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Primary pulmonary synovial sarcoma with epithelioid and rhabdoid morphology


Soft tissue synovial sarcoma (STSS) is a malignant tumor most commonly seen in young to middle-aged adults, arising in the deep soft tissues of the extremities, especially near joints. In 1995, the first case of a SS arising in the lung, known as a primary pulmonary synovial sarcoma (PPSS), was described. PPSS is a rare tumor, comprising 0.1 to 0.5% of primary lung malignancies, and the etiology is unknown. PPSS is found in young adults usually between 30 to 40-years-old, and there is no gender predilection. Patients may present with cough, dyspnea, or chest pain; however, some cases may be asymptomatic. Radiologically, PPSS show homogenous and heterogenous areas, often characterized by the “triple sign,” which denotes bright, dark, and gray zones. This variability corresponds to the gross examination findings of tumor necrosis, hemorrhage, and cystic areas. Histologically, PPSS show a uniform or monotonous population of cells arranged in a herringbone, storiform, or hemangipericytomatous (staghorn) pattern. Cytologically, cells are ovoid to spindled with minimal eosinophilic cytoplasm and inconspicuous nucleoli.

There are three morphologic variants of PPSS: monophasic, biphasic, and poorly differentiated. The monophasic type is described by predominantly spindle cells, whereas the biphasic type includes both a spindle cell and glandular component. The poorly differentiated type shows small round cells, which can mimic primitive lesions. Hemorrhage, necrosis, calcification, and cystic changes may also be seen in PPSS. Immunohistochemically, the lesional cells stain positively for Bcl2, CD99, and vimentin, and may show focal weak staining for epithelial markers such as pancytokeratin or EMA. INI-1 is usually retained.

The differential diagnosis for PPSS includes sarcomatoid carcinoma, malignant peripheral nerve sheath tumor (MPNST), solitary fibrous tumor (SFT), and mesothelioma. The spindle cells of sarcomatoid carcinoma stain strongly with pancytokeratin. MPNST show variable S100 and CD34 positivity, as well as H3K27me3 loss. SFT demonstrate strong CD34 and STAT6 positivity. Mesothelioma cells are highlighted by calretinin, WT1 and D2-40. When considering the diagnosis of PPSS, one of the best tests to order is FISH for the SYT/SS18 gene rearrangement [t(x;18)(p11.2;q11.2)], which is found in approximately 90% of PPSS cases. Treatment for PPSS includes surgical resection followed by chemotherapy, radiation, or combination chemoradiation. The prognosis is poor with a median survival of around two years.


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