Answer: Solitary fibrous tumor of the pleura

Histology: The tumor is centered in the visceral pleura, and does show significant infiltration of the lung parenchyma. It consists of a proliferation of spindle cells that grow in a haphazard fashion (i.e. “patternless pattern”). The cellularity is highly variable: zones of dense cellularity alternate with zones of low cellularity. Collagen deposition is prominent, and even in the densely cellular areas the individual tumor cells are separated by strands of dense pink collagen. The tumor is interrupted by numerous thin-walled gaping vascular channels demonstrating a “staghorn” configuration. The tumor cells are oval to spindled with indistinct cytoplasm. There is only minimal pleomorphism, and mitotic figures are not observed. By immunohistochemistry, the tumor cells are immunoreactive for CD34; and they are not immunoreactive for cytokeratin or calretinin.

Discussion: Solitary fibrous tumor of the pleura (SFT) are uncommon tumors that are usually first discovered as asymptomatic lesions on routine chest radiographs. Long believed to arise from the mesothelial cell, it is now recognized that SFT originates from some CD34 positive dendritic interstitial cell that is rather ubiquitously distributed throughout the body. Recognition of its non-mesothelial origin has undermined the notion that SFT is limited to the chest cavity and other mesothelial-lined sites. As a result, there has been a recent explosion of reports describing SFT in previously unrecognized sites including those entirely unassociated with a mesothelial lining. Further, the term “solitary fibrous mesothelioma” is no longer appropriate.

SFT is notorious for being misdiagnosed as some other spindle cell neoplasm (This potential for misdiagnosis is exacerbated when they are encountered at some unexpected extra pleural site.). The fact that SFTs (no matter where the primary site) are uniformly CD34 positive is a very helpful diagnostic feature. When dealing with tumors of the lung and pleura, the findings of strong CD34 staining and the absence of cytokeratin staining is helpful in excluding sarcomatoid carcinoma and mesothelioma from the differential diagnosis. Distinguishing SFT from hemangiopericytoma is more problematic since these two tumors may display considerable morphologic and even immunohistochemical overlap. Strong and diffuse CD34 positivity would, however, support the diagnosis of SFT.

About 10% to 20% of SFTs arising in the pleura locally recur. Infiltration into the parietal pleura and chest wall is an important predictor of recurrence. Histologic features of malignancy include high cellularity with overlapping nuclei, cellular pleomorphism, and 4 or more mitosis per 10 high-power fields.