Case of the Month

Answer to last month’s Case of the Month…

C. Solitary fibrous tumor

Solitary fibrous tumor (SFT) was originally described as a pleural tumor by Klemperer (1). Less commonly, it presents as an intrapulmonary neoplasm (2) .In both locations, it is positive for CD34, Bcl-2 and CD99. STAT6 is currently used as the most reliable marker for SFTs (3).

In the differential diagnosis of pulmonary SFT, one should consider other spindle cell tumors, including some that are listed here. Synovial sarcoma, the most common primary sarcoma of the lungs, is an obviously malignant, usually quite cellular spindle cell (or biphasic) tumor. Furthermore, it has a different immunohistochemical profile and, cytogenetically, it typically shows the diagnostic X;18 translocation. In most instances, benign fibrous histiocytoma and myopericytoma are skin tumors, usually found in the subcutis. Malignant mesothelioma may be composed of malignant spindle cells, but it is usually a pleural-centered tumor with secondary involvement of the lung parenchyma only in advanced stages of the disease. Almost invariably, it shows microscopic signs of malignancy. It has a  different immunohistochemical staining pattern to that of SFT, often showing at least some markers of epithelial and pleural cells, such as calretinin, CK5/6, WT1 or podoplanin (D2-40).

Microscopically, SFTs are composed of spindle-shaped cells forming fascicles that alternate with less cellular fibrous areas; this is often referred to as a “patternless pattern.” Another common feature of these tumors is a “hemangiopericytomatous growth pattern” characterized by close perivascular layering of tumor cells around prominent blood vessels. These vessels mostly have thin walls or show a “staghorn-like” branching appearance. Perivascular hyalinization may be prominent. Malignancy of SFT is usually marked by increased cellularity, nuclear atypia, mitoses, areas of necrosis, and invasive growth. However, even some tumors that have a benign appearance, like the one presented here, may recur and show invasive growth.

References

1. P Klemperer, CB Rabin, “Primary neoplasms of the pleura”, Arch Pathol; 11, 386–411 (1931).
2. N Rao et al., “Intrapulmonary solitary fibrous tumors: clinicopathologic and immunohistochemical study of 24 cases”, Am J Surg Pathol, 37, 155–166 (2013). PMID: 23108019.
3. JM Kim et al., “Comparison and evaluation of risk factors for meningeal, pleural, and extrapleural solitary fibrous tumors: a clinicopathological study of 92 cases confirmed by STAT6 immunohistochemical staining”, Pathol Res Pract, 213, 619–625 (2017). PMID: 28552537.