Anatomic Pathology: Bone and Joint Pathology

• Synovial sarcoma affects predominantly adolescents and young adults (age range, 15 to 40 years). It occurs around large joints, most commonly situated in the lower extremity, affecting the ankle and hip joints and most frequently the knee joint. The head and neck is the next most common site followed by the upper extremity. It may also arise in the chest and abdominal walls as well as various other locations.

• The tumor occupies the deep soft tissues causing a palpable mass associated with pain and tenderness. Tumors in unusual locations are often clinically misinterpreted, causing delays in diagnosis.

• The radiographic findings are most often a round or oval, deep-seated, lobulated mass of intermediate density located around large joints and showing spotty radiopacities owing to calcifications and less often ossification (15% to 20% of cases). Erosion of adjacent bone occurs frequently (20% of cases). CT and MRI have become indispensable in evaluating the margins of the mass and its extent into the soft tissues.

• A high-grade, monophasic fibrous tumor is characterized by an extremely cellular population of densely packed, spindle-shaped mesenchymal cells. The cells are small and uniform and arranged in sheets showing a fascicular growth pattern and pseudorosettes focally. Vascularity is abundant and hemangiopericytoid. Mitoses are frequent (three to four per high-power field). Zones of necrosis attest to the high level of cellularity and paucity of supporting stroma. This histologic type overlaps with other neoplasms discussed in the question. Immunohistochemically, vimentin is positive; endothelial monocyte antigen is focally positive; cytokeratin, CD45, S100, desmin, and smooth muscle actin are negative; and CD31 is positive in blood vessels. CD99, gene product of MIC2 gene, is frequently positive. Although some soft tissue sarcomas express cytokeratins (cytokeratin 8 and cytokeratin 18 in epithelioid sarcoma), synovial sarcoma uniquely expresses cytokeratin 7 and cytokeratin 19. Reverse transcriptase polymerase chain reaction shows a diagnostic chromosomal translocation t(x:18) (p11.2;q11.2) that causes a fusion between the SYT gene on chromosome 18 and SSX1 or SSX2 on chromosome X with the fusion transcript SYT/SSX1 (present in all biphasic tumors) or SYT/SSX2 (present in some monophasic tumors).

• The prognosis of synovial sarcoma was traditionally thought to be dismal. In recent years, the 5-year survival rate has been stated to be 50%. With factors favorable to the prognosis (young age, distal location, small size, absence of necrosis, low mitotic rate), the 5-year survival rate approaches 80%. The tumor frequently recurs, and distant metastases occur in regional lymph nodes and lungs.