Table 2. TNM classification and staging system for hidradenocarcinoma.

Primary tumour (T)
TX	Primary tumour cannot be assessed
T0	No evidence of primary tumour
Tis	Carcinoma in situ
T1	Tumour ≤ 2 cm in greatest dimension with < 2 high risk features†
T2	Tumour > 2 cm in greatest dimension or tumour of any size with ≥2 high risk features
T3	Tumour involving maxilla, mandible, orbit, temporal bone
T4	Tumour invading skeleton or perineural invasion in skull base

Regional lymph nodes (N)
NX	Regional lymph node cannot be assessed
N0	No regional lymph node metastasis
N1	Metastasis in single regional ipsilateral lymph node, not ≥3 cm in greatest dimension
N2a	Metastasis in single regional ipsilateral lymph node, > 3 cm but < 6 cm
N2b	Metastasis in multiple regional ipsilateral lymph nodes, none > 6 cm
N2c	Metastasis in bilateral or contralateral regional lymph nodes none > 6 cm
N3	Metastasis in a lymph node, greater than 6 cm in greatest dimension

Distant metastasis (M)
Mx	Distant metastasis cannot be assessed
M0	No distant metastasis
M1	Distant metastasis

Stage grouping
Stage	T	N	M
Stage 0	Tis	N0	M0
Stage 1	T1	N0	M0
Stage 2	T2	N0	M0
Stage 3	T3	N0	M0
	T1, 2, 3	N1	M0
Stage 4	T1, 2, 3	N2	M0
	T4	N3	M1

^†

High risk features:

• Depth/invasion- > 2 mm thickness, Clark > IV, perineural invasion
• Anatomic location-primary site – ear/non hair bearing lip
• Differentiation- poorly differentiated/ undifferentiated tumour