Microscopic description and IHC:
The mass is composed of a well circumscribed, firm, lobulated, gray-white mass with whorling foci and fasciculation. Microscopically the mass is cellular, composed of spindle fibroblast like cells admixed with abundant collagens in “keloid like” configuration. hemangiopericytoma-like or staghorn like vessels are scattered throughout. Necrosis are not present and only rare mitotic figures are found (2 per 2 mm^2).
The fibroblast like cells are positive for CD34 and STAT6 and negative for AE1/3.
Discussion:
The solitary fibrous tumor (SFT) of pleura is formally known as solitary fibrous mesothelioma. The patient is usually asymptomatic and is not associated with asbestosis. It can be associated with pulmonary osteoarthropathy, digital clubbing and hypoglycemia, which regress after tumor resection. The cells are most likely fibroblastic in origin, arising from submesothelial mesenchyme.
Poor prognostic factors are size > 10 cm, >4 mitoses/10 HPF, cellular atypia, positive margins, and malignant histology.
SFT are usually positive for CD34 and negative for S100. Signal transducer and activator of transcription 6, interleukin-4 induced (STAT6), was recently identified as a consistent finding in SFT (nuclear expression). It can be used e helpful to distinguish this tumor type from many histologic mimics including cellular angiofibroma, desmoid fibromatosis, DSFP, GIST, low-grade fibromyxoid sarcoma, MPNST, synovial sarcoma, sarcomatoid mesothelioma, schwannoma, soft tissue perineurioma, and spindle cell lipoma.