Merkel cell carcinoma

Histopathologic classification

• Hematoxylin and eosin (H&E) staining demonstrates a dermal tumor of small round blue cells.

• Immunohistochemistry should be used to exclude mimics:

  • Positive staining with CK20 (membranous or paranuclear dot pattern) and negative staining with thyroid transcription factor 1 (TTF-1) may be sufficient to confirm MCC.

  • Additional immunophenotyping with neuroendocrine markers or for MCPyV may be helpful.

• The College of American Pathologists (CAP) recommends a synoptic report including tumor depth in mm, tumor-infiltrating lymphocytes, tumor growth pattern, and presence of a second malignancy within the specimen, among other features.[39]Andea AA, Coit DG, Amin B, et al. Merkel cell carcinoma: histologic features and prognosis. Cancer. 2008 Nov 1;113(9):2549-58. https://acsjournals.onlinelibrary.wiley.com/doi/10.1002/cncr.23874 http://www.ncbi.nlm.nih.gov/pubmed/18798233?tool=bestpractice.com [69]Smoller BR, Bichakjian C, Brown JA, et al; College of American Pathologists. Protocol for the examination of specimens from patients with Merkel cell carcinoma of the skin. Version 4.1.0.0. Jun 2021 [internet publication]. https://documents.cap.org/protocols/Skin.Merkel_4.1.0.0.REL_CAPCP.pdf ​​​

American Joint Committee on Cancer (AJCC) TNM staging system (8th edition)[70]American Joint Committee on Cancer. AJCC cancer staging manual, 8th edition. ​Chicago, IL: American College of Surgeons / Springer; 2017.

The AJCC staging system describes the extent of disease based on the following anatomic factors: size and extent of the primary tumor (T); regional lymph node involvement (N) (assessed clinically [cN] and pathologically [pN]); and presence or absence of distant metastases (M).