14th July 2026

Merkel cell carcinoma

Background

Merkel cell carcinoma (MCC) is an aggressive primary cutaneous neuroendocrine tumour. Risk factors include UV exposure, increasing age and Merkel cell polyomavirus. Early aggressive treatment is needed. 

Epidemiology

MCC is a rare skin tumour with rising incidence rates. 

Aetiology

Commonly seen in elderly Caucasian individuals and appears in the sixth or seventh decade of life. Risk factors include increasing age, sun exposure, immunosuppression, and Merkel cell polyomavirus. 

Diagnosis

History and examination

Rapidly enlarging, shiny, smooth-surfaced solitary red nodule. Commonly seen on the head and neck. Ulceration is rare and typically only occurs in advanced stages. Metastasis and lymph node spread are common.

AEIOU can be used to aid diagnosis:

• A: asymptomatic/non-tender
• E: expanding rapidly (≤ 3 months)
• I: immunosuppressed
• O: older than 50 years
• U: UV-exposed sites

Dermoscopy:

• Linear, irregular, and polymorphous vessels, poorly focused vessels, milky pink/white areas, and architectural disorder. 

Immunohistochemistry: 

• Positive immunostaining with low molecular weight cytokeratins, e.g., cytokeratin 20 (CK20), and neuroendocrine markers such as neuron-specific enolase, Chromogranin A and synaptophysin are typically positive. 
• Thyroid transcription factor (TTF1) is usually negative.

Histopathology: 

• Uniformly small, round blue cells with scanty cytoplasm and vesicular nuclei

Prognosis

• Favourable prognostic factors include female gender, age <50, tumour size ≤  2cm, local disease, presence of Merkel cell polyomavirus or increased viral abundance. 
• Poor prognostic factors include those who are immunosuppressed and those with nodal involvement. Histology features that would suggest poor prognosis include tumour size ≥ 5mm, diffuse growth pattern and high mitotic rate.

Management

Dependent on the stage of the tumour, a multidisciplinary approach is needed.

Early aggressive treatment is needed. 

Early stage MCC: surgical excision with wide resection margins followed by sentinel lymph node biopsy and postoperative radiotherapy

Localised MCC: surgical excision followed by radiotherapy

Advanced/metastatic disease: radiotherapy, chemotherapy, immunotherapy +/- molecular-targeted therapy.

References

• https://bestpractice.bmj.com/topics/en-gb/269/differentials#collapseDiff-47fe7fd8-09aa-404f-806a-5a4954c1a525
• https://dermnetnz.org/topics/merkel-cell-carcinoma