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Once a cancer diagnosis is made, the next step is to determine whether the cancer has spread from its site of origin and, if it has, to locate all the places in the body where it is growing.

The process of accomplishing this is called the “staging workup,” because after it is completed, the cancer will be assigned a “stage.” The stage is extremely important to know because treatment plans are designed based on the extent and location of the cancer.

The stage of a cancer directly correlates with the likelihood of cure: in general, the higher the stage number, the more widespread the cancer and less favorable the outcome.

It is important to realize, however, that the relation between stage and prognosis is not absolute. Some stage I cancers can behave aggressively and return to take the patient’s life, whereas some stage IV cancers can be eradicated. Several factors in addition to stage affect the survivability of a cancer.

The staging workup typically involves the following: (1) imaging studies, such as X-rays, CT and MRI scans, bone scans, and sometimes PET scans, all of which locate cancer throughout the body (other kinds of testing may also be required, as dictated by the type and location of the cancer); (2) analysis of surgical results, especially if a cancer and its nearby lymph nodes are removed-in this case, the pathologist will be able to assign a “pathologic stage”; and (3) blood tests to measure tumor markers and how well the bone marrow, kidneys, and liver are functioning.

The staging system for the most common cancers (such as breast, lung, and colon cancers) recognizes that there is a direct relation between the extent of cancers and their ultimate curability.

The extent of a cancer is described by the TNM staging system, in which T stands for the size or extent of the primary Tumor, N stands for the number and location of lymph Nodes that contain cancer, and M stands for the presence or absence of distant Metastases.

Each cancer is staged according to its own TNM classification system. Once a cancer receives T, N, and M assignments, the three elements are combined to define the stage, which commonly has four categories: I, II, III, or IV. For example:

• A one-and-a-half-centimeter (half-inch) breast cancer primary tumor (T = 1) that has not spread to axillary lymph nodes (N = 0) or other parts of the body (M = 0) would be stage I based on a T1, N0, M0 designation. By contrast, a one-centimeter (one-third-inch) tumor (T = 1) that has spread to five axillary lymph nodes (N = 2) but nowhere else in the body (M = 0), would be stage III (T1, N2, M0).

• A T1 lung cancer measuring three centimeters (a little over an inch) or less that does not involve lymph nodes would be stage I (T1, N0, M0); if the cancer has spread to nearby lung lymph nodes, it would be stage II (T1, N1, M0); if it has spread farther to involve lymph nodes in the middle of the chest (mediastinum), it would be stage III (T1, N2 or N3, M0).

• In pancreatic cancer, the stage is determined mainly by the size and extent of the tumor rather than whether there is lymph node involvement. For example, cancers that extend beyond the pancreas but not into nearby arteries would be classified as T3, stage II, whereas cancers that do involve major arteries are classified as T4, stage III. In general, surgeons will be able to remove a stage II cancer but not a stage III cancer. (Sometimes chemotherapy and radiation can be given first to shrink the cancer and make surgery possible).

Your oncologist will explain how your cancer’s TNM stage was determined. It is important to understand that although cancer tends to spread first to nearby lymph nodes and then to more distant sites in the body, it does not always follow such an orderly or obvious path.

A person may be diagnosed with an early-stage cancer, not involving any lymph nodes, and still develop stage IV disease years later. The explanation for this is that cancer cells either bypassed the lymph nodes and spread through the bloodstream or did pass through the lymph nodes but left no traces behind.

Researchers are developing methods based on the genetic profile of a cancer that will enable oncologists to predict more accurately which early cancers have the potential to return and which can be cured by surgical removal alone.

In general, the lower the stage, the better the chances are that a cancer can be cured. Yet the fact that cancer relapses affect some individuals with low stages of cancer forms the basis for administering cancer treatments even after surgery has removed all visible evidence of the disease.

Such “adjuvant” therapy is discussed in depth later. The TNM staging system does not apply to cancers of the brain or to the blood and lymph cancers leukemia, lymphoma, and multiple myeloma. These cancers have their own unique staging systems because they behave very differently from the more common cancers.

For example, in leukemia, the cancer cells circulate in the bloodstream throughout the body; they would all be metastatic under the TNM system. In fact, most leukemias are not staged but are instead “classified” by the specific genetic defects they harbor.

Multiple myeloma, Hodgkin lymphoma, and non-Hodgkin’s lymphoma do have established staging systems.