Skin Cancer: Merkel Cell Carcinoma

Summary:
Wide local excision should always be followed by irradiation. This is an aggressive tumor with high likelihood of locoregional disease at presentation and high rates of recurrence. Adjuvant chemotherapy is controversial and generally not very effective.

Prognosis:
5 year overall survival
Stage I: 80%
Stage II: 60%
Stage III: 40%
Stage IV: 20%
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Etiology:
This is a neuro-endocrine tumor characterized by small-cell cancer cells on pathology. This tumor is generally rare, so high level evidence to guide practice is lacking. Mortality rates are high and double of that expected with melanomas (33 vs 15%).

There is a Merkel Cell Polyomavirus, which suggests a viral etiology. Additionally, immunocompromised individuals seem to be at greater risk.

Approximately 50% present with locoregional disease involving lymph nodes. 30% present with metastatic disease, while only 20% present with localized disease,

Work-up

• Complete history/physical
• CBC, LFTs, RFTs, PT/PTT/INR
• Biopsy
• CT chest
• Anatomical CT for assessment of nodes
  

TNM
T1: < 2 cm
T2: 2 - 5 cm
T3: > 5 cm
T4: Invades bone, muscle, cartilage

N1a:micrometastasis
N1b: macrometastasis
N2: In transit metastasis (between tumor and regional LN or distal to primary)

Staging
Stage IA: T1pN0
Stage IB: T1cN0
Stage IIA: T2-3pN0
Stage IIB: T2-3cN0
Stage IIC: T4N0
Stage IIIA: TxN1a
Stage IIIB: TxN1b; TxN2
Stage IV: TxNxM1

Management:
Wide local excision with 1 - 2 cm margins.
Sentinel lymph node biopsy is bare minimum in all cases. A full lymph node dissection is indicated in the presence of a clinically detectable node (physical exam or CT) or in the presence of a positive SLNBx.

Adjuvant radiation is indicated in all Merkel Cell Cases regardless of margin status or LN status. Doses should be similar to head and neck doses.
Gross disease = 70 Gy
Positive margins or extra-capsular extension = 66 Gy
Negative margins = 60 Gy
Elective nodal irradiation = 50-56 Gy

Volumes:
GTV = gross tumor volume
CTV High Dose = GTV + 1 cm + any LN level with positive LNs
CTV Int Dose = First echelon LN's adjacent to gross disease
CTV Low Dose = Elective nodal irradiation
PTVs = 0.5 - 0.7 cm around CTVs

Chemotherapy:
Regimens are cisplatin and etoposide based as this is a small-cell neuroendocrine tumor. Outcomes and response rates aren't great for these tumors, so it's controversial when chemotherapy is best started.