Answer: Solitary fibrous tumor (SFT)

Histology: There is a spindle cell proliferation in the submucosa. The lesion is circumscribed but nonencapsulated. The overlying squamous epithelium is non-ulcerated and without dysplasia. The spindle cells are haphazardly arranged and have a bland and uniform appearance with finely dispersed chromatin and scanty cytoplasm. Mitotic activity is very low. The spindle cells are intimately entwined between thin and thick bands of collagen. In areas, the collagen has a kelloid-like quality. In areas, the tumor has a hemangiopericytoma-like vascular pattern seen as irregularly dialated thin-walled vessels.

Discussion: Solitary fibrous tumor (SFT) is a tumor that most frequently arises from the pleural surface of the lung. Once regarded as a tumor of mesothelial derivation (hence the old term “localized fibrous mesothelioma”), it is now recognized that SFT is derived from a more ubiquitously distributed CD34+ dendritic cell. As a result, these tumors have been reported in a growing number of anatomic sites unassociated with a mesothelial lining including the oral cavity.

For SFTs arising in the oral cavity, the buccal mucosa is the most commonly involved site. In this location, they tend to circumscribed and non-infiltrative. Of the limited number of intraoral SFTs so far reported with significant patient followup (about 8 cases), all have behaved in a benign fashion without evidence of recurrence or metastasis.

SFT can exhibit a range of histologic patterns, and it morphologically overlaps with a broad spectrum of mesenchymal neoplasms. Immunohistochemistry plays a helpful role in the recognition of SFT. SFTs are consistently immunoreactive for the markers CD34 and bcl-2; but they are generally negative (or only weakly positive) for markers of muscle (actin and desmin) and neural (S100) differentiation. A panel including these markers would be helpful in excluding leiomyoma (CD34 negative, actin/desmin positive) and fibroma (CD34 negative) from the differential diagnosis. Distinguishing SFT from HPC is perhaps the greatest diagnostic challenge. SFT and HPC demonstrate tremendous morphologic and even immunohistochemical overlap. HPCs are bcl-2 immunoreactive, and some may even demonstrate immunoreactivity of CD34. In light of a growing believe that HPC may simply represent a histologic pattern rather than a true biologic entity, there has been a recent trend to classify most HPCs as SFTs. This tends to be our approach for those “HPCs” that are CD34 positive.