Merkel Cell Carcinoma Lecture Notes

Overview

• Merkel Cell Carcinoma: Also known as primary cutaneous neuroendocrine carcinoma.
  • Typically arises in sun-exposed areas in older adults.
  • Recognized for its aggressive behavior and high chance of metastasis.
  • Differentiation from basal cell carcinoma is crucial due to differences in treatment and prognosis.

Characteristics of Merkel Cell Carcinoma

• Appears as a "blue tumor" in the dermis, often confused with basal cell carcinoma at low power.
• Histological Features:
  • Neuroendocrine carcinoma arising in the skin.
  • Similar features to small cell carcinoma of the lung.
  • Cells arranged in nests or aggregates, sometimes sheet-like or diffuse.
  • Lack of palisading nuclei, unlike basal cell carcinoma.
  • Nuclei have pale or fine smudgy chromatin, often described as "salt-and-pepper," "stippled," or "dusty."
  • Nuclei mold together, squishing to fit neighbors, not perfectly round.
  • May show rosette-like structures, although not a primary diagnostic feature.

Mitotic Activity

• Extremely mitotically active tumors.
  • High number of mitoses per square millimeter compared to melanomas and other cancers.
  • Example: Up to 140 mitoses per square millimeter observed.

Diagnosis

• Histological examination confirms classic appearance.
• Immunohistochemistry:
  • Cytokeratin 20: Almost always positive in Merkel cells with a perinuclear dot-like (Golgi) pattern.
  • Other markers: Neurofilament, synaptophysin, chromogranin (neuroendocrine markers).
  • Negative TTF-1 and CK7 can rule out metastatic small cell carcinoma of the lung.

Differential Diagnosis

• Metastatic small cell carcinoma (lung): Very uncommon in skin.
  • CK20 is a reliable marker for distinguishing between Merkel cell and metastatic small cell carcinoma.
  • TTF-1 positivity may suggest need for lung imaging.

Clinical Implications

• Important to accurately diagnose due to the aggressive nature and potential for metastasis.
• Ensuring correct identification prevents mismanagement and improves patient outcomes.

Diagnostic Considerations

• Use of additional stains if CK20 is negative or unclear.
• Immunohistochemistry as a mixed science and art; approaches may vary.

Note: These notes aim to emphasize the importance of recognizing and correctly diagnosing Merkel Cell Carcinoma due to its aggressive nature and the differences in treatment and prognosis compared to similar appearing carcinomas.