How are basal and squamous cell skin cancers staged?

Staging is a process of finding out how widespread a cancer is. Because basal cell cancer
is almost always cured before it spreads to other organs, it is seldom staged unless the
cancer is very large. Squamous cell cancers have a somewhat greater (although still quite
small) risk of spreading, so staging may sometimes be done, particularly in people who
have a high risk of spread. This includes people with suppressed immune systems, such
as those who have had organ transplants and people infected with HIV, the virus that
causes AIDS.
The tests and exams described in the section called “How are basal and squamous cell
skin cancers diagnosed?” are the main ones used to help determine the stage of the
cancer. In rare cases, imaging tests such as x-rays, CT scans, or MRI scans may be used
as well.

The American Joint Committee on Cancer (AJCC) TNM 
system 
A staging system is a way to summarize how far a cancer has spread. This helps members
of the cancer care team determine a patient's prognosis (outlook) as well as the best
treatment options.
The system most often used to stage basal and squamous cell skin cancers is the
American Joint Commission on Cancer (AJCC) TNM system. (Merkel cell carcinoma
has a separate AJCC staging system, which is not described here.)
Physical exams and other tests may be used to assign T, N, and M categories and a
grouped stage. The TNM system for staging contains 3 key pieces of information:
• T stands for tumor (its size, location, and how far it has spread within the skin and to
nearby tissues).
• N stands for spread to nearby lymph nodes (small bean-shaped collections of immune
system cells, to which cancers often spread first).
• M is for metastasis (spread to distant organs).
T categories 
The possible values for T are:
TX: Primary tumor cannot be assessed.
T0: No evidence of primary tumor.
Tis: Carcinoma in situ (tumor is still confined to the epidermis).
T1: The tumor is 2 centimeters (cm) across (about 4/5 inch) or smaller and has no or only
1 high-risk feature (see below).

T2: Tumor is larger than 2 cm across, or is any size with 2 or more high-risk features.
T3: Tumor invades into facial bones, such as the jaw bones or bones around the eye.
T4: Tumor invades into other bones in the body or into the base of the skull.
High-risk features: These features are used to distinguish between some T1 and T2
tumors.
• Tumor is thicker than 2 millimeters (mm).
• Tumor has invaded down into the lower dermis or subcutis (Clark level IV or V).
• Tumor has invaded into tiny nerves in the skin (perineural invasion).
• Tumor started on an ear or on hair-bearing lip.

• Tumor cells look very abnormal (poorly differentiated or undifferentiated) when
viewed under a microscope.
N categories
The possible values for N are: 
NX: Nearby lymph nodes cannot be assessed.
N0: No spread to nearby lymph nodes.

and other organs in the body is combined in a process called stage grouping. The stages
are described using the number 0 and Roman numerals from I to IV. In general, patients
with lower stage cancers tend to have a better prognosis for a cure or long-term survival.
Stage 0  Tis, N0, M0
Stage I  T1, N0, M0
Stage II  T2, N0, M0
Stage III  T3, N0, M0
T1 to T3, N1, M0
Stage IV  T1 to T3, N2, M0

N1: Spread to 1 nearby lymph node which is on the same side of the body as the main
tumor and is 3 centimeters (cm) or less across.
N2a: Spread to 1 nearby lymph node which is on the same side of the body as the main
tumor and is larger than 3 cm but not larger than 6 cm across.
N2b: Spread to more than 1 nearby lymph node on the same side of the body as the main
tumor, none of which are larger than 6 cm across.
N2c: Spread to nearby lymph node(s) on the other side of the body from the main tumor,
none of which are larger than 6 cm across.
N3: Spread to any nearby lymph node that is larger than 6 cm across.
M categories
The M values are:
M0: No spread to distant organs.
M1: Spread to distant organs.
Stage grouping 
To assign a stage, information about the tumor and whether it has spread to lymph nodes


and other organs in the body is combined in a process called stage grouping. The stages
are described using the number 0 and Roman numerals from I to IV. In general, patients
with lower stage cancers tend to have a better prognosis for a cure or long-term survival.
Stage 0  Tis, N0, M0
Stage I  T1, N0, M0
Stage II  T2, N0, M0
Stage III  T3, N0, M0
T1 to T3, N1, M0
Stage IV  T1 to T3, N2, M0

Any T, N3, M0
T4, any N, M0
Any T, any N, M1