Extrapleural SFTs are most commonly diagnosed between the fifth and seventh decades of life. They are typically slow growing and asymptomatic. Symptomatic tumors are typically secondary to the locally invasive nature of the tumor and compression and/or impingement of nearby structures. Common symptoms seen with intra-abdominal and pelvic SFTs include pain, palpable mass, abdominal distention, urinary retention, hematuria, constipation and bowel obstruction . Overall there is an equal distribution among men and women, however certain SFTs have been shown to have a gender predilection (for example, bladder SFT is 3.5 times more common in males) . A small subset of tumors may cause a paraneoplastic syndrome, the most common of which is hypoglycemia secondary to insulin-like growth factor secreted by the tumor, or so-called Doege-Potter syndrome .
Because of the wide variety in clinical presentation, the work-up and diagnosis of SFT can be challenging. Although non-specific, certain imaging characteristics have been observed. A sonography can reveal a hypo- or heterogeneous echogenic mass with well-defined margins. A CT scan typically shows a well-enhanced, circumscribed heterogeneous mass. Prominent vasculature due to mass effect and calcification can be observed in larger lesions, as seen in our case . Regions of hemorrhage and necrosis can also be observed, specifically in cases of malignant SFT. A magnetic resonance imaging scan typically shows variable (low-to-intermediate) signal intensity on T1 and T2 weighted images, dependent on collagenous and fibrous stroma content, vascularity and chronicity of the tumor . Attempts at an image-guided biopsy have been described, but have been largely unsuccessful due to sampling error. As such, surgical resection is often necessary for diagnostic and therapeutic purposes.
Macroscopic examination of solitary fibrous tumors typically reveals a well-circumscribed, tan-colored rubbery mass with a white whorled appearance on cut sections. The mass is often tethered by a pedicle and encapsulated. Most cases of bladder SFT are described as being intravesicular, growing from the submucosa, however there have been isolated cases of pelvic SFT growing from the serosal surface of the bladder, as was seen with our patient. Microscopically, SFT is classically described as having a ‘patternless pattern’, with juxtaposed hypercellular (spindle to ovoid cells) and hypocellular zones (hyalinized collagen) and a prominent branching vasculature [4,7]. Mitoses and microscopic necrosis are rare in SFTs. Because there are numerous other spindle cell lesions that can display a similar cellular architecture (such as gastrointestinal stromal tumor, leiomyoma, leiomyosarcoma, malignant fibrous histiocytoma, benign and malignant nerve sheath tumor and so forth), the immunohistochemical staining plays an important role in the diagnosis, as seen in our case. The tumor cells are often positive for CD34, CD99, vimentin and BCL-2, and negative for CD117 (in gastrointestinal stromal tumors), smooth muscle actin (SMA) and desmin (in smooth muscle tumors) and S-100 protein (in nerve sheath tumors) [6,7]. Recently, genomic studies have revealed a gene fusion that may have diagnostic and therapeutic implications for patients with SFT. The NAB2 gene which indirectly represses transforming growth factor (TGF) - β, and the STAT6 gene which is a transcriptional factor that modulates signaling through interleukin-4 and interleukin-13, have both been described as oncogenic. A recent study of 53 patients undergoing whole exome tumor sequencing revealed that when fused, these two genes (located on chromosome 12) represent a distinct molecular feature in SFT. This characteristic fusion transcript occurred in 55% of tumors, creating the opportunity to develop drugs that specifically target the fusion gene product. Interestingly, a cytogenetic analysis did not reveal the NAB2-STAT6 fusion in our patient . Moreover, an immunohistochemistry analysis for STAT6 and GRIA2 was also negative. An analysis of 44 SFTs by Mohajeri et al. revealed that in addition to the NAB2-STAT6 fusion transcript, GRIA2 was the top up-regulated protein found by immunohistochemistry using tissue microarrays .
Malignant criteria for SFTs include large tumor size (>10cm), hypercellularity, nuclear atypia, tumor necrosis, more than four mitoses per 10 high power fields and infiltrative margins . However, it is important to note that malignant histologic features are not always an indicator of aggressive tumor behavior, as benign tumors can act locally aggressive and recur, while alternatively malignant tumors can proceed with an indolent course [4,7]. Nonetheless, with respect to benign SFTs, malignant SFTs do account for higher rates of local recurrence (63 versus 8%) and metastasize to distant soft tissue sites and the lungs . With respect to bladder SFT, most reported cases have pursued a benign course, even in the presence of malignant histologic features .
Because of the variable clinical behavior and unpredictable nature of SFTs, surgery is considered the treatment of choice. Furthermore, long-term follow-up is strongly recommended for all cases of SFT, although no specific surveillance strategy has proved to be superior. Surgical resectability is the most important prognostic factor, and the five-year survival rate is high, with some authors claiming close to 100% with complete surgical excision (R0 resection) . For unresectable disease, chemotherapy and radiation therapy have been described as having variable success. In one retrospective study of 21 patients with late-stage SFT, 16 of the 18 patients who received first line chemotherapy (with doxorubicin, gemcitabine or paclitaxel) had no disease progression for a median of 4.6 months . Prospective randomized studies are needed to substantiate these results. Going forward, anti-angiogenic therapy (such as bevacizumab, a monoclonal antibody against vascular endothelial growth factor) has shown promising early results in the treatment of patients with advanced unresectable tumors .