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Synovial sarcoma

Dr Henry Knipe and Dr Frank Gaillard et al.

Synovial sarcomas are relatively common intermediate to high grade malignant soft tissue tumour often with an initial indolent course.

Epidemiology

Synovial sarcomas typically present in adolescents and young adults (15-40 years of age). There may be a mild (M:F 1.2:1) male predilection.

Clinical presentation

Presentation is most often with a slowly enlarging soft tissue mass which may have been noted for a number of years ⁴.

The most common location for these tumours is within the soft tissues adjacent to large joints, e.g the knee and popliteal fossa. While these tumour arise near joints, it is rare form 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.

Pathology

Synovial sarcomas account for 2.5 - 10% of all soft tissue sarcoma ^2-4. Its name is derived 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 does not have normal synovium.

Macroscopically they have non-specific appearances. They are well or poorly defined heterogeneous masses with frequent areas of haemorrhage and necrosis.

They are divided histologically into 4 sub types:

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.

Location

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%
- hypopharynx or parapharyngeal space
- conjunctiva ³
- CNS
chest wall : rare 6
viscera : rare
- lung ², kidney, prostate, oesophagus, heart, pericardium, pleura

Radiographic features

Plain film

X-rays may be normal, unless the mass is large or contains calcification, which are found in up to 30% of cases ¹. These calcifications are non-specific 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 frank bony invasion is seen in 11 - 50% of cases, with aggressive bony features (permeative, geographic, or moth-eaten appearances) ⁴.

Ultrasound

Non-specific findings, with a heterogeneous predominantly hypoechoic mass.

CT

Similarly CT is non-specific demonstrating a soft tissue mass of heterogenous density and enhancement. CT is of course more sensitive to calcifications that either x-rays or MRI.

MRI

MRI is the modality of choice to locally stage the tumour, 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
- very heterogeneous with areas of necrosis and bands of fibrosis
- fluid-fluid levels are seen in up to 10-25% of cases
C + (Gd) - enhancement is usually prominent and can be diffuse (40%) heterogeneous (40%) or peripheral (20%) ¹

Bone scan

Typically synovial sarcomas have heterogeneously increased dynamic and blood pool uptake, with some increase in delayed images, but usually less intense.

Angiography / DSA

Is not usually performed. These tumours are usually hypovascular ¹.

Treatment and prognosis

Treatment involves a combination of surgery and usually adjuvant radiotherapy +/- chemotherapy. Radiotherapy is particularly useful in treating tumours where an adequate clear margin cannot be achieved, and ideally radiotherapy is administered pre-operatively.

Good prognostic variables include:

small size
located in extremity
younger age < 20 years of age
solid homogenous mass
presence of calcification
biphasic histology (controversial)

Poor prognostic variables include:

large size (> 5cm) : most important factor
located in the trunk or head and neck
older patients
cystic / haemorrhagic components
marked heterogeneity
histology
- poorly differentiated histology
- rhabdoid cells
- extensive tumor necrosis
- high nuclear grade
- p53 mutations
- high mitotic rate (> 10 mitoses/10 high-power field)