Synovial Sarcoma

Synovial sarcoma is a malignant soft tissue tumour defined by the t(X;18) SS18-SSX chromosomal translocation. Despite its name it does not arise from joint synovium, and it shows no synovial differentiation. It accounts for 5-10% of all soft tissue sarcomas and is one of the commonest non-rhabdomyosarcoma soft tissue sarcomas of adolescents and young adults.

Retire the old ranking. The line "third commonest after liposarcoma and malignant fibrous histiocytoma" fails on two counts. Liposarcoma and undifferentiated pleomorphic sarcoma peak in the fifth to seventh decades, so the ranking does not describe the young population synovial sarcoma actually affects, and malignant fibrous histiocytoma is an abandoned diagnosis, reclassified as undifferentiated pleomorphic sarcoma.

The name. The tumour was named in the early 20th century for its microscopic resemblance to synovial tissue and its frequent occurrence near joints. Modern molecular studies confirm that it arises from primitive mesenchymal cells, and although 60-70% lie near a joint, it can occur anywhere: up to 30% occur away from joints entirely.

Who. Adolescents and young adults, with a peak at 15-35 years and an equal sex distribution. Children under 15 account for 15-20% of cases, and the tumour is uncommon over 50.

Where. The lower limb carries 60-70%, most often around the knee, ankle and foot, and the upper limb 15-20% (hand, wrist, forearm). The table breaks the sites down further, with the typical presentation and prognosis of each.

Origin. Synovial sarcoma arises from pluripotent mesenchymal stem cells with the capacity for epithelial differentiation.

The translocation. The t(X;18) translocation is present in over 95% of synovial sarcomas and is pathognomonic. It fuses the SS18 gene (formerly SYT) on chromosome 18 to SSX1 or SSX2 on the X chromosome. The SYT-SSX fusion protein functions as an aberrant transcription factor, disrupting chromatin remodelling and gene expression, and it drives the tumour.

The fusion variants. There are two main fusion types, and each travels with a histological pattern:

• SYT-SSX1, about 61-65%: almost all biphasic tumours carry it
• SYT-SSX2, about 35-37%: almost all SSX2 tumours are monophasic, and the male-to-female ratio shifts to roughly 1:2
• SYT-SSX4, a rare variant, under 1%

Fusion type carries prognostic information that is real but conditional, and it is not the same variable as histology; both points are set out under Prognosis.

Growth and spread. The tumour is slow-growing initially, then progresses rapidly, which is why symptoms have often run for months to years before diagnosis. It infiltrates along fascial planes and neurovascular bundles, so wide margins are essential and neurovascular sacrifice may be needed. Metastatic potential is high: 40-50% metastasise within 5 years, and 5-10% already have metastases at presentation. The pattern of metastasis:

• Lung - 90% of metastases, the most common site
• Bone - 10-20% (vertebrae, pelvis, ribs)
• Lymph nodes - 5%, rare but a higher rate than other sarcomas

The WHO recognises three histological subtypes.

Biphasic (20-30%). The classic appearance, with epithelial and spindle cell components both clearly visible. Gland-like structures of columnar or cuboidal cells are intimately admixed with monotonous spindle cells in fascicles, yet the two components remain clearly distinct. When both are prominent the pattern is diagnostic, which makes the diagnosis easier. Almost all biphasic tumours carry SYT-SSX1 (all 12 in Kawai 1998).

Monophasic (60-70%). Spindle cells only on H&E: monotonous, uniform cells with a haemangiopericytoma-like vascular pattern. The epithelial component is inconspicuous or absent on routine staining but may be demonstrated by immunohistochemistry. SYT-SSX2 is enriched here (all SSX2 tumours in Kawai 1998 were monophasic).

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Poorly differentiated (10-15%). A high-grade variant with areas of dedifferentiation. High-grade round/small-cell sarcoma areas make up more than 50% of the tumour, with pleomorphic cells, a high mitotic rate and sometimes necrosis. It can resemble other high-grade sarcomas, and the t(X;18) translocation, still present, is what confirms the diagnosis.

Biphasic against monophasic is not a prognostic question. Histological subtype had no impact on survival in the Ladanyi series (P=0.8). Where biphasic tumours seem to do worse, the outcome is tracking the SSX1 fusion and its association with metastatic presentation, not the biphasic pattern; the subtype that behaves badly is the poorly differentiated one.

Immunohistochemistry

The conventional panel. Of these markers, nuclear TLE1 staining is the most sensitive, but none is specific: TLE1 in particular is also positive in MPNST and some other sarcomas. On a conventional panel, molecular confirmation of the fusion by RT-PCR or FISH remains the gold standard, and it should be requested on every suspected case.

The fusion-specific antibody. The genuine recent advance is a monoclonal antibody to the SS18-SSX fusion junction, clone E9X9V. It gives strong, diffuse nuclear staining only when the fusion protein is present, and in validation studies its specificity approaches 100% and its sensitivity is roughly 95% for synovial sarcoma, clearly outperforming TLE1. A companion SSX (C-terminus) antibody can be run alongside it, and concordant positivity further increases confidence.

What this changes. A positive fusion-specific stain can confirm synovial sarcoma on a small core biopsy within hours, and in many laboratories it now substitutes for FISH/RT-PCR in classic cases. Molecular testing is then reserved for equivocal stains or for when the fusion variant (SS18-SSX1 versus SSX2) is wanted. Asked how you would confirm the diagnosis, the modern answer is no longer just "TLE1 then molecular".

Differential Diagnosis

Monophasic synovial sarcoma is a classic mimic of other monomorphic spindle cell tumours. The combination of clinical context, immunohistochemistry and molecular testing resolves the differential.

History. Most patients present with a painless mass (60-70%). The mass is painful in 30-40% (larger tumours, neurovascular involvement), and joint symptoms (pain, stiffness) are reported in 30% when the tumour lies near a joint. Growth is slow, over months to years before diagnosis; rapid growth suggests a high-grade or poorly differentiated tumour.

Delay. The average time from symptom onset to diagnosis is 2-4 years. The slow growth and the location near a joint lead to misdiagnosis as a benign lesion such as a ganglion cyst, lipoma or tendon sheath tumour.

Examination. The median size at diagnosis is 5cm. The mass is firm to hard, unlike a soft lipoma, and deep lesions are often fixed to fascia; 30-40% are tender on palpation. Skin changes are rare except over superficial lesions, and large tumours may show venous engorgement. A complete examination provides the baseline for treatment planning, so document:

• The location of the mass relative to the joint, and its relation to neurovascular structures
• Range of motion of the adjacent joint
• Neurovascular status distal to the mass, including signs of nerve compression or vascular compromise
• Muscle strength and function
• Regional lymph nodes (nodal metastases are rare but possible, in 5%)
• The chest (lung metastases are uncommon at presentation)

The standard is multimodal. Wide surgical resection, chemotherapy and radiotherapy together. Unlike many soft tissue sarcomas, synovial sarcoma is chemosensitive, and ifosfamide-based chemotherapy improves survival, particularly for high-risk patients (larger than 5cm, high grade, deep). How large that benefit is remains the most debated question in the disease (see Controversies), so the intensity of each component is set by risk:

• Surgery - wide excision, with a 2cm margin as the goal
• Chemotherapy - ifosfamide-based, doxorubicin/ifosfamide in the matrix below
• Radiotherapy - 50-66 Gy, before or after surgery

Intensity follows risk. The full multimodal package is the high-risk pathway, with chemotherapy given both before and after surgery. For a low-risk tumour the margin aimed for is 1-2cm, adjuvant chemotherapy is controversial and considered case by case, and radiotherapy is given for a close margin or a local-risk concern.

Algorithm

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Preoperative or postoperative radiotherapy. Local control is equivalent either way (O'Sullivan, NCIC SR2), so the choice is a trade of early wound morbidity, worse with preoperative treatment, against late functional morbidity, worse with postoperative treatment. It is individualised at the sarcoma MDT. Preoperative radiotherapy is often favoured for large or deep tumours near critical structures, where field size and late function matter most.

Planning. Review the MRI with the radiology and surgical team to define tumour extent and neurovascular involvement, plan the resection margins (2cm is the goal) and decide whether vascular or nerve reconstruction will be needed. Then plan the approach and the reconstruction:

• Mark the biopsy tract for en bloc excision and plan the incision to incorporate it
• Avoid transverse incisions, which limit extensile approaches, and consider drain sites as contaminated
• Anticipate the size and depth of the defect and arrange soft tissue coverage (flap, graft)
• Plan tendon transfers if muscle resection is needed, and have vascular surgery available if a vessel must be reconstructed

Resection. A longitudinal incision along the biopsy tract, proximal and distal control of the neurovascular structures, and the tumour extent identified from the preoperative MRI while critical structures (vessels, nerves, bone) are protected. The tumour is removed en bloc with a cuff of normal tissue and the biopsy tract, aiming for a 2cm margin in all dimensions and accepting a closer margin on critical structures. Major vessels and nerves are preserved if they are not encased and sacrificed if margin clearance demands it.

The specimen. Orient it for the pathologist:

• Mark the margins with sutures (superior, lateral, deep)
• Photograph it with orientation markers
• Request margin assessment on all surfaces
• Send fresh tissue for molecular testing if the initial biopsy was inconclusive

Closure. Primary closure if possible, local flaps for moderate defects and free tissue transfer for large ones, with a drain for dead space. Functional reconstruction is essential for quality of life.

Margins and re-excision. The final margin sets the local recurrence risk and the next step.

Recovery. From the ward to the end of adjuvant treatment:

• Days 0-7 (inpatient) - watch the wound (flap viability, haematoma, infection); LMWH for DVT prophylaxis; multimodal analgesia; remove the drain when output is under 30ml per 24 hours
• Weeks 2-6 (clinic) - check wound healing and remove sutures at 2-3 weeks; review the final pathology (margins, grade, molecular) and plan adjuvant chemotherapy and/or radiotherapy
• Months 1-6 (adjuvant) - chemotherapy, 4-6 cycles over 3-4 months; radiotherapy, 5-7 weeks to 50-66 Gy; monitor toxicity and maintain physiotherapy throughout

Surveillance. The high metastatic rate requires vigilant imaging. Review is most frequent in years 1-2, the high-risk period, and 70% of recurrences occur in the first 3 years; late recurrences are possible, and late metastases can occur at 10 years or more.

Suspected metastasis. High-resolution lung CT for nodule detection, PET-CT when recurrence is suspected to assess resectability, and a bone scan for bone pain, bone being the site of 10-20% of metastases.

Fusion type, quoted precisely. In the 243-patient multi-institutional Ladanyi series, unadjusted overall survival was better with SYT-SSX2 (median 13.7 years, 5-year 73%) than with SYT-SSX1 (6.1 years, 53%; P=0.03). The effect was no longer significant once stratified for disease status at presentation, partly because SSX1 tumours presented with metastatic disease more often. On Cox regression across the whole cohort, only disease status and tumour size were independently predictive.

Where fusion does count. Within the localised-disease subgroup, fusion type became the single independent significant factor (P=0.04), although the unadjusted difference there was itself only a trend (P=0.08). Keep fusion apart from histology: the two are tightly associated, but a viva answer that says "biphasic does worse" is quoting the association and missing the finding.

Global Epidemiology

Side-by-Side Guidance from Major Societies

High- vs Limited-Resource Practice Variation

Adjuvant chemotherapy. The single most debated issue. The Italian trial showed a survival benefit in high-risk extremity sarcoma, but pooled meta-analyses and longer follow-up are less consistent, and the EORTC 62931 trial of doxorubicin plus ifosfamide showed no overall-survival benefit. Most centres reserve chemotherapy for young patients with large, deep, high-grade tumours, decided case by case at MDT.

The prognostic weight of fusion type. SYT-SSX1 versus SSX2 correlates with histology and survival in retrospective series, yet on multivariate analysis tumour size and stage often dominate. Fusion type is best regarded as one input among several rather than a standalone treatment driver.

Lung metastasectomy. Resection of pulmonary metastases is widely practised for oligometastatic, resectable disease and is associated with longer survival in selected patients, but no randomised trial confirms a survival benefit. Patient selection (resectability, disease-free interval, fitness) is decisive.

Access to immunotherapy. Engineered TCR T-cell therapy (afami-cel) is a genuine advance but is limited to HLA-A*02-positive, MAGE-A4-expressing tumours and to specialist centres, raising equity-of-access questions globally.