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Unless you're an oncology nurse, cancer staging can seem like a foreign language. The stage classifies the anatomic extent of a malignancy, which helps determine treatment.

There are two types of staging: clinical and pathologic. Clinical staging usually occurs at diagnosis and relies on physical examination, imaging studies, lab tests, and biopsies. Pathologic staging occurs after a patient has surgery to remove the cancer.

Staging is crucial, according to oncology nurse practitioner Tracy Ruegg, RN, MS, of Ohio State University's James Cancer Hospital in Columbus. Staging, she said, helps healthcare providers plan the patient's treatment, estimate the prognosis, and collect data on treatment outcomes in clinical trials and for cancer registries.

"The same type of cancer may need different treatment at different stages," says Ruegg, whose expertise is in lung cancer.

In lung cancer, for example, early-stage tumors usually can be resected, but late-stage tumors most often cannot, Ruegg says.

Even if a cancer grows or spreads, the clinical stage typically does not change, the American Joint Committee on Cancer reports. For some cancers, if the cancer returns after remission or the tumor decreases after preoperative therapy, the tumor may be restaged, Ruegg says.

Solid-Tumor Staging

There are many cancer staging systems. Most staging systems consider these aspects, according to the National Cancer Institute:

• Location of the primary (original) tumor

• Size and number of tumors

• Lymph node involvement

• Presence or absence of distant metastases (the spread to other parts of the body)

One of the most common systems used throughout the world is TNM, or tumor-node-metastasis. Before its use began 50 years ago, there was little standardization in cancer staging. Now, TNM is the internationally accepted system for staging most solid-tumor cancers, according to the AJCC. Along with the International Union Against Cancer, the AJCC updates the TNM system as new evidence becomes available about the clinical behavior of malignant tumors. About 50 sites or types of cancer rely on this classification system.

Despite TNM serving to unify cancer staging, there are differences in the staging rules among the many cancer types, Ruegg says, because various cancers have diverse clinical behavior and outcomes.

Cancers that are not staged using the TNM system include brain tumors, which are staged by cell type and grade, and cancers of the blood, bone marrow, and lymph system. The Ann Arbor System is an example of one staging system used for lymphomas. Gynecologists stage gynecologic tumors using the International Federation of Gynecology and Obstetrics system, but they translate the stage to correspond to TNM.

ABCs of TNM

The first step in TNM staging is to classify the size and extent (depth of invasion and involvement of the nearby tissues) of the primary tumor (T), the presence or absence of cancer in nearby lymph nodes (N), and the presence or absence of distant metastases (M). A number follows each letter to indicate the extent of each of these factors. Also, sometimes TNM categories have subcategories for a more detailed description (indicated with lowercase letters). For instance, in ovarian cancer, T1a is a tumor confined to one ovary and T1b is in both ovaries.

After determining the TNM categories, the clinician combines them to assign the cancer a stage number, called stage grouping. Most cancers are staged as 0, I, II, III, and IV, using Roman numerals, which indicate whether a cancer is noninvasive or invasive and to what extent.

For example, a T1N0M0 breast cancer designates a small tumor with disease-free lymph nodes and no spread to structures outside the breast. It is stage I, according to breast cancer staging criteria.

Stages also may have subcategories, such as A and B, where differences in tumor size and node status combinations are considered. For example, in breast cancer, T2N0M0 is stage IIA and T2N1M0 is stage IIB.

However, different types of cancer use different staging criteria. The same TNM combination — for instance, T3N0M0 — can be stage II for lung cancer and stage III for bladder cancer.

Some experts say the TNM classification is limited because nonanatomic factors also play an important prognostic role in some cancers, such as estrogen receptors in breast cancer. Ruegg, however, says, "TNM is not going away."

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Kathleen Louden is a freelance writer. To comment, e-mail editorNTL@gannetthg.com.

Editor's Note: For more information, see TNM Online at www3.interscience.wiley.com/cgi-bin/mrwhome/104554799/home or www.cancerstaging.org