorn-like vessels (Fig. 3B). STAT6 exhibited strong nuclear expression (Fig. 3C), confirming the diagnosis of an ovarian solitary fibrous tumor (SFT). Based on the risk model by Demicco et al. [5], incorporating patient age (>55 years), mitotic count (>4 per 10 HPFs) and tumor size (>15 cm), the metastatic risk was categorized as high. The postoperative course was uneventful; however, the patient was lost to follow-up. Fourteen months later, she returned with recurrent symptoms, including abdominal distension and lower limb edema. Contrast-enhanced MRI revealed a recurrent, heterogeneously enhancing mass of comparable size to the initial tumor. During a second exploratory laparotomy, the tumor was found to be firmly adherent to the right external iliac vein, ureter and ileum, preventing complete resection. Consequently, an ileostomy and nephrostomy were performed. The resected mass weighed approximately 6000 grams. Given the patient and her family’s preference to avoid further aggressive treatment, palliative care was initiated. The patient’s condition gradually deteriorated, and she passed away eight months after the second surgery.

Fig. 3.Histopathological findings of the left ovarian tumor. (A) Histological features of SFT showing haphazardly arranged spindle to ovoid fibroblast-like tumor cells in a patternless architecture, with variable cellularity and staghorn-like vasculature (hematoxylin and eosin, original magnification ×100). (B) Immunohistochemical staining for CD34 demonstrating cytoplasmic positivity in SFT (original magnification ×100). (C) Strong and diffuse nuclear positivity for STAT6 (original magnification ×400).

3. Discussion

Solitary fibrous tumor (SFT) is classified by the World Health Organization as an intermediate-grade neoplasm with fibroblastic/myofibroblastic differentiation and a low metastatic potential [6]. SFT predominantly affects adults, with peak incidence between the ages of 40 and 70, and no significant sex predilection. This tumor can arise in a wide range of anatomical locations, including the pleura, lung, retroperitoneum, pelvis, meninges, extremities, head and neck, and trunk, with a preference for deep-seated over superficial soft tissues [6]. SFTs involving the female genital tract are exceedingly rare, with reported cases primarily occurring in the vulva, vagina and cervix [1]. Ovarian SFTs are particularly uncommon, with only a few documented cases in the literature [1, 2, 3, 4]. In this case, the patient presented with abdominal distension and lower limb edema, symptoms likely resulting from tumor compression of the external iliac vein. Many SFTs are asymptomatic and detected incidentally on imaging. However, radiologic findings are often non-specific [2], as seen in this case where the initial MRI suggested a degenerative fibroma or fibrosarcoma. Unlike sex cord-stromal tumors, SFTs do not produce characteristic tumor markers such as alpha-fetoprotein, beta-human chorionic gonadotropin or inhibin [2]. Histopathologically, SFTs may resemble fibromas, sarcomas, adult granulosa cell tumors and sclerosing stromal tumors, necessitating immunohistochemical analysis for definitive diagnosis [1]. SFTs typically express STAT6, CD34, B-cell lymphoma (Bcl)-2 and CD99, while lacking expression of smooth muscle actin (SMA), desmin and inhibin [1, 2, 5]. Although fibromas and sarcomas may exhibit weak focal STAT6 positivity, fibromas generally show at least weak inhibin expression, which is absent in SFTs [1]. Granulosa cell tumors are strongly positive for inhibin but lack STAT6 expression [1], The nuclear overexpression of STAT6, resulting from NAB2-STAT6 gene fusion, serves as a highly specific and sensitive marker for SFT [6]. However, STAT6 immunohistochemistry has limitations due to variability in detecting fusion transcripts and its use should be confined to cases with classical histological features of SFT [1]. In this patient, the presence of spindle-to-ovoid fibroblastic cells in a staghorn-pattern vascular background, along with positive STAT6 and CD34 staining, supported the diagnosis of ovarian SFT.

To date, only a limited number of ovarian SFT cases have been reported. Yang et al. [1] analyzed 25 gynecologic SFTs diagnosed via immunohistochemistry, including two ovarian cases, one of which exhibited high mitotic activity (56 mitoses per 10 HPFs) and recurred at 81 years old. Meena et al. [2] described an ovarian SFT in a 28-year-old woman treated laparoscopically, which showed no mitotic activity or recurrence at nine months postoperatively. Huang et al. [3] reported an 18-cm ovarian SFT classified as intermediate-risk, with no recurrence at six months postoperatively. Additionally, Tian et al. [4] documented a case of bilateral ovarian SFT with no recurrence after 96 months of follow-up. Although SFTs are generally regarded as benign [2], their clinical course remains unpredictable, with potential for both local and distant recurrence [2]. Malignant variants have also been reported [7]. Demicco et al. [5] proposed a risk stratification model based on patient age, mitotic count and tumor size, estimating metastasis rates of 10% for low-risk, 12.5% for intermediate-risk and 25% for high-risk gynecologic SFTs. According to this model, the tumor in our patient was classified as high-risk for metastasis.

The major limitation of this report is that it describes a single case of ovarian solitary fibrous tumor, reflecting the extreme rarity of this neoplasm. Due to the limited data available, it is not possible to draw generalized conclusions regarding its clinical, radiological, or pathological characteristics, nor to establish optimal treatment strategies.

4. Conclusion

This case highlights the potential aggressiveness of ovarian SFT, particularly upon recurrence. Given its rarity, further case studies are necessary to better characterize the clinical course and establish optimal management strategies for ovarian SFTs.

Abbreviations

SFT, solitary fibrous tumor; NAB2, NGFI-A binding protein 2; STAT6, signal transducer and activator of transcription 6; MRI, magnetic resonance imaging; HPFs, high-power fields; CD34, cluster of differentiation 34; Bcl-2, B-cell lymphoma; CD99, cluster of differentiation 99; SMA, smooth muscle actin.

Availability of data and materials

The data are contained within this article

Author contributions

KT—Data curation; formal analysis; original draft writing. RM—Data curation; formal analysis. MM, AY, YY, MS, HM—Review & editing. SK—Immunohistochemical investigation.

Ethics approval and consent to participate

All clinical information and images used in this paper were approved by the ethical committee of Kobe Medical Center (approval number: 1367). The patient gave her written informed consent to publish her case.

Acknowledgment

Not applicable.

Funding

This research received no external funding.

Conflict of interest

The authors declare no conflict of interest.