S100 and Melanoma
S100 protein is most strongly associated with melanoma (malignant melanoma). This marker is highly sensitive for melanocytic tumors and serves as both a diagnostic immunohistochemical marker and a serum biomarker for disease monitoring in metastatic melanoma 1, 2.
Diagnostic Applications
Immunohistochemistry
S100 protein, along with SOX10, represents the most sensitive immunohistochemical markers for confirming melanocytic origin in tumors 2. These markers are particularly valuable when:
- Tumors lack typical morphologic features (pigmentation, nesting, pagetoid scatter)
- Distinguishing melanoma from other malignancies in metastatic settings
- More specific melanocytic markers (Melan-A/MART1, HMB45, tyrosinase) may be used for confirmation, though they are less sensitive 2
Important caveat: S100 is not entirely specific to melanoma. The marker can also be positive in 1, 3:
- Clear cell sarcoma
- Malignant peripheral nerve sheath tumors (MPNST)
- Gliomas
- Schwannomas and neurofibromas 4
- Occasionally in breast carcinomas (48% in one study) 5
Serum Monitoring
Rising serum S100 levels provide the most accurate blood test for monitoring melanoma progression 6, 7:
- Stage IV disease: 79.4% of patients with disseminated melanoma show elevated serum S100 8
- Stage III disease: Only 21.4% of patients with lymph node metastases have detectable levels 8
- Stage I/II disease: Rarely elevated (4% positivity) 8
S100 has higher specificity for disease progression than LDH, making it the preferred serologic marker when blood testing is performed during follow-up 6, 7.
Other Cancers with S100 Expression
While melanoma shows the strongest association, S100 positivity occurs in other malignancies:
Neural Tumors
- Benign nerve sheath tumors (neurofibromas, schwannomas) show extensive S100 expression 9
- Loss of S100 expression in these tumors is actually worrisome, suggesting transformation to higher-grade lesions like MPNST 9
- Only 50% of malignant schwannomas retain S100 positivity, indicating loss correlates with dedifferentiation 4
Other Malignancies (Less Common)
- Breast carcinoma: 48% show some S100 positivity, particularly lobular (60%) and medullary (80%) subtypes 5
- Lung adenocarcinoma: S100A2 and S100A4 show differential expression patterns 10, 11
- Colorectal cancer: Various S100 family members involved in progression 12
Clinical Pitfalls
False positives can occur with:
- Liver cirrhosis (63% show elevated serum S100) 13
- Renal failure (45% show elevation) 13
- Liver metastases from non-melanoma cancers 13
When evaluating metastatic disease of unknown primary, S100 positivity should prompt consideration of melanoma but requires correlation with additional markers (SOX10, Melan-A, HMB45) and clinical context to avoid misdiagnosis 1, 3, 14.
The combination of high sensitivity for melanoma, utility in both tissue diagnosis and serum monitoring, and established prognostic value makes melanoma the cancer most definitively associated with S100 protein expression.