Role of Immunohistochemistry (IHC) in Soft Tissue Sarcomas

1. Differentiating Sarcoma from Other Anaplastic Malignancies

• Vimentin is broadly positive in almost all mesenchymal tumors, making it a non-specific but useful screen for STS when other markers are negative.
• Cytokeratin positivity can occur in synovial sarcoma and epithelioid sarcoma - this is an important pitfall that can mimic carcinoma.

2. IHC Markers by STS Subtype

Myogenic Tumors

• Desmin+ (skeletal muscle marker)
• MyoD1+ and Myogenin (myf-4)+ - these are the most specific nuclear markers for skeletal muscle differentiation
• Muscle-specific actin (MSA)+
• Note: Alveolar RMS (ARMS) carries t(2;13) fusing PAX3-FOXO1, confirmable by FISH or RT-PCR

• Smooth muscle actin (SMA)+
• Desmin+
• h-Caldesmon+ (highly specific for smooth muscle)

Liposarcoma

• MDM2+ (IHC for MDM2 protein reflects underlying 12q14-15 amplification)
• CDK4+
• Lipoblasts (signet ring cells) on histology
• RB1 deletion by molecular testing

Synovial Sarcoma

• TLE1+ (sensitive nuclear marker, most useful)
• CD99+
• EMA+
• Pan-cytokeratin+ (epithelial component in biphasic type)
• Negative: Desmin, S100, DOG1
• Molecular confirmation: t(X;18) producing SS18-SSX1/SSX2/SSX4 fusion (monophasic variant = SS18-SSX2; biphasic = SS18-SSX1)

Malignant Peripheral Nerve Sheath Tumor (MPNST)

• S-100+ (but often focal/patchy, unlike schwannoma where it is diffuse)
• SOX10+
• May arise de novo or in the setting of NF1
• Loss of H3K27me3 (trimethylation at histone H3 lysine 27) is a useful marker for high-grade MPNST

Angiosarcoma / Vascular Tumors

• CD34+ (endothelial/vascularity)
• CD31+ (more specific endothelial marker)
• ERG+ (nuclear endothelial marker, highly sensitive)
• FLI1+
• Association: Stewart-Treves syndrome (post-mastectomy lymphedema)

Solitary Fibrous Tumor (SFT)

• STAT6+ (nuclear - reflects NAB2-STAT6 gene fusion, highly specific)
• CD34+ (strong diffuse)
• Negative: TLE1, EMA, pan-keratin

Epithelioid Sarcoma

• Loss of INI1 (SMARCB1) - characteristic loss of nuclear staining
• Keratin+
• EMA+
• CD34+ (especially proximal type)
• Most common STS of the hand in young adults

Fibromatosis (Desmoid Tumor)

• Beta-catenin+ (nuclear staining, reflects CTNNB1 mutation or APC mutation in FAP-associated cases)
• SMA+

Ewing Sarcoma / PNET

• CD99+ (MIC-2, membranous, "diffuse strong" pattern)
• FLI1+ (nuclear)
• NKX2.2+ (highly specific)
• Molecular: t(11;22) producing EWSR1-FLI1 (most common), confirmed by FISH or RT-PCR

GIST (Gastrointestinal Stromal Tumor)

• DOG1+ (ANO1, highly sensitive and specific)
• CD117 (c-KIT)+
• CD34+
• Negative: Desmin, S100
• Note: GIST was historically misclassified as leiomyosarcoma before molecular characterization

3. IHC in Determining Site of Origin of Metastatic Sarcoma

4. IHC for Prognostic and Therapeutic Targets

• ALK: ALK-rearranged inflammatory myofibroblastic tumor (IMT) - amenable to ALK inhibitors (crizotinib)
• MDM2 amplification (WDLPS/DDLPS): IHC can screen before FISH confirmation
• INI1 loss (epithelioid sarcoma, rhabdoid tumor): predictive of sensitivity to EZH2 inhibitors (tazemetostat)
• PD-L1: Emerging biomarker in undifferentiated pleomorphic sarcoma and angiosarcoma for immunotherapy eligibility

5. Practical Summary Table

Key Limitations to Remember

1. IHC alone is rarely sufficient - molecular/cytogenetic testing (FISH, RT-PCR, NGS) is increasingly required for definitive classification, especially for translocation-associated sarcomas.
2. Cross-reactivity is common - cytokeratin in synovial sarcoma, S100 in clear cell sarcoma (mimics melanoma), CD99 in many tumors.
3. IHC panels must be interpreted in the clinical/morphologic context - no single marker is pathognomonic for most sarcomas.
4. Undifferentiated pleomorphic sarcoma (UPS) is a diagnosis of exclusion: by definition, tumor cells lack lineage-specific IHC expression and diagnostic genetic abnormalities.