Learning Objectives
Master the specific Immunohistochemical (IHC) Stains used to determine the primary site of origin for metastatic or poorly differentiated tumors. Identify the cellular targets—from epithelial cytokeratin to neural crest S-100—and their corresponding diagnostic value.
1. Epithelial & Mesenchymal Framework
The first step in tumor classification is often distinguishing between carcinomas (epithelial) and sarcomas (mesenchymal).
| Stain | Target | Tumors Identified |
|---|---|---|
| Cytokeratin | Epithelial cells | Epithelial tumors (e.g., Squamous cell carcinoma). |
| Vimentin | Mesenchymal tissue | Mesenchymal tumors (Sarcomas), endometrial CA, RCC, meningioma. |
2. Neural, Glial & Neural Crest Stains
Tumors of the nervous system and those derived from the neural crest have highly specific markers that aid in sub-classification.
| Stain | Target | Tumors Identified |
|---|---|---|
| S-100 | Neural crest cells | Melanoma, Schwannoma, Langerhans cell histiocytosis. |
| GFAP | NeuroGlia | Astrocytoma, Glioblastoma. |
| Neurofilament | Neurons | Neuronal tumors (e.g., Neuroblastoma). |
Activity:
3. Specific Lineage & Vascular Markers
Certain stains pinpoint the exact cell type, such as muscle fibers or the endothelial lining of blood vessels.
| Stain | Lineage | Key Tumors |
|---|---|---|
| Desmin | Muscle | Muscle tumors (e.g., Rhabdomyosarcoma). |
| PECAM-1 (CD31) | Endothelial cells | Vascular tumors (e.g., Angiosarcoma). |
| Chromogranin / Synaptophysin | Neuroendocrine | Small cell lung CA, Carcinoid, Neuroblastoma. |
| TRAP | Acid phosphatase | Hairy cell leukemia. |
Activity
High-Yield Mnemonics & Tips:
- S-100: Remember that Melanocytes come from the Neural Crest. This makes S-100 the classic marker for Melanoma.
- Desmin: Think of “Des-muscle” — it marks muscle-derived tumors.
- GFAP: The “G” stands for Glia. It helps identify brain tumors like glioblastomas and astrocytomas.
- TRAP: Remember “TRAP the Hairy cell.” This stain is very specific for Hairy Cell Leukemia.