Table 2.

Summary of the 2010 American Joint Committee on Cancer Merkel Cell Carcinoma staging system.

Primary tumor (T)
Tx	Primary tumor cannot be assessed
T0	No evidence of primary tumor
Tis	In situ primary tumor
T1	Primary tumors less than or equal to 2 cm maximum tumor dimension
T2	Primary tumors greater than 2 cm but not more than 5 cm maximum tumor dimension
T3	Primary tumors over 5 cm maximum tumor dimension
T4	Primary tumor invades bone, muscle, fascia, or cartilage
Regional lymph nodes (N)
Nx	Regional lymph nodes cannot be assessed
N0	No regional lymph node metastasis
cN0	Nodes negative by clinical exam (no pathologic node exam performed)
pN0	Nodes negative by pathologic exam
N1	Metastasis in regional lymph node(s)
N1a	Micrometastasis
N1b	Macrometastasis
N2	In-transit metastasis
Distant metastasis (M)
M0	No distant metastasis
M1	Metastasis beyond regional lymph nodes
M1a	Metastasis to skin, subcutaneous tissues, or distant lymph nodes
M1b	Metastasis to lung
M1c	Metastasis to all other visceral sites
Anatomic stage/prognostic groups
IA	Primary tumor ≤2 cm; regional LN negative by pathologic examinationa
IB	Primary tumor ≤2 cm; regional LN negative by clinical examination onlyb
IIA	Primary tumor >2 cm; regional LN negative by pathologic examinationa
IIB	Primary tumor >2 cm; regional LN negative by clinical examination onlyb
IIIA	Primary tumor any size; positive micrometastasis in regional LNc
IIIB	Primary tumor any size; clinically detectable regional LN metastasis and/or in-transit metastasisd
IV	Primary tumor any size; any distant metastasis

^aNegative sentinel lymph node biopsy (SLNB) or elective lymph node dissection (ELND).

^bNo pathologic LN evaluation (SLNB or ELND).

^cPositive micrometastasis by SLNB or ELND.

^dConfirmed pathologically by biopsy or therapeutic lymph node dissection.

LN: lymph nodes.