Synovial sarcoma accounts for about 5-10% of all soft tissue sarcomas. They mostly occur in teenagers and young adults (90% before age 50) and are more common in men.
The tumour is locally destructive. It can spread to regional lymph nodes. About 40% metastasise and are fatal. Local recurrence is also common (~ 50%). Despite the name, these tumours do not arise from or resemble synovium, but they are commonly found in close proximity to joints.
Tumour/Normal Tissue Features
The tumour is usually tan or grey in colour and may be multinodular.
There are several common forms seen:
- Biphasic SS consists of spindle and epithelioid cells. The epithelioid cells form glands or papillae. The spindle cells are small and relatively uniform with indistinct borders. Mitoses are uncommon
- Monophasic SS consists of spindle cells only.
- Calcified SS is uncommon but confers a better prognosis with excellent 5 year survival
- Poorly differentiated SS contains areas resembling a small round blue cell tumour with high mitotic rate and necrosis. It is associated with a poorer prognosis.
The two cell populations stain differently. The epithelial portion stains for cytokeratins and epithelial membrane antigen. The spindle cell component stains for S100 and CD99 in some cases.
Important: Synovial sarcoma almost always carries a characteristic translocation between chromosome X and chromosome 18 t(X;18). This affects the SSX1, SSX2 or SSX4 from the X chromosome and SS18 from chromosome 18.