Malignant Solitary Fibrous Tumor.
Solitary fibrous tumor (SFT) was first described in 1931 by Klemperer and Rubin. The most common site of SFT is the pleura, but it is been described in a large variety of sites including head and neck area (5%), sinonasal tract and oral cavity being the most commonly involved site(1)
SFTs are highly variable in appearance depending upon the relative proportion of cells and fibrous stroma. The cellular areas consist of tightly packed round to fusiform cells arranged around a continuous ramifying vascular network (staghorn or antler like). The large vessels commonly are invested with thick coat of collagen which extends into the interstitium. Myxoid change is common. The other areas show spindle cells arranged randomly (patternless pattern) with areas of hyalinization. The cells are arranged singly or small parallel clusters next to the dense collagen. Artificial cracks develop between the cells and the collagen. They usually express CD34 (80-90%), CD99 (70%), bcl2 (30%), EMA (30%), and actin (20%). They are negative for desmin, cytokeratin, and S-100.
SFTs are a part of spectrum of the tumor ranging from benign to intermediate to malignant, depending on histopathologic features. Malignant SFTs are usually hypercellular lesions, showing at least focally moderate to marked cytological atypia, tumor necrosis, numerous mitosis (> 4 mitosis / 10 HPF) and/ or infiltrative margins. Rare case show abrupt transition from conventional benign appearing SFT to high grade sarcoma, likely representing a form of dedifferentiation(2)
SFT-hemangiopericytoma (HPC) debate: The term Hemangiopericytoma has been used over the years to describe wide variety of neoplasms with certain morphological features: monotonous appearance at low power, moderate to high cellularity, and presence of numerous, variably thick-walled, branching "staghorn' vessels. The characteristic morphological features of HPC were actually not specific for a given entity, benign or malignant. Roughly speaking, up to 15% of soft tissue neoplasms show HPC-like features, at least focally. The term HPC is getting increasingly unpopular because the pericytic differentiation could be confirmed only in minority of cases ultrastructurally and actin as a marker of pericytes was infrequently present in these lesions(3). Many entities previously called as HPC are now recognized as SFT. Currently, there is a consensus that SFT is a distinctive mesenchymal neoplasm that characteristically shows HPC-like features, and that SFT is barely distinguishable from HPC on light and electron microscopic examinations. Ultrastructurally, both lesions display varying features of pericytic, fibroblastic and/or myofibroblastic differentiation and, immunohistochemically, they both variably express CD34, CD99, and bcl-2 antigens.
The cases which were considered previously as HPC of sinonasal tract may fall into two categories. The first category (10-15% of cases) corresponds to cellular SFT. In the second category (about 85-90% of cases), tumor cells are consistently reactive to muscle specific actin and SMA, but are negative for desmin and CD34 and demonstrate myoid differentiation in keeping with true pericytic differentiation. These lesions correspond to what are now designated as glomangiopericytomas. TREATMENT: Total resection of the tumour is the gold standard of treatment. Radiotherapy may be considered as adjuvant therapy in cases of positive surgical margins, although with skepticism (4-6).
BEHAVIOR: The behavior of extrapleural SFTs is not entirely predictable, though the aforementioned criteria for malignancy do predict malignant behavior (present in 10-15% of cases of all SFTs) to some extent. However, occasional tumors with indolent histologic features behave aggressively and conversely, occasional malignant SFT have a favorable outcome. The quality of the initial excision, with free margins and the presence histologically malignant component are the important indicators of clinical outcome in pleural and extrapleural SFTs(7).
Contributed by Sarika Jain, MD and Raja Seethala, MD.