Synovial sarcomas are relatively common intermediate-to-high grade malignant soft tissue tumors, often with an initial indolent course, affecting young patients, and most commonly involving the soft tissue surrounding the knees.
Synovial sarcomas typically present in adolescents and young adults (15-40 years of age). There may be a mild male predilection (M:F = 1.2:1). They account for 2.5-10% of all soft tissue sarcoma 2-4.
The presentation is most often with a slowly-enlarging soft tissue mass which may have been noted for some years and gives a false impression of a benign process 4.
Synovial sarcoma derives its name from the fact that microscopically it resembles normal synovium. However, it stains for epithelial markers (e.g. epithelial membrane antigen and cytokeratin), which synovium does not. Additionally, it has been found in many locations that do not normally have synovium.
The most common location for these tumors is within the soft tissues adjacent to large joints, e.g. knee and popliteal fossa. While these tumors arise near joints, it is rare for them to arise from the joint itself and despite their name, they do not arise from synovial structures, e.g. joints, tendon sheaths and bursae.
Macroscopically they have non-specific appearances. They are well or poorly-defined heterogeneous masses with common areas of hemorrhage and necrosis.
They are divided histologically into four subtypes:
- biphasic: 20-30%
- monophasic fibrous: 50-60%
- monophasic epithelial: very rare
- poorly differentiated: 15-25%
Cytogenetic aberration of the t(X;18) translocation is highly specific, seen in over 90% of cases 1-4.
Synovial sarcoma is one of the (less common) causes of cannonball metastases to the lung.
- extremities: 80-95% 1
- lower limb: 60-70%
- upper limb: 15-25%
- from soft tissues: 90-95%
- from joint: 5-10%
- head and neck: 5%
- chest wall: rare 6
- viscera: rare
- lung 2, kidney, prostate, esophagus, heart, pericardium, pleura
A soft tissue mass near but not in a joint in a young patient (15-40 years old), particularly if dystrophic calcification is present, are very suggestive.
Radiographs may be normal unless the mass is large or contains dystrophic calcifications, which are found in up to 30% of cases 1. These calcifications are non-specific within the periphery of the lesion and not usually osteoid or chondroid in appearance.
Occasionally, smooth pressure erosions may be visible on the underlying bone, which should not be mistaken for a sign of a benign process.
Alternatively, direct bony invasion is seen in ~30% (range 11-50%) of cases, with aggressive bone features (permeative, geographic, or moth-eaten appearances) 4.
Non-specific findings, with a heterogeneous predominantly hypoechoic mass. Intra-lesion flow is usually associated with more aggressive lesions.
CT is non-specific, similar to ultrasound. It appears as a soft tissue mass of heterogeneous density and enhancement. CT is, of course, more sensitive to calcifications than either radiographs or MRI.
MRI is the modality of choice to stage the tumor locally, although again imaging findings are not pathognomonic. The mass is usually large and variably well-defined (smaller lesions tend to be better circumscribed).
- iso- (slightly hyper-) intense to muscle
- mostly hyperintense
- the markedly heterogeneous appearance of synovial cell sarcomas on fluid-sensitive sequences results in so-called "triple sign" which is due to areas of necrosis and cystic degeneration with very high signal, relatively high signal soft tissue components and areas of low signal intensity due to dystrophic calcifications and fibrotic bands
- due to the high tendency of lesions to bleed, there might be areas of fluid-fluid levels known as "bowl of grapes" are seen in up to 10-25% of cases
T1 C + (Gd)
- enhancement is usually prominent and can be diffuse (40%), heterogeneous (40%), or peripheral (20%) 1
On bone scan, synovial sarcomas typically have heterogeneously increased dynamic and blood pool uptake, with some increase in delayed images, but usually less intense.
This imaging test is not usually performed. These tumors are usually hypovascular 1.
Treatment and prognosis
Treatment involves a combination of surgery and usually adjuvant radiotherapy +/- chemotherapy. Radiotherapy is particularly useful in treating tumors where an adequate clear margin cannot be achieved, and ideally, radiotherapy is administered pre-operatively.
Good prognostic variables include:
- small size
- located in the extremity
- younger age <20 years of age
- solid homogeneous mass
- presence of calcification
- biphasic histology (controversial)
Poor prognostic variables include:
- large size (>5 cm): the most important factor
- located in the trunk or head and neck
- older patients
- cystic/hemorrhagic components
- marked heterogeneity
- poorly differentiated histology
- rhabdoid cells
- extensive tumor necrosis
- high nuclear grade
- p53 mutations
- high mitotic rate (>10 mitoses per 10 high-power field)
Overall 5-year survival is between 36-76%. Both local recurrence (30-50%) and distant metastases are frequent (40-70%), most commonly to the lungs (~80%), bones (~15%), regional lymph nodes (~10%), followed by chest wall / abdomen (~7.5%) 7,8.
General imaging differential considerations include:
- 1. Murphey MD, Gibson MS, Jennings BT et-al. From the archives of the AFIP: Imaging of synovial sarcoma with radiologic-pathologic correlation. Radiographics. 26 (5): 1543-65. doi:10.1148/rg.265065084 - Pubmed citation
- 2. Terasaki H, Niki T, Hasegawa T et-al. Primary synovial sarcoma of the lung: a case report confirmed by molecular detection of SYT-SSX fusion gene transcripts. Jpn. J. Clin. Oncol. 2001;31 (5): 212-6. doi:10.1093/jjco/hye045 - Pubmed citation
- 3. Votruba M, Hungerford J, Cornes PG et-al. Primary monophasic synovial sarcoma of the conjunctiva. Br J Ophthalmol. 2002;86 (12): 1453-4. Br J Ophthalmol (link) - Free text at pubmed - Pubmed citation
- 4. Meyers SP. MRI of bone and soft tissue tumors and tumorlike lesions, differential diagnosis and atlas. Thieme Publishing Group. (2008) ISBN:3131354216. Read it at Google Books - Find it at Amazon
- 5. O'sullivan PJ, Harris AC, Munk PL. Radiological features of synovial cell sarcoma. Br J Radiol. 2008;81 (964): 346-56. doi:10.1259/bjr/28335824 - Pubmed citation
- 6. Fujimoto K, Hashimoto S, Abe T et-al. Synovial sarcoma arising from the chest wall: MR imaging findings. Radiat Med. 15 (6): 411-4. - Pubmed citation
- 7. Krieg AH, Hefti F, Speth BM et-al. Synovial sarcomas usually metastasize after >5 years: a multicenter retrospective analysis with minimum follow-up of 10 years for survivors. Ann. Oncol. 2011;22 (2): 458-67. doi:10.1093/annonc/mdq394 - Pubmed citation
- 8. Spurrell EL, Fisher C, Thomas JM et-al. Prognostic factors in advanced synovial sarcoma: an analysis of 104 patients treated at the Royal Marsden Hospital. Ann. Oncol. 2005;16 (3): 437-44. doi:10.1093/annonc/mdi082 - Pubmed citation