Merkel cell cancer classification

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Classification

Histologically, MCC has been classified into three distinct subtypes:

• Trabecular: classic pattern, large-cell type, high density or granules on ultrasound examination.
• Intermediate: solid pattern (most common).
• Small cell: diffuse, few high density granules on ultrasound examination (second most common).

Staging

• According to the [TNM staging system], there are four stages of Merkel cell carcinoma based on the tumor size, presence of lymph nodes and metastasis. Each stage is assigned a letter/number.
• The stages of Merkel cell carcinoma are shown in the table below:

Stage	Primary Tumor	Lymph Node	Metastasis
0	In situ primary tumor	No regional lymph node metastasis	No distant metastasis
IA	Less than or equal to 2 cm maximum tumor dimension	Nodes negative by pathologic exam	No distant metastasis
IB	Less than or equal to 2 cm maximum tumor dimension	Nodes negative by clinical exam (no pathologic node exam performed)	No distant metastasis
IIA	Greater than 2 cm tumor dimension	Nodes negative by pathologic exam	No distant metastasis
IIB	Greater than 2 cm tumor dimension	Nodes negative by clinical exam (no pathologic node exam performed)	No distant metastasis
IIC	Primary tumor invades bone, muscle, fascia, or cartilage	No regional lymph node metastasis	No distant metastasis
IIIA	Any size tumor (includes invading tumors)	Micrometastasis (diagnosed after sentinel or elective lymphadenectom)	No distant metastasis
IIIB	Any size tumor (includes invading tumors)	Macrometastasis (defined as clinically detectable nodal metastases confirmed by therapeutic lymphadenectomy or needle biopsy) OR In transit metastasis (a tumor distinct from the primary lesion and located either between the primary lesion and the draining regional lymph nodes or distal to the primary lesion)	No distant metastasis
IV	Any size tumor (includes invading tumors)	Any lymph node metastasis	Metastasis beyond regional lymph nodes

Adapted from AJCC's Cancer Staging Manual 2009