FINAL DIAGNOSIS: SYNOVIAL SARCOMA
Synovial sarcoma is a soft tissue tumor occurring in the biphasic or monophasic type and consisting of epithelial and/or spindle cell components. The differential diagnosis includes malignant peripheral nerve sheath tumors, muscle-derived sarcomas, and biphasic mesotheliomas.
The reported incidence of synovial sarcomas ranges from 5.6% to 10% among all soft tissue sarcomas. According to the AFIP, synovial sarcoma ranks as the fourth most common sarcoma after malignant fibrous histiocytoma, liposarcoma and rhabdosarcoma.
Although it can occur at any age, the peak incidence is ages 10 to 35. There is a slight male predominance. The majority of synovial sarcomas occur in the extremities with a predilection for the lower extremities. Less frequently, it can occur in the head and neck, abdominal wall, pleura, lung and mediastinum and has been found in almost all anatomical sites. The lesions often grow close to joints, particularly the knee, however the intra-articular tissues are only rarely involved.
Synovial sarcomas vary in size and are often deep-seated. Slow-growing tumors tend to be well- circumscribed, while rapidly growing tumors tend to be more variegated. Grossly, the tumor is yellow-tan to gray. The cut surface is soft to firm. Calcification can be grossly apparent and may be detected radiographically. Cyst formation may be prominent.
Microscopically, synovial sarcomas contain two morphologically different cell types: epithelial cells and spindle cells. Depending on the prominence of the cell types, the synovial sarcomas can be further classified on a spectrum including 1) biphasic type (containing both epithelial and spindle cells in varying proportions), 2) monophasic spindle cell, 3) monophasic epithelial type (rare), and 4) poorly differentiated round cell sarcoma. The biphasic and monophasic spindle cell are equally common, the remaining two types are much less common.
The epithelial cell component consists of large, pale cells with abundant cytoplasm. The cells are cuboidal or tall columnar and are arranged in solid cords, nests or glandular structures. The nuclei are round and vesicular. The glandular lumens contain eosinophilic or granular secretions, which stain with PAS with diastase. The spindle cell component is made of small, uniform spindle cells arranged in fascicles. The cells have small amounts of indistinct cytoplasm and oval dark-staining nuclei. Three characteristic findings in the spindle cell component are distinctive, but not always present: 1) thick collagen bands separating adjacent malignant cells, 2) calcification (variable amounts), and 3) a hemangiopericytoma-like vascular network. Mitotic figures may occur in either cell type component, however only poorly differentiated synovial sarcoma exhibits more than two mitotic figures per high power field. Mast cells are also commonly seen in synovial sarcomas. They tend to be more numerous in the spindle cell component.
Both elements of synovial sarcomas show immunoreactivity for cytokeratins and less-intensely for epithelial membrane antigen (EMA). The following stains may prove to be useful.
Cytogenetically, 90% of synovial sarcomas are associated with a chromosomal translocation, t(X;18)(p11.2;q11.2), occurring in both biphasic and monophasic forms. The translocation results in two fusion genes: SYT-SSX1 and SYT-SSX2.
The 5-year survival rate is 50% and up to 82% in heavily calcified tumors. The 10-year survival rate is 20-30%. Better prognosis is related to the following factors: small tumor size (<5cm), early clinical stage, early age at presentation, presence of calcification and distal extremity location. Synovial sarcomas can recur locally or metastasize distantly to the lung and lymph nodes in particular. Tumors >5cm are more likely to metastasize. Treatment consists of local excision with wide margins, followed by high dose radiation therapy. Adjuvant chemotherapy may also play a role.
Contributed by Regina Schmidt, MD, Uma Rao, MD, Sam Yousem, MD.