Which histologic findings are characteristic of malignant solitary fibrous tumors (SFTs) of the pleura?

Updated: Dec 25, 2019
  • Author: Joseph F Tomashefski, Jr, MD; more...
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Answer

The histological features of SFTs are quite variable. [3, 8, 9, 59]

Benign tumors are composed of bland, short, fibroblastlike spindle cells with ovoid nuclei, interspersed amidst a fibrocollagenous stroma. The most common histological pattern has been termed the “patternless pattern” of Stout, in which spindle cells and collagen bundles are randomly dispersed throughout the tumor. In the diffuse sclerosing pattern, dense collagen bundles are the dominant feature. Collagen may assume an eosinophilic wirelike appearance similar to that seen in desmoplastic mesothelioma. A storiform or hyalinized arrangement may also be seen.

Cellularity varies from minimal to diffuse sheets of round to ovoid cells. Cellularity varies inversely with the degree of collagen matrix. Some SFTs have a hypocellular myxoid appearance. [9, 59] Prominent thin-walled vessels with staghorn configuration resemble hemangiopericytoma. Multiple histological patterns may occur within a single SFT. [9, 59]

Pathology of nonmesothelial cancers of the pleura. Pathology of nonmesothelial cancers of the pleura. "Patternless pattern" of benign solitary fibrous tumor showing short spindle cells insinuated between coarse bundles of collagen.

Histological features associated with malignancy include the following: [3, 9]

  • Increased numbers of mitotic figures (>4 per 10 HPF)

  • Nuclear pleomorphism and atypia, including frankly sarcomatous areas within an SFT

  • Areas of tumor necrosis (as opposed to ischemic necrosis due to torsion)

An interesting variant of malignant SFT showing lipomatous differentiation has recently been described. [60]

Pathology of nonmesothelial cancers of the pleura. Pathology of nonmesothelial cancers of the pleura. Malignant solitary fibrous tumor. Note the hypercellular appearance and area of necrosis on the left side of the image.
Pathology of nonmesothelial cancers of the pleura. Pathology of nonmesothelial cancers of the pleura. Pleomorphic sarcomatous appearance with bizarre tumor giant cells. On immunohistochemical staining, the neoplastic cells were positive for CD34 and negative for pankeratin.

There is no apparent difference in the histology of intrathoracic versus extrathoracic SFTs. The atypical histological features associated with aggressive behavior in thoracic SFTs are also associated with aggressive behavior in extrathoracic tumors. [43, 61] Histologic features tend to be more atypical in recurrent or metastatic SFT than in the primary tumor. [43] SFTs can be accurately diagnosed presurgically with transthoracic needle biopsy. [62] Assessment of malignant potential, however, usually requires sampling of the resected tumor. Diagnosis with fine-needle aspiration is difficult owing to nonspecific cytologic features. [62]

Pathology of nonmesothelial cancers of the pleura. Pathology of nonmesothelial cancers of the pleura. (A) Malignant solitary fibrous tumor. A 2 kilogram pleural solitary fibrous tumor in a 36-year-old female. The collagenous area is typical of that seen in low-grade tumors.
Pathology of nonmesothelial cancers of the pleura. Pathology of nonmesothelial cancers of the pleura. (B) Another field from the primary pleural solitary fibrous tumor shown in the previous image (A) from the same patient. This image reveals a hypercellular area with minimal pleomorphism. Note the mitotic figure near the center of the field.
Pathology of nonmesothelial cancers of the pleura. Pathology of nonmesothelial cancers of the pleura. (C) Metastatic solitary fibrous tumor involving the lung in the same patient as in the previous two images (A) and (B), 3 years after resection of the primary pleural tumor. The tumor has dedifferentiated and has a pleomorphic high-grade sarcomatous appearance with numerous mitotic figures.

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