Table 2. Comparison of spindle cell thymoma and its closest mimickers.
| Tumor type | Clinical | Histology | IHC | Molecular | Prognosis |
|---|---|---|---|---|---|
| Spindle cell thymoma | 5th–7th decade; M = F |
Heterogeneous; classic type with storiform or diffuse growth; bland spindle/oval cells; scant lymphocytes | CK+, CK5/6+, p63/p40+, Pax8 (polyclonal)+ | GTF2I mutation | Good |
| Spindle cell carcinoid tumor | 5th decade; M > F | Monotonous spindle cells arranged in organoid patterns | CK+, synaptophysin+, chromogranin+, CD56+ | – | Poor |
| Sarcomatoid thymic carcinoma | 6th decade; M > F | Solid or nested proliferation of malignant spindle cells; dense collagenous stroma; ± areas of spindle cell thymoma | CK+, CK5/6+, CK7+, p63/p40+, Pax8 (polyclonal)+, CD5+/−, c-kit+/− | – | Poor |
| SFT | 6th decade; M = F | Bland spindle cells in patternless pattern; keloid-like stroma; staghorn-like vasculature | CD34+, STAT6+, CK− | NAB2::STAT6 fusion | Good |
| Synovial sarcoma | 4th decade; M > F | Cellular proliferation of monotonous spindle cells; high mitotic activity; inconspicuous stroma; mast cells; staghorn-like vasculature | TLE1+, SS18-SSX+/SS18_CT+, CK+/− | SYT::SSX fusion | Poor |
| Dedifferentiated liposarcoma | 5th–6th decade; M = F |
Areas of well differentiated liposarcoma and non-lipogenic sarcoma (spindle cell proliferation with variable mitotic activity, nuclear pleomorphism) | MDM2+ | MDM2 amplification | Poor |
| Angiosarcoma | 3rd–4th decade; M = F |
Spindle cell proliferation with variable degrees of vasoformation; increased mitotic activity; necrosis | CD31+, CD34+, ERG+, CK+/− | – | Poor |
| Schwannoma | 4th–6th decade; M = F |
Bland spindle cells with wavy appearance; Antoni A and B areas; hyalinized blood vessels; lymphoid aggregates | S100+, SOX10+ | – | Good |
| Inflammatory myofibroblastic tumor | Children/young adults; M = F | Spindle cells with indistinct cell borders; fascicular growth pattern; mixed chronic inflammatory cell infiltrate; low mitotic activity | SMA+/−; ALK+/− | ALK gene rearrangement | Good |
| Sarcomatoid mesothelioma | Older adults; M > F |
Spindle cells with nuclear atypia, prominent nucleoli, increased mitotic activity | CK+; other mesothelioma markers variable | – | Poor |
| Intimal sarcoma | 5th decade; F > M |
Spindle cells with varying degrees of atypia and cellularity; subset with distinct differentiation; heterologous elements possible | Variable | MDM2 amplification | Poor |
SFT, solitary fibrous tumor; M, male; F, female; IHC, immunohistochemistry; CK, cytokeratin; ERG, erythroblast transformation specific related gene; SMA, smooth muscle actin; ALK, anaplastic lymphoma kinase.