When a tumor is detected, the physician takes a biopsy by removing a sample of the tissue. The biopsy sample is inspected under a microscope to determine if the tumor is benign or malignant. Cancerous cells usually appear abnormal in shape and no longer orient themselves in orderly configurations. If the tumor is cancerous, the physician assigns it a stage, indicating how far cancer has spread.

The stage is a key factor in determining both the cancerís treatment and prognosis. Oncologists, physicians who specialize in the diagnosis and treatment of cancer, use several different staging systems. In one system, tumors are grouped into four stages denoted by Roman numerals I through IV. Stage I cancers are small localized cancers that are usually curable. Stage II and III tumors are usually locally advanced and may or may not have invaded nearby lymph nodes, and stage IV tumors have usually metastasizedóthat is, spread to distant tissues in the body.

The most widely used staging system is the Tumor, Lymph Node, and Metastasis system, commonly abbreviated TNM. This system uses numbers between zero and three to assess the size of the tumor (T), the extent that it has spread to nearby lymph nodes (N), and the extent that it has spread throughout the body (M). A cancerís stage depends on a combination of these numbers. For example, a T-1, N-0, and M-0 tumor is a stage 1 tumor. This tumor is 2 cm (1 in) or less (T-1) and has not spread to nearby lymph nodes (accounting for N-0) or metastasized (M-0). The five-year survival rate for a patient with this stage tumor is accordingly excellent. A T-3, N-1, and M-0 tumor is a stage 3 tumor. This tumor is greater than 5 cm (2 in) and has spread to nearby lymph nodes, but there is no evidence that the cancer has spread to distant tissues. The five-year survival rate for a patient with this tumor is not as high as the T-1, N-0, M-0 patient. Stage 4 tumors are distinguished by an M-1 number. This means they have progressed to the point where metastasis is widespread, and the prognosis is usually quite poor.

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