Synovial sarcoma

Last revised by Mohammad Taghi Niknejad on 29 Feb 2024

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 is a misnomer because this tumor shows no synovial differentiation but reveals somehow epithelial differentiation, so 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 biphasic variant is composed of glandular components (which may be dominant or hidden) and spindle cells. Spindle cells are quite uniform and arranged in fascicular pattern.

Monophasic synovial sarcoma composed of spindle cells and is quite resemble to MPNST, fibrosarcoma, mesenchymal chondrosarcoma and cellular SFT. For establishing a correct diagnosis, an IHC study for keratin and EMA is very helpful. Nearly 90% of monophasic variants are at least focally positive for keratin or EMA.

SS18-SSX fusion antibody is 95% sensitive and specific for synovial sarcoma.10

The most common location for these tumors is within the deep 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.

A soft tissue mass near but not in a joint in a young patient (15-40 years old), particularly if dystrophic calcification is present, is very suggestive.

Radiographs may be normal unless the mass is large or contains dystrophic calcifications found in up to 30% of cases 1. These calcifications are non-specific within the lesion's periphery and are usually not 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).

  • T1

    • iso- (slightly hyper-) intense to muscle

    • heterogeneous

  • T2

    • 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 scans, 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 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

  • histology

    • 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 as cannonball metastases (~80%), bones (~15%), regional lymph nodes (~10%), followed by chest wall/abdomen (~7.5%) 7,8.

General imaging differential considerations include:

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