Synovial sarcoma

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Tumor biology and incidence

  • Malignant soft tissue sarcoma that usually involves tendon sheaths, bursae, and joint capsule; rarely invades joint itself
    • Joint cavity involvement has been reported to occur in 5-10% of patients
  • Commonly found in close proximity to large joints of extremities
  • May arise from the pluripotential mesenchyme of the limb bud
  • Fourth most common sarcoma (primary three being MFH, liposarcoma, and rhabdomyosarcoma)
    • Most common soft tissue sarcoma of foot and second most common malignant soft-tissue tumor in pediatric patients
    • Accounts for 5-10% of all soft tissue sarcomas; approximately 800 new cases diagnosed each year in the US

Age

Adolescents and young adults between ages 15 and 40

Gender

M:F = 1.2:1

Presentation

  • Roughly >50% of cases present with a palpable, deep swelling or mass; may be associated with pain or tenderness
    • Unusual for pain/tenderness to be only presenting symptom
  • Radiating or referred pain and paresthesias as secondary symptoms due to local mass effect on nerves
  • Typically a delayed presentation, often confused with recent or remote history of injury to extremity
  • May be confounded by mechanical symptoms at involved joint (ie, snapping, locking, JLT at knee joint)

Physical findings

  • Tenderness or pain associated with a deep tissue mass
  • Tissue mass may be mobile or fixed and of variable size
  • Masses are generally located in the extremities
  • Typically chronic symptoms specific to involved joint
  • Mechanical symptoms involving joint not uncommon
  • Paresthesias may be involved as well

Plain films

  • X-ray essential - roughly 25% of patients present with images containing evidence of calcification of tumor mass, varying from stippling to dense/amorphous deposition
  • Tumors may present with lobulated, round, or oval radiographic shadow on X-ray
Site

Knee most common site, followed by ankle/foot, elbow, and upper arm/shoulder

Size
  • May grow to >15 cm, but on average measure 3 to 5 cm
  • Lesions <1 cm have been reported.
Tumor effect on bone
  • Underlying bone tends to be uninvolved, although 15-20% of patients have bone erosion, invasion, or periosteal reaction
  • Massive bony destruction uncommon and seen only with large, extensive, poorly differentiated tumors
  • Changes in bone are secondary to pressure atrophy

CT scan

Useful to delineate size, calcification, and bony invasion/destruction (if present)

MRI

  • Nonspecific; has appearance of a benign lesion
  • Seen in extra-articular locations
  • Usually shows heterogeneous septated mass of low to intermediate signal on T1-weighted sequences and high signal on T2 images
  • High signal on T1 and T2 images has been reported, hemorrhage within tumor may be evident as fluid levels

Differential diagnosis

  • Myositis
  • Hematoma
  • Synovitis
  • Tendonitis
  • Bursitis

Natural history

  • Insidious, slow growth of tumor with potential for metastasis
  • Primary location for metastasis is lungs; however, lymph node involvement occurs in 3-27% of patients
  • Time to surgery from onset of symptoms generally 2-4 years
  • Poor prognosis; some molecular markers can be used to as prognostic indicators to predict course of disease

Pathology

  • Initially named due to a similar appearance to normal synovium microscopically; however, the tumor bears no resemblance to synovium, either ultrastructurally or immunohistochemically
  • Gross: Yellow to gray-white
  • Four distinct histologic types: classic biphasic, monophasic fibrous, monophasic epithelial, and poorly differentiated
    • Classic biphasic
      • Epithelial cells characterized by large, round, or oval vesicular nuclei and abundant pale-staining cytoplasm with distinctly outlined cellular borders
      • Cells present in solid cords, whorls, or nests
      • Spaces lined by single layer of epithelial cells bearing close resemblance to normal synovium
      • Surrounding component consists mostly of well-oriented, spindle-shaped cells of uniform appearance with small amounts of indistinct cytoplasm and oval dark-staining nuclei
      • Generally indistinguishable from fibrosarcoma except for absence of herringbone pattern
    • Monophasic fibrous
      • Positive immunostaining of spindle cells for keratin and epithelial membrane antigen
      • Shares identical morphological features to spindle-cell portion of the biphasic type
    • Monophasic epithelial
      • Exhibits glandular structures lined with epithelial cells
      • Differential diagnosis includes metastatic and adnexal carcinoma, malignant melanoma, malignant epithelioid schwannoma, and epithelioid sarcoma
    • Poorly differentiated
      • Solidly packed oval or spindle-shaped cells of small size that seem to be intermediate in appearance between epithelial and spindle cells
  • Less-differentiated variants grow more rapidly and tend to be poorly circumscribed, with multiple areas of hemorrhage, necrosis, and cystic formation
    • Implies a worse prognosis; however, large studies have not shown that the histologic analysis of synovial sarcoma has a significant prognostic or therapeutic importance
  • Distinguishing feature from other sarcomas is potential presence of two distinct cell types: epithelial (seen in carcinoma) and fibrosarcoma-like spindle cells

Immunohistochemistry and cytogenetics

  • Both cellular elements show reactivity for cytokeratins and, less intensely, for epithelial membrane antigen
  • Positive immunostaining for keratin seen in nearly all biphasic synovial sarcomas and many of the monophasic fibrous type
  • Other positive stains include desmoplakin, Leu-7, and S-100 protein
  • Characteristic balanced translocation between chromosomes X and 18, t(X;18)(p11.2;q11.2) in most cases
    • Translocation fuses the SYT gene from chromosome 18 to either of two highly homologous genes at Xp11, SSX1, or SSX2
    • SYT-SSX1 and SYT-SSX2 function in aberrant transcriptional regulation.
    • Translocation occurs in all histological variants

Diagnosis and treatment

  • Most commonly misdiagnosed soft tissue tumor, due to variable size, slow growth, benign appearance on imaging studies, and similarity of pain symptoms seen with simple trauma
  • Imaging relatively non-specific; biopsy needed to confirm diagnosis
  • Diagnosis based on clinical and imaging information such as patient age, symptoms, location of the tumor, presence of calcification or ossification, and MRI appearance
  • Treatment is includes surgical wide resection
    • No minimal acceptable margin has been established
    • However, there is susceptibility for microscopic infiltration of tumor cells into pseudocapsule of tumor
    • Suggested margins include 1-3 cm in adults (may be impossible in most children and in deep-seated tumors adjacent to bone or important neurovascular structures)
  • Adjuvant chemotherapy controversial, not standard practice
    • However, preoperative systemic or isolated limb perfusion chemotherapy may enable limb-sparing surgery in most patients
  • Local control may be improved with perioperative radiation, especially in patients with marginal rather than wide-resection 
    • Recommendations for radiation depend on primary site and size of tumor, histology, patient age, and extent of disease before and after surgical resection
    • In general, with conventional fractionation (1×1.8 to 2 Gy/day), radiotherapy doses between 50 and 70 Gy should be administered
    • In most cases, the radiation field includes the initial tumor plus 2- to 3-cm margins
  • Chemotherapy and/or radiation typically reserved for high risk patients

Prognosis

  • Factors determining high risk and associated with a worse prognosis include:
    • Size >5 cm
    • Deep seated
    • Inadequate surgical resection
    • Local recurrence
    • Patient age >20 years
    • Monophasic subtype
    • Mitotic activity of >10 per high power field
  • Variant of translocation may imply prognostic value: Two studies with cohorts of 104 and 243 synovial sarcoma patients showed the overall 5-year metastasis-free survival for patients with SYT-SSX1 ranged from 42% to 53%, versus 73% to 89% for those with SYT-SSX2.
  • Non-metastatic synovial sarcoma: 5-year disease-specific survival rate was 62.9% in one study of 128 patient . 5-year overall survival and the 5-year metastatic-free survival rate in another study of 271 patients was 71% and 51%, respectively. 

Recommended reading

Siegel HJ, et al. Synovial sarcoma: clinicopathologic features, treatment, and prognosis. Orthopedics. 2007 Dec;30(12):1020-5.
Schwartz HS. OKU- Musculoskeletal Tumors 2.  2007.


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