Solitary fibrous tumors (SFTs) are uncommon neoplasms that can be benign or malignant. They were first described by Klemperer and Rabin, who observed a mesothelial tumor arising from the pleura. It was established that SFTs could also be found in numerous extra-pleural sites. In fact, extra-pleural SFTs (ESFTs) are universal neoplasms with pleural and extra-pleural distribution. ESFTs have been described within the head and neck region (e.g., petrous bone, skull, cervical spine, infratemporal fossa, nasal cavity, orbit, thyroid salivary gland, face, neck, parapharyngeal space, nasopharynx, larynx, and oral cavity), and other organs (e.g., breast, bladder, thigh, the retroperitoneum, and the genitourinary tract).
Although reports of ESFTs have become more common in recent years, reports in the skeleton are rare. To our knowledge, SFTs of bone were only found in the petrous bone, skull, thigh, and spine. This is the first case of an ESFT in the ilium.
2 Case report
This Health Insurance Portability and Accountability Act (HIPAA) compliant study was approved by the institutional review board of the First Affiliated Hospital, Zhejiang University, China. The requirement for informed consent was waived because of the study's retrospective nature.
A 33-year-old man visited our hospital on December 2, 2014, due to repeated right ilium pain for 3 months (Table 1). The pain was dull and bearable, with no hip joint dyskinesia. The relevant physical examinations are negative. The patient was healthy before and had a negative family history. A pelvic x-ray showed an area of low density in the right iliac inhomogeneously. The cortical bone was discontinuous with an obscure boundary (Fig. 1). The computed tomography (CT) scan of the pelvis revealed that there was an 8.5 cm × 6.1 cm × 8.7 cm mass with slightly low attenuation, similar to muscle, in the right ilium. An osteophyte was seen in the mass and the cortical bone was discontinuous at the margin. The mass extended to the adjacent soft tissue with a well-defined margin while pushing on the surrounding tissue (Fig. 2). The magnetic resonance imaging (MRI) scan showed an abnormal 8.8 cm × 6.1 cm × 8.7 cm mass in the right ilium, which was slightly hypointense on T1-weighted images and hyperintense on T2-weighted images, with some areas of much higher intensity within the mass. The mass showed obvious inhomogeneous enhancement with mottling and a hypointense band in the center. The adjacent tissue was compressed (Fig. 3). One week later, the patient was given an “Incision biopsy of the right ilium.” Histological analysis demonstrated spindle cell lesions accompanied by collagen fibers, the tumor may be solitary fibrous tumors (Fig. 4). On December 19, 2014, the patient was given an “Extended resection of tumor” (Table 1). The immunohistochemical staining of the tumor cells was strongly and diffusely positive for CD34, BCL-2, and CD99; and negative for S-100, CD31, CKpan, EMA, P63, and SMA. The Ki-67 was 20% (Fig. 4, Table 1). The tumor was finally diagnosed as a SFT. The patient was followed-up by computed tomography of pelvis in local hospital every 6 mouths, and there is no recurrence and any symptoms (Table 1).
SFTs are uncommon neoplasms that were originally reported as arising from mesothelial cells exclusively from the surface of the thoracic pleura. The most common hypothesis suggests that they originate from undifferentiated mesenchymal cells. Although SFTs most commonly occur in the pleura, in recent years, they have been found to occur anywhere in the body. The most common sites of ESFTs are the retroperitoneum, deep soft tissues of the proximal limbs, abdominal cavity, and the head and neck. There are only a few cases reported in the skeleton, such as the skull, cervical spine, and infratemporal fossa. This is the first report of an ESFT of the ilium. SFTs rarely occur in children of both sexes, and occur during the fourth to seventh decades of life with equal incidence by sex.
The imaging findings of SFTs are nonspecific. They are usually large (varying from 2 to 26 cm in diameter), solid with well-defined margins, polylobulated contour masses, and tend to displace adjacent structures. On CT, SFTs can show low or high attenuation respective to the muscle, depending on the collagen content. Calcifications may be present in 10% cases and bone erosion may also be present. Hemorrhage and central necrosis may be visible in larger lesions. In this case, the mass measured 8.8 × 6.1 × 8.7 cm, with clear margins and had a polylobulated appearance. Bone erosion was also present, which is consistent with the imaging findings for SFTs in the literature. On MRI, SFTs are usually isointense or slightly hyper-intense on T1WI and variable on T2WI to skeletal muscle; SFTs are also hypervascular tumors that are vigorously enhancing. Enhanced patterns may be homogenous, heterogeneous, or patchy. However, the mass in this article was heterogeneous and we can see few necrosis as Garcia-Bennett et al reported. A vascular pedicle, although unspecific, is a useful feature for distinguishing SFT. Heterogeneous signal intensity, which is often caused by necrosis and internal hemorrhage, is observed in malignant tumors.
SFTs are typically composed of bundles of spindle cells irregularly arranged with a background of collagen and a dilated “staghorn-like” vascular network. SFTs usually stain positive for CD34 (95%) and CD99 (70%). They may be positive for BCL2, EMA and smooth muscle actin (SMA) (∼30%). A strong immunoreactivity for CD34 can not only diagnose but also allow the distinction of SFT from most of the other spindle cell neoplasms. An immunohistochemical profile showing strong expression of CD34 and bcl-2, with a weaker but still positive reaction for CD99, is diagnostic for an SFT. In this case, the tumor showed the same immunohistochemical profile as described in several previous cases. The proliferation marker Ki-67 can be used to stratify lesions according to their clinical outcome. Recently, a cut off level of a proliferation rate of 12% (Ki-67) was suggested to distinguish benign and malignant lesions. In our report the proliferation rate of Ki-67 was 20%, so the lesion may be malignant.
The symptoms of SFTs depend on their location, and size, however, the clinical behavior of SFTs is unpredictable. In this case, the pain was a unique symptom. SFTs present as slow-growing, painless masses and are frequently found incidentally based on radiological findings or symptoms related to their location. Two paraneoplastic syndromes have been reported: non islet cell tumor hypoglycemia and hypertrophic osteoarthropathy, neither of which was present in our patient. The World Health Organization (WHO) classifies SFTs as intermediate risk for aggressive behavior. The behavior is usually benign but can be difficult to predict. The percentage of SFTs that are found to be malignant varies in the literature, with most studies reporting ranges from 5 to 26% of cases, though even benign SFTs have indeterminate malignant potential. Malignancy potential increases with the presence of necrosis, four or more mitotic figures per 10 high power fields, increased cellularity, and cellular polymorphism. Increased tumor size (>10 cm), the presence of hemorrhage, infiltrative margins, and the presence of anaplastic foci also point to malignancy. Local recurrence after resection can also occur, with reported rates of around 6% in benign disease and around 23% in malignant disease. Malignant SFTs are diagnosed with atypical histologic features such as nuclear atypia, hypercellularity, a large necrotic portion, and a high mitotic activity (C4 mitoses per 10 high-power fields).
The preoperative diagnosis for this case is giant cell tumor (GCT). GCTs mainly affect young adults between the ages of 20 and 35 years. The most frequent locations are the distal femur and proximal tibia, however, some reports say that they can also occur in ilium. The imaging findings of the GCT are the following: (1) the lesion grows expansively and has shell formation; (2) the lesion has sclerotic margins; (3) the “soap bubble sign,” which is a characteristic sign of a GCT; (4) the cortex is discontinuous, a soft tissue mass is present, and there is no periosteal reaction. This case report contained several imaging findings consistent with GCT, with the exception of sclerotic margins. It can be seen that SFT and GCT of the ilium are difficult to distinguish.
The main treatment of SFT is surgical, and postradiotherapy may useful and SFTs are regarded as chemo resistant. It has been reported in SFTs that about 6% of patients with histologically benign tumors have the possibility of recurrence. Long-term clinical follow-up is recommended for all patients with a SFT. The potential adverse biological behavior of this tumor may lead to repeated recurrences or malignant transformation.
To our knowledge, this is the first case of a SFT in the ilium. The radiological findings for an SFT were difficult to differentiate from other tumors such as GCTs. Positive immunohistochemical findings such as strong immunoreactivity to CD34 can facilitate diagnosis and can prompt a consideration of a SFT.
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