Synovial sarcoma is a rare, highly malignant soft tissue tumour in young adults often demonstrating a “triple signal” on T2-weighted MRI sequences.
Description
Synovial sarcoma is a high-grade, malignant tumour of the soft tissues, part of the family of soft tissue sarcomas. Despite its name, it does not typically arise from synovial structures but is so-called due to its histologic resemblance to the synovium. It occurs predominantly in adolescents and young adults.
Pathogenesis
The exact pathogenesis of synovial sarcoma is unclear. It arises from pluripotent mesenchymal cells, not synovial cells as its name suggests. Almost all cases are characterised by a specific chromosomal translocation, t(X;18)(p11.2;q11.2), resulting in the SS18-SSX fusion gene, which appears to be a key driver in its pathogenesis.
Subtypes
Synovial sarcoma is categorised into three main histologic subtypes based on the morphology of the neoplastic cells and the presence of a biphasic or monophasic pattern:
- Monophasic fibrous: Consisting only of spindle cells.
- Monophasic epithelial: Composed entirely of epithelial cells.
- Biphasic: Contains both spindle cell and epithelial components.
Epidemiology, Risk Factors & Associations
- Third most common soft tissue sarcoma in adolescents and young adults (15-40 years of age).
- No significant gender predilection.
- No established environmental or genetic risk factors apart from the characteristic t(X;18) translocation.
Clinical Features
- Presents typically as a slow-growing, deep-seated mass, often in close association with joints.
- Pain or tenderness is relatively common.
- Neurologic symptoms may occur due to nerve compression.
Complications
- Local recurrence occurs in ~30-50% of cases.
- Metastases most commonly occur in the lungs (70 – 94% of cases). Other less common sites include lymph nodes and bones.
Pathological Features
Histopathology
- Macroscopic: Synovial sarcomas are typically well-circumscribed, lobulated masses that may show areas of haemorrhage and necrosis.
- Microscopic: Characterised by monomorphic spindle cells arranged in a fascicular pattern with hemangiopericytoma-like vascular pattern. Biphasic type shows glandular structures lined by cuboidal cells.
Serology
- No specific markers
Biochemistry
- No specific markers
Radiological Features
General Features
- Well-defined, lobulated soft tissue mass, often situated in the deep intermuscular or subfascial planes.
- Commonly demonstrates adjacent bone erosion, cortical scalloping or periostitis.
- Calcification present in ~30% of cases, typically punctate, stippled, or ring-like; more frequently seen in myxoid or ossifying variants.
- Lesion may extend along fascial planes and show infiltration into neurovascular bundles.
- Characteristic triple signal intensity pattern on T2-weighted MRI, considered highly suggestive:
- High signal: necrosis, cystic change, or myxoid stroma
- Intermediate signal: solid tumour cellularity
- Low signal: fibrous tissue, calcification, or haemorrhage
- Mostly located in the extremities (95%), lower more than upper
- Arises near a joint (90%), rather than from the joint itself
XR
- Appears as a non-specific soft tissue mass, often obscured without calcification.
- When present, calcifications are fine, stippled, or speckled, often peripheral.
- May show pressure erosion on adjacent bone or joint margin.
CT
- Non-contrast:
- Soft tissue mass with heterogeneous attenuation
- Calcifications more conspicuous than on radiographs
- Pulmonary metastases may mineralise
- Contrast-enhanced:
- Variable enhancement, ranging from mild to intense
- Enhancement may be heterogeneous, reflecting internal necrosis or cystic components
- Post-contrast perilesional oedema or fascial plane infiltration may be visible
MRI
- T1:
- Lesion is typically isointense or mildly hypointense relative to muscle
- Areas of intralesional haemorrhage may appear hyperintense
- Split fat sign: thin rim of fat around mass when intermuscular or along neurovascular bundle
- T2:
- Heterogeneous hyperintensity, often multilobulated
- Triple signal intensity pattern is highly suggestive:
- High signal (fluid, necrosis, myxoid stroma)
- Intermediate signal (viable tumour tissue)
- Low signal (fibrous bands, calcification, or haemorrhagic debris)
- T1 C+:
- Heterogeneous enhancement, often intense in solid components
- Peripheral or septal enhancement may occur in myxoid variants
- DWI/ADC:
- Restricted diffusion is common due to high tumour cellularity
- Corresponding low ADC values support malignancy
US
- B-mode:
- Well-circumscribed, hypoechoic or heteroechoic mass
- May show internal lobulations or cystic areas
- Colour Doppler:
- Increased internal and peripheral vascularity
- Chaotic flow patterns support malignant vascular supply
- Resistive index (RI) may be >0.7 in highly vascular regions
NM
- PET FDG:
- High FDG avidity, correlating with grade and cellular activity
- Standardised Uptake Value (SUV) often elevated (>5–10), though variable
- Useful for staging and assessing metastatic spread (especially lungs, bone)
- May assist in guiding biopsy to most metabolically active area
Associated Findings
- Lung metastases are the most common site of distant spread, occurring in the vast majority of cases (>80%)
- Modality: High-resolution CT chest is the imaging modality of choice for staging
- Findings: Multiple, bilateral, variable-sized pulmonary nodules; may cavitate in aggressive subtypes
- Bone metastases may occur, particularly in high-grade tumours
- Modality: PET-CT or bone scintigraphy
- Findings: Lytic or mixed lesions, often without sclerosis
- Lymph node involvement is rare but may be seen in large, high-grade or recurrent lesions
- Local recurrence is common and best assessed by MRI surveillance, particularly if resection margins were close or positive
Grading and Staging
The FNCLCC (Fédération Nationale des Centres de Lutte Contre le Cancer) system is used for grading, while the AJCC (American Joint Committee on Cancer) TNM system is used for staging.
Diagnosis
The diagnosis is confirmed by histopathologic examination and identification of the specific SS18-SSX fusion gene by molecular techniques.
Differential Diagnosis
- Fibrosarcoma: Typically occurs in older adults, lacks “triple signal” on MRI, and does not demonstrate the specific chromosomal translocation.
- Leiomyosarcoma: Immunohistochemistry shows smooth muscle markers.
- Hemangiopericytoma: Demonstrate prominent vascular pattern on imaging, but lack the specific chromosomal translocation.
- Neurofibroma: Characteristic target sign on MRI.
Management
Management typically involves surgical resection with a wide margin, followed by radiotherapy. Chemotherapy may be used for metastatic disease.