49 year old male with a 7 x 4.5 x 4.0 cm
lobulated mass in the left abductor longus muscle of the lower extremity
Discussion:
This tumor occurs primarily in the para-articular region with no apparent relation to
synovial structures. There may be a history of trauma and the tumor is
often associated with pain. It is characteristically found in young
adults, in the extremities near a joint, but rarely in joint cavities. It
may, however, be found in other areas like the parapharyngeal region, pleura,
or heart. It has been described in virtually all anatomic sites.
Synovial sarcoma may be biphasic, monophasic (fibrous or epithelial) or
poorly differentiated. There is a characteristic radiologic finding in
15-20% of cases of synovial sarcoma in which there are multiple, small,
spotty radiopacities, caused by focal calcification or bone formation.
Radiopacities are not observed in most other forms of sarcoma, save
extra-osseous osteosarcoma.
Histologically, there is coexistence of morphologically different, but
histogenetically related epithelial cells (solid cords, nests or
glandular structures) and fibroblast-like spindle cells.
Commonly, cellular portions of synovial sarcoma alternate with less
cellular areas. There may be hyalinization, myxoid change, calcification
or mast cell infiltrate. In monophasic variants the epithelial or the
fibroblast-like spindle cells predominate.
This particular case is classic for synovial sarcoma and should present
no diagnostic difficulties.
The best outcomes are in pediatric patients, tumors less than 5 cm in
diameter, less than 10 mitoses / 10 hpf, no necrosis and when the tumor is
eradicated locally. There is no difference in prognosis between monophasic
and biphasic tumors or in relation to immunophenotype. However, patients
with the SS18/SSX2 variant gene, found mostly in monophasic synovial
sarcoma, have a better prognosis.
Immunohistochemistry: Synovial sarcoma shows positive immunoreactivity
with Cytokeratin (both epithelial and mesenchymal cells are
typically positive); CK7 and CK19 are expressed in synovial sarcoma in contrast to
other spindle cell sarcomas; however, because some synovial sarcomas stain
for epithelial membrane antigen (EMA), but not cytokeratin, and vice
versa, both EMA and CK should be used in an attempt to demonstrate
epithelial differentiation in these tumors; S100 is positive in up to 30% of cases;
BCL-2 protein is diffusely expressed in all synovial sarcomas,
especially in spindle cells; CD99 can be detected in 60-70% of cases (cytoplasm/membrane). It
is always negative for CD34.
There is a specific balanced translocation in synovial sarcoma, t(X;18)
which involves– SSY gene (chromosome 18) and SSX gene (chromosome X).
References:
- Verbal discussions - Dr. Kristen Atkins
- Weiss, S.W., Goldblum, J.R.,
Enzinger, F.M. (2001). Enzinger and
Weiss's Soft Tissue Tumors (4th ed.).
Philadelphia, PA: Mosby
Publishing.
