= (MALIGNANT) SYNOVIOMA = TENDOSYNOVIAL SARCOMA = SYNOVIOBLASTIC SARCOMA = SYNOVIAL ENDOTHELIOMA

= [misnomer related to appearance of cells not their origin] slow-growing expansile malignant tumor originating from mesenchymal tissue (not synovium, named for its histologic resemblance of synovium) with extensive metastatic potential

Origin: pluripotential mesenchymal cell of variable epithelial differentiation; it has been proposed to rename tumor carcinosarcoma / spindle cell sarcoma of soft tissue

Frequency: 4^th most common soft-tissue sarcoma (after malignant fibrous histiocytoma, liposarcoma, rhabdomyosarcoma); 7–10% of all primary soft-tissue sarcomas

Histo: in 90% positive staining for keratin (epithelial marker) pankeratin, EMA, CK7 (absent in malignant peripheral nerve sheath tumor + Ewing sarcoma)

1. biphasic (20–30%): mesenchymal spindle cell + epithelial component usually forming glands
2. monophasic (50–60%): spindle cell component with fascicular interlacing growth pattern predominates
3. poorly differentiated (15–25%): generally epitheloid with high mitotic activity + geographic necrosis

Cytogenetics: t(X;18) translocation + SYT-SSX gene fusion products (identified by FISH / RT-PCR studies)

Median age: 30–38 (84% between 15 and 50) years; M÷F = 1.2÷1.0

• slow-growing occasionally painful palpable soft-tissue mass (2–4 years average duration of symptoms) often mistaken for benign indolent process

Site: in synovial lining / bursa / tendon sheath adjacent to joint (40–50%) / within 5 cm of joint (60–75%); uncommonly intraarticular (in 5–10%)

Location: 80–95% in extremities; pelvis (8%); trunk (7%); head & neck (5%, pharynx); retroperitoneum (0.3%); rare in: chest wall, mediastinum, heart, lung, pleura; usually solitary

1. lower extremity (²/³): thigh, popliteal fossa (most common), hip, foot & ankle (18%)
2. upper extremity (¹/³): elbow, wrist, hands, feet

• large spheroid well-defined soft-tissue mass:
  • homogeneous / heterogeneous dependent on degree of hemorrhage / necrosis
  • amorphous punctate calcifications / ossification (30–40%), often eccentric or at periphery of tumor
    • Calcifications in other soft-tissue connective tissue sarcomas are uncommon!
• lesion about 1 cm removed from joint cartilage
• often indolent nonaggressive appearance of involvement of adjacent bone (11–20%):
  • periosteal reaction
  • bone remodeling (pressure from tumor)
  • invasion of cortex with wide zone of transition
  • infiltration of adjacent soft tissue (infrequent)
• juxtaarticular osteoporosis

CT:

• heterogeneous deep-seated multinodular soft-tissue mass with attenuation slightly less than muscle ← necrosis / hemorrhage (in 50%)
• multinodular morphology with well-defined (53%) / irregular (47%) margins
• areas of lower attenuation represent necrosis / hemorrhage
• predominantly areas of low attenuation mimicking hematoma / cystic mass (in 6%)
• calcifications (in 27–41%)
• bone erosion / marrow invasion (25%)
• heterogeneous enhancement (89–100%)

MR:

• predominantly well-defined mass of homogeneous texture for lesions <5 cm (rare) mimicking a benign process
• prominently heterogeneous soft-tissue mass with SI similar to / slightly higher than muscle on T1WI
• heterogeneously increased SI on T2
• “triple signal intensity” sign on T2WI (in 35–57%) = marked heterogeneity with a mixture of areas of
  1. hypointensity ← calcified / fibrotic collagenized tissue,
  2. isointensity ← solid cellular elements, and
  3. hyperintensity ← hemorrhage / necrosis (in 40%)
• “bowl of grapes” sign = large multilocular multilobulated cystic spaces + prominent hemorrhagic foci separated by septa (67–75%) on T2WI
• fluid-fluid levels (10–25%) ← previous hemorrhage
• bone marrow invasion / cortical erosion (in up to 21%)
• neurovascular encasement (17–24%)

CEMR:

• prominent heterogeneous (83%) / homogeneous (17%) enhancement
• peripheral / nodular enhancement for necrotic tumor
• serpentine vascular channels (1/3)
• initially rapid progressive linear increase in SI followed by washout (60%) / late sustained increase (40%)

US:

• focal nodular round / lobulated hypoechoic solid soft-tissue mass (66%)
• heterogeneous texture with irregular margins

Angio:

• hypervascular tumor displacing native vessels
• arteriovenous shunting (in 24%)

NUC:

• prominently increased uptake on blood flow + blood pool images of bone scan ← increased tumor vascularity
• heterogeneous mild uptake (← mixture of viable + necrotic tissue) perhaps associated with calcifications

PET:

• markedly increased activity with high SUV

Spread: distant metastases develop in 41% within 2–5 years; lung (94%) >lymph nodes (4–18%) >bone (8–11%)

• Metastasis present in 16–25% at presentation

Rx: local excision / amputation + radiation + chemotherapy

Prognosis: local recurrence in 30–50% within 2 years after Rx; 36–76% 5-year survival rate; 20–63% 10-year survival rate

Poor prognosticators: tumor size >5 cm, trunk >peripheral location, poorly differentiated areas