The histological appearance of SFT may resemble and overlap with other benign and malignant diagnostic entities, such as hemangiopericytoma, leiomyoma, nodular fasciitis, inflammatory myofibroblastic tumor, fibromatosis, and benign peripheral nerve sheet tumor.3
It must be noted that tumors with similar features have been described in literature with different names, thereby creating some confusion among pathologists and clinicians.
The unifying morphological criterion of all these lesions is represented by a well-circumscribed, tan-colored, rubbery mass, which is often tethered by a pedicle and partially encapsulated. Microscopically it is described as a patternless
proliferation of bland-looking spindly to oval epithelioid cells that form short fascicles and/or clusters, admixed with thick or thin collagen bands, and a prominent branching vasculature. Mature adipocytes and giant multinucleated stromal cells may be present.3,4
Mitoses are generally scarce, rarely exceeding 3 mitoses per 10 hp fields.
Malignant counterparts are usually hypercellular lesions, showing variable cytologic (focally moderate to marked) atypia, tumor necrosis, infiltrative margins and higher frequency of mitoses.5
Immunohistochemical staining for vimentin, Bcl2, CD 99 and CD34 immunoreactivity isperformed in order to discriminate SFT from other neoplasms; it is usually completely negative for S-100 protein, cytocheratin AE1/AE3 and neurofilaments. In some cases immunohistochemical negativity has been reported for c-kit, CAM 5.2, factor XIIIa, HMB-45, AE-1, SMA, CD31, and Fli-1.6,7
One classification distinguishes SFT into two subcategories: fibroblastic and myofibroblastic. The latter strongly express alfa smoothmuscle actin and desmin, while the former do not.8
Solitary fibrous tumors are karyotipically variable, and currently available data do not evince any anomalies that are common to or characteristic of these lesions. Cytogenetic data are very limited and suggest predominantly structural abnormalities and numerical imbalances.9
The tumour is difficult to detect because it has no typical radiologic features: on X-ray it appears as a moderately radio-opaque mass; on ultrasonography as a nodule with generally well-defined borders and a homogeneous echostructure. In addition to these means, second-level imaging techniques, such as contrast-enhanced CT scan and MRI, are usually performed: the former reveals a well-defined capsule surrounding a nearly homogeneous mass with progressive enhancement from the arterial to the venous phase and occasionally multiple small non-enhancing portions.10
Despite their unpredictable behavior, 10–15% of SFTs are aggressive. Local recurrence or onset of metastasis depends mainly on the following prognostic parameters: histological features (nuclear atypia, increased cellularity, necrosis, and more than 4 mitoses per 10 HPF), localization (approximately 10 to 15% of SFTs located outside the thoracic cavity are recurrent or metastatic), dimensions (>10 cm) and resectability (the most important).
Although most SFTs are characterized by a non-aggressive clinical course, malignant transformation and a large size have been associated with a poor outcome, thus making long-term follow-up in all cases strongly advisable. Lesions located in the mediastinum, abdomen, pelvis or retroperitoneum tend to behave more aggressively than those in the limbs. Metastases are most often reported in the lungs, bone and liver.
Non islet-cell hypoglycemia (Doege Potter syndrome) due to the production of high molecular weight insulin-like growth factors11
is the most frequent associated paraneoplastic syndrome: it is described in only 40 of all cases of SFT reported in the literature,11
but in nearly 70% of these it has a malignant behavior or a poor survival from diagnosis (30% have a benign behavior or no reported deaths during follow-up). Bladder localizations with associated hypoglycemia are very rare.
Because of the unpredictable behavior that SFTs display and the lack of consistent data in literature, surgery is considered the treatment of choice whenever possible. The use of chemotherapy is reserved to metastatic or symptomatic non-resectable SFTs, but there are no standard chemotherapeutic indications or regimens. Even though SFTs are considered relatively chemoresistant, the most effective drugs seem to be anthracyclines and ifosfamide, followed by gemcitabine and dacarbazine, which are commonly used in soft tissue sarcomas.12,13
Radiation therapy is of some benefit, when applicable, also in combination with chemotherapy.14
The novel targeted drug imatinib mesilate seems to exert some activity on SFT expressing the wild type platelet-derived growth factor receptor-β: its in vitro15
and in vivo16
inhibitory activity in chemo- and radioresistant malignant SFT has recently been reported.