MERKEL CELL CARCINOMA OF SKIN (ICD‐O‐3 C44.0–9, C63.2)

Rules for Classification

The classification applies only to Merkel cell carcinomas. There should be histological confirmation of the disease.

Regional Lymph Nodes

The regional lymph nodes are those appropriate to the site of the primary tumour. See Regional Lymph Nodes under Skin Tumours.

TNM Clinical Classification

T – Primary Tumour

TX	Primary tumour cannot be assessed
T0	No evidence of primary tumour
Tis	Carcinoma in situ
T1	Tumour 2 cm or less in greatest dimension (Fig. 374)
T2	Tumour more than 2 cm but not more than 5 cm in greatest dimension (Fig. 375)
T3	Tumour more than 5 cm in greatest dimension (Fig. 376)
T4	Tumour invades deep extradermal structures, i.e., cartilage, skeletal muscle, fascia or bone (Fig. 377)

Schematic illustration of tumour 2 cm or less in greatest dimension. — **Fig. 374**

Schematic illustration of tumour more than 2 cm but not more than 5 cm in greatest dimension. — **Fig. 375**

Schematic illustration of tumour more than 5 cm in greatest dimension. — **Fig. 376**

Schematic illustration of tumour invades deep extradermal structures, i.e., cartilage, skeletal muscle, fascia
or bone. — **Fig. 377**

N – Regional Lymph Nodes

NX	Regional lymph nodes cannot be assessed
N0	No regional lymph node metastasis
N1	Regional lymph node metastasis
N2	In transit metastasis without lymph node metastasis
N3	In transit metastasis with lymph node metastasis

Note
In‐transit metastasis: a tumour distinct from the primary lesion and located between the primary lesion and the draining regional lymph nodes and/or distal to the primary lesion

M – Distant Metastasis

M0	No distant metastasis
M1	Distant metastasis
	M1a	Skin, subcutaneous tissues or non‐regional lymph node(s)
	M1b	Lung
	M1c	Other site(s)

pTNM Pathological Classification

The pT category corresponds to the T category.

pNX	Regional lymph nodes cannot be assessed
pN0	No regional lymph node metastasis
pN1	Regional lymph node metastasis
	pN1a(sn)	Microscopic metastasis detected on sentinel node biopsy
	pN1a	Microscopic metastasis detected on node dissection
	pN1b	Macroscopic metastasis (clinically apparent)
pN2	In transit metastasis without lymph node metastasis
pN3	In transit metastasis with lymph node metastasis
pM1	Distant metastasis microscopically confirmed

Note
pM0 and pMX are not valid categories.

pN0

Histological examination of a regional lymphadenectomy specimen will ordinarily include 6 or more lymph nodes. If the lymph nodes are negative, but the number ordinarily examined is not met, classify as pN0.