June 2022 – Dermatopathology

An 84-year-old-man with history of squamous cell carcinoma and Merkel cell carcinoma underwent neck dissection. One lymph node was found to be involved by chronic lymphocytic leukemia/small lymphocytic lymphoma (a), metastatic Merkel cell carcinoma (b), and metastatic basaloid squamous cell carcinoma colonized by Merkel cell carcinoma (c). The Merkel cell carcinoma cells are arranged in a nodular or nested pattern and have granular “salt and pepper” chromatin with indistinct nucleoli.

Image: 10x Neck Lymph Node With Label

Which of the following is true regarding Merkel cell carcinoma (MCC)?

  • It is not associated with UV radiation.
  • The tumor cells are typically positive for synaptophysin and chromogranin, and negative for cytokeratins.
  • Immunodeficiency and secondary malignancy are not associated with MCC.
  • It can be associated with an oncogenic virus.

The correct answer is ...

It can be associated with an oncogenic virus.

Merkel cell carcinoma is an aggressive primary cutaneous neuroendocrine malignancy. The discovery of Merkel cell polyoma virus (MCV) has led to greater understanding of the pathogenesis of MCC. MCV was first described in 2008 (Feng, et al). In this study, the viral sequence identified in their lab was detected in 80% of the tested MCC tumors compared to 8% of control tissues from various body sites and 16% in control skin tissues. There is a smaller subset of cases that are not associated with this virus and appear to be driven by UV radiation exposure.

As demonstrated in this case, immunodeficiency such as concurrent CLL/SLL increases the risk of MCC development. Secondary malignancy is not uncommon in patients with MCC, with squamous cell carcinoma occurring most frequently.

MCC is typically positive for synaptophysin, chromogranin, and cytokeratin 20. CK20 will often show a paranuclear dot pattern. Small cell carcinoma of the lung may have similar morphology and is also typically positive for synaptophysin and chromogranin; however, cytokeratin 20 should be negative.

UV radiation exposure is a known risk factor for MCC. It frequently involves sun-exposed sites, such as the head and neck, and typically affects elderly, fair-skinned males.


  1. Feng H, Shuda M, Chang Y, Moore PS. Clonal integration of a polyomavirus in human Merkel cell carcinoma. Science. 2008 Feb 22;319(5866):1096-100. doi: 10.1126/science.1152586. Epub 2008 Jan 17. PMID: 18202256; PMCID: PMC2740911.
  2. Coggshall K, Tello TL, North JP, Yu SS. Merkel cell carcinoma: An update and review: Pathogenesis, diagnosis, and staging. J Am Acad Dermatol. 2018 Mar;78(3):433-442. doi: 10.1016/j.jaad.2017.12.001. Epub 2017 Dec 9. PMID: 29229574.
  3. Walsh NM, Cerroni L. Merkel cell carcinoma: A review. J Cutan Pathol. 2021 Mar;48(3):411-421. doi:10.1111/cup.13910. Epub 2020 Dec 3. PMID: 33128463.
  4. Harms PW. Update on Merkel Cell Carcinoma. Clin Lab Med. 2017 Sep;37(3):485-501. doi: 10.1016/j.cll.2017.05.004. Epub 2017 Jun 13. PMID: 28802497.

Amanda J. Nguyen, M.D.
Resident, Anatomic and Clinical Pathology
Mayo Clinic

Ruifeng (Ray) Guo, M.D., Ph.D.
Consultant, Anatomic Pathology
Mayo Clinic
Assistant Professor of Laboratory Medicine and Pathology
Assistant Professor of Dermatology
Mayo Clinic College of Medicine and Science

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This post was developed by our Education and Technical Publications Team.