Solitary Fibrous Tumor with Entrapment of Minor Salivary Gland Tissue: An Unusual Presentation That Requires Exclusion of Pleomorphic Adenoma
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We report two unusual cases of solitary fibrous tumor (SFT) of minor salivary glands that microscopically mimicked pleomorphic adenoma. One of these lesions presented in the retromolar region and the other in the buccal mucosa. The microscopic features of these two tumors and their intimate relationship with regional mucous minor salivary glands posed a diagnostic challenge. Awareness of the morphological diversity of SFT coupled to a judicious use of appropriate immunohistochemical probes should prove valuable to accurately segregate SFT from pleomorphic adenoma.
KeywordsIntraoral solitary fibrous tumor Pleomorphic adenoma Minor salivary gland neoplasm Differential diagnosis
Solitary fibrous tumor (SFT) is a rare benign spindle cell mesenchymal neoplasm that was originally described in the pulmonary pleura . Today, SFT is considered a ubiquitous tumor, having been reported at many extra-pleural sites . In the head and neck area, SFT has been reported in upper airways, nasal cavity, paranasal sinuses, orbit, all major salivary glands, thyroid gland, skin and meninges . SFT infrequently involves the oral cavity with only approximately 70 cases reported in the English literature. Intraorally, the buccal mucosa is the most common site of involvement and the tumor develops as a slow-growing, asymptomatic, smooth submucosal growth of varying sizes that occurs in a wide age range .
Regardless of a pleural or extra-pleural location, SFT retains similar microscopic features. SFT appears as a well-circumscribed tumor showing a wide histological spectrum. However, in 1997, Chan  delineated a set of diagnostic criteria. The essential diagnostic features of SFT are those of a tumor characterized by the presence of hypercellular and hypocellular areas composed of bland, spindle or ovoid cells with scanty and poorly defined cytoplasm. The tumor cells are usually distributed haphazardly, but storiform and fascicular patterns of mesenchymal spindle cells are not uncommon and may suggest a neural genesis. The tumors cells may be intertwined amidst thin and thick collagen fibers. Areas of abundant stromal hyalinization and regions of vascular proliferation with a staghorn pattern may also be present. In addition, SFT displays a low mitotic activity .
The array of microscopic features displayed by SFT may pose a diagnostic challenge . The present paper reports two exceptional cases of SFT of the oral cavity involving minor salivary glands. The microscopic findings were unusual and resembled those of pleomorphic adenoma with cystic changes and stromal hyalinization.
The post-surgical recovery was uneventful and no evidence of recurrence has been documented at a 3-years follow-up.
After local infiltration, an excisional biopsy was performed. During the surgical procedure, the lesion appeared to be encapsulated. The lesion was removed intact from the surrounding tissue and was submitted for microscopic examination. The specimen was fixed in 10% buffered formalin. Gross examination showed a specimen measuring 2.0 × 0.9 × 0.8 cm in its largest dimensions with a firm consistency and white color.
Based on the microscopic findings and the immunohistochemical profile, a final diagnosis of solitary fibrous tumor was rendered. No recurrence was noted at a 3-year follow-up.
Originally described as a “primary neoplasm of the pleura” by Kemperer and Rabin  in 1931, SFT has subsequently been reported to present in a variety of anatomic sites . Pleural  and extrapleural SFT  may attain large size and can display malignant microscopic features and biological behavior. It has been reported that 13–23% of the pleural SFT are malignant . In contrast, intraoral SFT, has an excellent prognosis associated with a benign clinical behavior .
Early reports uniformly regarded SFT as a tumor of mesothelial origin . This view was gradually modified with the observation that SFT can affect other areas of the body . Electron microscopy and immunohistochemistry have shown that SFT tumor cells are of mesenchymal origin . Therefore, the World Heath Organization (WHO) currently classifies SFT as an intermediate fibroblastic tumor .
The diagnosis of SFT may be difficult  in extrapleural locations due to the its wide histological spectrum. Moreover, a small biopsy specimen will further hinder the ability of the pathologist to recognize this tumor despite a strict adherence to Chan’s criteria and the use of adequate immunohistochemical markers .
Nerve sheath tumor
Spindle cell lipoma
Giant cell angiofibroma
Benign fibrous histocytoma
Monophasic and myxoid synovial sarcoma
The major diagnostic dilemma lies mainly in distinguishing SFT from hemangiopericytoma (HPC) due to their overlapping histological features . Both tumors share the “staghorn-like” vascular pattern and cellular morphology . In addition, SFT and HPC have been reported to show similar immunohistochemical and ultrastructural profiles . SFT and HPC display immunoreactivity for CD34, Bcl-2 and CD99 antigens . Ultrastructural studies have demonstrated pericytic, fibroblastic, and myofibroblastic differentiation in both tumors . These findings have led pathologists to consider HPC and SFT as a spectrum of the same entity . Consequently, usage of the term hemangiopericytoma/solitary fibrous tumor to describe these lesions has started to populate the literature .
Rarely, SFT of the oral cavity may show entrapment of adipose tissue, skeletal muscle, and salivary gland tissue that may also complicate the microscopic diagnosis. The presence of adipose tissue has been described in several SFTs of the oral cavity. This finding has previously reported in other areas of the body. In fact, the prominent entrapment of adipocytes prompted Weiss et al.  to coin the term lipomatous hemangiopericytic-solitary fibrous tumor. The differential diagnosis of a SFT of the oral cavity with adipose cells should include spindle cell lipoma and myxoid liposarcoma . Interestingly, the presence of skeletal muscle fibers in SFT has not prompted the designation of these tumors as “myogenous producing” lesions. Rather, the presence of muscle fibers in SFT has been interpreted as a reflection of the intramuscular genesis of SFT. Similarly, it appears to be that the presence of minor salivary glands in some SFTs may represent the development of this tumor within the stroma of salivary gland parenchyma .
Reported cases of solitary fibrous tumor involving minor salivary glands
Swelam et al. 
NED at 2 years
O’Regan et al. 
Lost to follow up
Left buccal mucosa
NED at 12 years
Right retromolar region
NED at 3 years
NED at 3 years
Microscopic features of pleomorphic adenoma that may resemble other tumors. Modified from Speight and Barrett 
Polymorphous low grade adenocarcinoma
Bilayered ducts and cribiform pattern
Adenoid cystic carcinoma
Bilayered ducts with clear outer cells
Sheets of epithelioid or basaloid cells
Basal adenoma or adenocarcinoma
Myxoma, neural tumors
Spindled myoepithelial cells
Benign neural tumors and spindle cell sarcomas
Squamous cell carcinoma
Benign lipomatous tumors
Ductal and spindle cell proliferation with interspersed hyalinization (our cases)
Solitary fibrous tumor
Similarly to PA, SFT can show diverse morphological features that may cause diagnostic difficulties. Examining multiple sections and the use of appropriate immuno-probes can ameliorate these difficulties.
The most consistent and reliable immunohistochemical marker for SFT is CD34 . The CD34 antigen is strongly expressed in both, pleural, and extrapleural SFTs , including intraoral lesions . Both of our cases showed a strong CD34 immunoreactivity. Bcl2 and CD99 markers have also been reported to be of value for the diagnosis of SFT. However, inconsistent expression of CD99 and lack of specificity of bcl2 in SFT lessens the value of these probes [3, 4, 8, 14]. CD34 has been used in pleomorphic adenomas to evaluate the stromal vascularization .
Some reports have described the cytogenetic alterations displayed by SFT and PA. SFT has been shown to display trisomy 8 and 21 [22, 23], while 70% of pleomorphic adenomas show abnormal karyotypes . Thus, cytogenetic studies may be of value to differentiate these two tumors.
Intraoral SFT is usually treated by complete surgical excision and recurrence is unlikely. Two case series of intraoral SFTs, with extensive follow-up information, showed no signs of local recurrence and/or malignant transformation after treatment [3, 8]. In contrast, malignant and metastatic SFTs have been reported to occur in extraoral sites [6, 7]. Our two cases have demonstrated the expected biological behavior for intraoral SFT, with no evidence of recurrence at a three-year follow-up.
Our cases are unique not only because of their interglandular location but also because both mimicked pleomorphic adenoma at the microscopic level. Reporting this uncommon presentation of intraoral SFT should help the pathologist to resolve a potential diagnostic pitfall.
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