Synovial Sarcoma

Synovial sarcoma is a distinct high-grade soft tissue sarcoma characterised by a specific chromosomal translocation and a biphasic or monophasic histological pattern. Despite its name, it does not arise from synovium — it originates from primitive pluripotent mesenchymal cells. It is one of the most common soft tissue sarcomas in young adults and has a unique combination of features that make it a high-yield topic in orthopaedic oncology.

• Accounts for approximately 5–10% of all soft tissue sarcomas; third most common STS overall
• Age: most common in adolescents and young adults (15–40 years); peak incidence in third decade; one of the most common STS in patients under 30 years
• Gender: slight male predominance
• Location: lower extremity in 70% of cases — especially around the knee (popliteal fossa, thigh, lower leg); upper extremity (15%); trunk; rarely intra-articular
• Despite the name, synovial sarcoma does NOT arise from synovial tissue — it arises from primitive mesenchymal cells near but not within joints; the name reflects histological resemblance to synovium, not cellular origin; true intra-articular synovial sarcoma is rare
• Prognosis: 5-year overall survival approximately 50–60% for localised disease; 15–20% for metastatic disease; high rate of late metastasis (lung) up to 10–20 years after initial treatment

• Pathognomonic translocation: t(X;18)(p11;q11) — present in virtually all (>95%) synovial sarcomas; this translocation fuses the SS18 gene (chromosome 18) with SSX1, SSX2, or SSX4 genes on the X chromosome; produces SS18-SSX fusion oncoproteins that disrupt chromatin remodelling and drive oncogenesis
• Fusion subtype: SS18-SSX1 → more commonly biphasic histology; SS18-SSX2 → more commonly monophasic histology; SSX2 fusion may be associated with slightly better prognosis in some studies
• Detection: FISH (fluorescence in situ hybridisation) or RT-PCR on tumour tissue; confirmation of t(X;18) is diagnostic and clinically useful when histology is uncertain
• This translocation is highly specific — not found in other soft tissue tumours; molecular confirmation is particularly important in monophasic variants which can be histologically confused with fibrosarcoma, MPNST, or spindle cell carcinoma

• Immunohistochemistry: positive for EMA and cytokeratins (epithelial markers — unusual for a soft tissue sarcoma, reflecting epithelial differentiation); TLE1 (highly sensitive and specific for synovial sarcoma); CD99 positive; S100 negative; desmin negative
• TLE1 positivity: highly sensitive (approximately 90%) and specific immunohistochemical marker for synovial sarcoma; useful in monophasic variants and on small core biopsies; combined with t(X;18) FISH is essentially diagnostic

• Presentation: slowly growing, often painful soft tissue mass near a joint in a young adult; long history before diagnosis (months to years) as it may be mistaken for a benign ganglion cyst or soft tissue haematoma
• Pain: present in approximately 50% at presentation — unlike most sarcomas which are painless; the presence of pain near a joint in a young patient with a mass should heighten suspicion
• Plain radiograph: soft tissue mass with calcification in approximately 25–30% of cases (punctate or stippled calcification within the mass) — an important radiological clue; periosteal reaction of adjacent bone in some cases
• MRI: the imaging cornerstone — heterogeneous mass with "triple signal" pattern on T2: areas of high signal (haemorrhage/necrosis), intermediate signal, and low signal (fibrous or calcified areas); well-defined pseudocapsule; close proximity to joint or neurovascular bundle; no intra-articular extension in most cases
• CT chest: mandatory for staging — lung metastases at presentation in approximately 15–25% of cases; solitary or multiple pulmonary nodules
• PET-CT or whole-body MRI: for complete staging; lymph node metastases in approximately 5–10% (higher than most STS)

• Synovial sarcoma has one of the highest rates of late recurrence and late metastasis among STS — metastases can appear 10–20 years after initial treatment; long-term surveillance (chest CT) is mandatory for at least 10 years; patients must be counselled about this risk

• Surgery: wide excision with tumour-free margins is the cornerstone of treatment — achieving an Enneking wide margin; limb-salvage in most cases; amputation reserved for cases where wide margins cannot be achieved without sacrifice of critical neurovascular structures
• Radiotherapy: adjuvant radiotherapy (pre-operative 50 Gy or post-operative 60–66 Gy) reduces local recurrence; synovial sarcoma is relatively radiosensitive compared to many other STS; combined wide excision + RT achieves local control in approximately 85–90%
• Chemotherapy: synovial sarcoma is one of the more chemosensitive STS — doxorubicin + ifosfamide first-line for metastatic or advanced disease; neoadjuvant chemotherapy considered for large (>5 cm) high-risk tumours; response rates of approximately 30–50% with ifosfamide-based regimens
• Ifosfamide sensitivity: synovial sarcoma is more sensitive to ifosfamide than many other STS — high-dose ifosfamide monotherapy can achieve responses in relapsed disease; this is an important management pearl distinguishing synovial sarcoma from less chemo-responsive STS subtypes
• Afami-cel (afatinib): T-cell receptor (TCR)-based cellular therapy targeting SSX antigen in HLA-A*02:01 positive patients; first tumour-specific TCR-T cell therapy approved for synovial sarcoma; approved by FDA 2024; represents a new treatment paradigm

• Synovial sarcoma vs ganglion cyst: the most dangerous diagnostic mistake — synovial sarcoma near a joint in a young adult is frequently misdiagnosed as a ganglion cyst and excised without imaging; the lesion shells out through the pseudocapsule (marginal excision), the reactive zone is left behind with residual tumour; always MRI any presumed ganglion before excision if there is diagnostic doubt, if it is deep to fascia, if it is >5 cm, or if it is in an atypical location
• Lymph node surveillance: synovial sarcoma has the highest lymph node metastasis rate among extremity STS (approximately 5–10%); sentinel lymph node biopsy is not routinely performed but regional lymph nodes should be assessed by imaging; clinically suspicious lymph nodes should be biopsied or dissected
• Management of pulmonary metastases: surgical resection of isolated pulmonary metastases improves survival in selected patients — patients with limited (1–3) metachronous metastases and good performance status should be considered for pulmonary metastasectomy; 5-year survival after complete resection approximately 30–40%; multiple metastases or incomplete resection — systemic chemotherapy
• Recurrence pattern: local recurrence in approximately 25–40% after wide excision alone; re-resection for local recurrence if technically feasible; repeated pulmonary metastasectomy for isolated lung recurrence in selected patients

• Synovial sarcoma: NOT from synovium; primitive mesenchymal origin; near joints in young adults; lower extremity around knee most common
• Translocation: t(X;18) — SS18-SSX fusion; present in >95%; pathognomonic; detected by FISH or RT-PCR
• Histology: biphasic (epithelial glands + spindle cells) or monophasic (spindle cells only); TLE1 positive; EMA and cytokeratin positive (unusual for STS)
• Calcification on plain radiograph: 25–30% — important clue to diagnosis; stippled/punctate within mass
• Late metastasis: can occur 10–20 years after treatment — long-term chest CT surveillance mandatory for at least 10 years
• Chemosensitive STS: more responsive to ifosfamide than most STS; doxorubicin + ifosfamide first-line for advanced disease
• Diagnostic trap: synovial sarcoma misdiagnosed as ganglion → marginal excision → tumour cells left behind; always MRI before excising deep or atypical "ganglion"
• Surgical treatment: wide excision + adjuvant RT; limb-salvage in most cases
• 5-year survival: approximately 50–60% localised; 15–20% metastatic; poorly differentiated subtype worst prognosis
• Lymph node metastasis: 5–10%; higher than most STS; assess regional nodes on staging imaging