Definition
• Uncommon mesenchymal tumors of the lung; probable fibroblastic origin; often shows hemangiopericytoma (HPC)-like vascular pattern
Clinical features
Epidemiology
• Most often affects adults older than 40 years
• No gender predilection
Presentation
• May be asymptomatic or present with fever, chest pain, shortness of breath, hemoptysis, and weight loss
• Rarely, it may present with paraneoplastic symptoms: hypertrophic osteoarthropathy or tumor-associated hypoglycemia (Doege-Potter syndrome)
• Chest x-ray film reveals the intrapulmonary mass
Prognosis and treatment
• Vast majority are benign and have a favorable prognosis
• Rare tumors show malignant potential with locally aggressive growth and distant metastasis
Pathology
Gross
• Well-circumscribed lobulated mass with a firm, whitish and tan-gray cut surface
• Some may show foci of degeneration with mucoid changes, hemorrhage, and calcification
• Locally aggressive or malignant tumors may show necrosis and infiltrative margins
Histology
• Alternating hypercellular and hypocellular areas separated by fibrous stroma and branching HPC-like vessels
• Hypercellular area with fusiform cells arranged in random arrays (patternless architecture)
• Hypocellular area may be highly hyalinized with keloidal collagen
• Degenerative changes may be present, including myxoid changes, hemorrhage, and calcification
• Malignant SFTs are characterized by high cellularity, moderate to marked cellular atypia, high mitotic activity (>4 mitoses per 10 high-power fields), and infiltrative growth pattern
Immunopathology/special stains
• Majority are positive for CD34, BCL2, CD99, and vimentin
• Typically negative for keratin, epithelial membrane antigen (EMA), actin, desmin, S100, and CD31
Main differential diagnoses
• Sarcomatoid mesothelioma: malignant spindle cells arranged in fascicles or a haphazard distribution; may contain areas of osteosarcoma, chondrosarcoma, or other sarcomas; usually positive for keratin, focally positive, or may be negative for EMA, CK5/6, calretinin, WT-1, and CD34
• Desmoplastic mesothelioma: dense collagenized stroma interspersed with malignant spindle tumor cells; usually positive for keratin, focally positive, or may be negative for EMA, CK5/6, calretinin, WT-1, and CD34
• Sarcomatoid carcinoma: spindle cell carcinoma; positive for keratin, EMA
• Fibrosarcoma: malignant spindle cell tumor arranged in sweeping fascicles with herringbone pattern, no HPC-like vascular pattern, negative for CD34
• Synovial sarcoma: highly cellular biphasic tumor, positive for keratin, with characteristic t(X;18) translocation
• Malignant peripheral nerve sheath tumor (MPNST): a malignant spindle cell tumor arises from a peripheral nerve or extraneural soft tissue; half of MPNSTs are associated with NF1. Tumor cells are positive for S100 with less expression in high-grade tumor, and negative for CD34
Fig 1 Solitary fibrous tumor of the lung. The tumor is composed of cellular and less cellular areas with branching HPC-like vessels.
Fig 2 Solitary fibrous tumor of the lung. Round- to spindle-shaped tumor cells have scant cytoplasm and bland nuclei with minimal atypia.
Fig 3 Solitary fibrous tumor of the lung. Myxoid changes are common.
Fig 4 Solitary fibrous tumor of the lung. Hypocellular area is composed of thick bands of hyalinized, keloidal collagen.
Fig 5 Solitary fibrous tumor of the lung. Invasion into the lung parenchyma creates a phyllodes tumor–like pattern, with residual type 2 pneumocytes lining the leaflike surfaces.
Fig 6 Solitary fibrous tumor of the lung. Even in benign SFT, hypercellular areas with thick bands of collagen are seen. Note lack of mitosis, atypia, and necrosis.
Fig 7 Solitary fibrous tumor of the lung. Cut surface of malignant SFT shows a variegated appearance with areas of necrosis and hemorrhage.
Fig 8 Solitary fibrous tumor of the lung. Low power of malignant SFT shows a hypercellular tumor.
Fig 9 Solitary fibrous tumor of the lung. The tumor cells in malignant SFT have high nuclear/cytoplasmic ratio and frequent mitoses.
Fig 10 Solitary fibrous tumor of the lung. Focal cytological atypia is evident in this malignant SFT.
Fig 11 Solitary fibrous tumor of the lung. Benign SFT with strong diffuse CD34 immunoreactivity.
Fig 12 Solitary fibrous tumor of the lung. Malignant SFT with strong diffuse CD34 immunoreactivity.
Fig 13 Solitary fibrous tumor of the lung. This malignant SFT is strongly positive for BCL2.