Merkel Cell Carcinoma, Skin

Nayela Keen, MD

Christine M. Glastonbury, MBBS

Key Facts

Terminology

Neuroendocrine skin malignancy derived from Merkel cells
Rare but aggressive, rapidly growing cutaneous malignancy

Imaging

Primary tumor variable but may be small or not discernible on clinical exam
Imaging important for assessing nodes
CT or MR: Useful for assessing nodal spread
MR may be more sensitive for intralymphatic spread and deep extent of tumor
PET: High FDG avidity makes ideal staging agent

Top Differential Diagnoses

Skin basal cell carcinoma
Skin SCCa
Skin melanoma

Pathology

Associated with sun exposure
Merkel cell polyomavirus associated with most
Immunocompromised patients have higher incidence, often present in later stage and worse prognosis
MCC AJCC staging system introduced in 2010

Clinical Issues

Most patients > 65 years
Painless, firm, red nodule that grows rapidly
Early and frequent metastasis to regional lymph nodes
Nodal metastasis most important predictor of outcome
Wide local excision is important; XRT has role
2x the mortality rate of melanoma
5-year relative survival if N0 at diagnosis ˜ 75%, if nodal mets ˜ 45%

(Left) Clinical photograph demonstrates subtle, small lesion of midline face, just above the upper lip

, which at biopsy proved to be Merkel cell carcinoma. Also notice marked fullness of left lower cheek

. (Right) Axial CECT in same patient with skin marker demonstrates enlarged left facial node

without necrosis or calcifications. This corresponds to lower left cheek palpable and visible mass. Additional small bilateral submandibular nodes were also evident on this CECT.

(Left) Axial T2WI FS MR in same patient obtained almost 4 weeks after initial CT shows marked increase in size of left facial node

without necrosis. Case well illustrates the potential for rapid growth of Merkel cell carcinoma. (Right) Axial fused PET/CT obtained 1 week prior to MR shows avid FDG uptake in small left submandibular node

and large right IB node

. Larger left facial node shown on CT above and MR to left was also markedly FDG avid (SUV = 15).

TERMINOLOGY

Abbreviations

Merkel cell carcinoma (MCC)

Synonyms

Neuroendocrine carcinoma of skin

Definitions

Neuroendocrine skin malignancy derived from Merkel cells
- Part of the mechanoreceptor complex in skin
- Located at basal layer of epidermis
- Probably derived from neural crest cells

IMAGING

General Features

Best diagnostic clue
- Nodal mass within neck associated with possible focal skin thickening
  - Primary site may be subtle clinically as well as radiographically
Location
- Most commonly arises on facial skin
  - 50% of cases occur in head and neck
    - Cheeks, nose, perioral, eyelids, periocular
  - 30% occur on extremities
  - 5-7% occur on trunk
  - < 15% unknown primary site
  - Even more rarely arises from mucosal surfaces
Size
- Median diameter of primary at diagnosis = 1.1 cm
- May present from adenopathy without discernible primary
Morphology
- If primary visible, appears as exophytic subcutaneous nodule

CT Findings

CECT
- Primary: Skin thickening or subcutaneous mass
- Solid nodal disease, moderately enhancing

MR Findings

Primary site is typically small and may not be evident on imaging
If lesion seen, typically T2 hyperintense and moderately to markedly enhances with contrast
May see abnormal adjacent nodularity or hyperintensity of subcutaneous fat to suggest intralymphatic spread
Involved nodes of variable size, typically solid and moderately enhance with contrast

Nuclear Medicine Findings

PET/CT
- Merkel cell carcinoma is highly FDG avid
  - Useful for staging, surveillance, restaging recurrent disease and distant metastasis
  - Even small nodes may show increased FDG uptake
Lymphoscintigraphy may be used to locate sentinel nodes
- Increasingly important role in MCC treatment