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Skin Cancer
 Skin cancers are broadly classified as melanoma and nonmelanomas. More than 1 million cases of nonmelanoma are diagnosed each year, making it the most common cancer in the United States. Melanoma occurs less frequently, with approximately 60,000 new cases diagnosed in 2007.
The number of people affected by both types of skin cancer has increased substantially over the past several decades. Increased exposure to the sun is thought to be the cause of this increase.
Melanomas and nonmelanomas are cancers that originate in the outer layer of the skin, or the epidermis. The epidermis is made up of three kinds of cells. Skin cancers are defined by the type of cell from which they develop. Squamous cell carcinoma arises from squamous cells, — thin flat cells that make up the top layer of the epidermis. Basal cell carcinoma develops in basal cells — round cells that lie beneath the layer of squamous cells.
These two types of skin cancer are referred to as nonmelanoma to distinguish them from melanoma, which develops in melanocytes. Melanocytes form the bottom layer of the epidermis and determine the skin color of an individual. Nonmelanomas are relatively slow growing and rarely spread (metastasize). In contrast, melanoma cells are much more likely to invade nearby lymphatic vessels and/or spread to other parts of the body, making them potentially more harmful. They are the focus here.
A biopsy must be done to confirm the diagnosis of a nonmelanoma or melanoma. An excisional biopsy or a deep-shave biopsy is most often used if the lesion is thought to be melanoma, as this method involves excision (removal) of the entire lesion.
The factors considered in classifying the melanoma according to the tumor (T) staging category in the AJCC system are the thickness of the lesion and the presence or absence of ulceration. (Ulceration means that the layer of skin over the melanoma is disrupted.)
The T categories are as follows:
| Tx: |
Melanoma cannot be evaluated (because of lack of information) |
| T0: |
No cancer is found |
| Tis: |
Melanoma cells are found in only the outer layer of skin, but have not yet invaded into the next layer (the dermis) of the skin (also known as melanoma in situ) |
| T1: |
Melanoma is no more than 1 mm thick |
| T2: |
Melanoma is more than 1 mm thick but no more than 2 mm thick |
| T3: |
Melanoma is more than 2 mm thick but no more than 4 mm thick |
| T4: |
Melanoma is more than 4 mm thick |
Each T classification is also subcategorized according to whether ulceration is absent (subcategory a) or present (subcategory b). For example, an ulcerated lesion that is 1 mm thick is classified as T1b. The pathologist will also report on the level of invasion, a measure of how deep into the dermis the melanoma has invaded.
The node (N) classification is used to describe the spread of the melanoma to lymph nodes in terms of both the number of lymph nodes involved and the amount of cancer cells in the lymph nodes. If the cancer cells in the lymph node are found with use of a microscope, the metastasis is considered to be microscopic. If there are enough cancer cells in the lymph node to make the node appear as a palpable mass, it is said to be macroscopic.
The N categories are as follows:
| Nx: |
Lymph nodes cannot be evaluated (because of lack of information) |
| N0: |
No spread of melanoma to lymph nodes |
| N1: |
Melanoma found in one lymph node |
| N2: |
Melanoma found in two or three lymph nodes |
| N3: |
Melanoma found in four or more lymph nodes or two or more lymph nodes appear to be joined together (known as matted lymph nodes) |
The N1, N2 and N3 categories include subcategories to indicate whether metastasis to the lymph nodes is microscopic; that is, not clinically detectable (subcategory a); or macroscopic; that is, detectable by clinical or x-ray exam (subcategory b). In addition, N2 and N3 also include a subcategory of c to indicate in-transient or satellite metastasis, which is the presence of cancer cells in the lymphatic vessels leading to a lymph node.
Lastly, the melanoma is classified according to whether the cancer has metastasized. The M categories are as follows:
| Mx: |
Metastasis cannot be evaluated (because of lack of information) |
| M0: |
No metastasis |
| M1: |
Metastasis |
| M1a: |
Melanoma has spread outside the region where it first started to other parts of the skin, under the skin, or any distant lymph nodes; |
| M1b: |
Melanoma has spread to the lungs |
| M1c: |
Melanoma has spread to any other internal organ in the body. |
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