After the difficult job of delivering the news of a cancer diagnosis, one of the first questions I am routinely asked is, “Doctor, what is the stage?”
While you may not be familiar with the specifics of cancer staging, you probably understand that strategies exist to classify cancer and that those classifications can impact treatment decisions, follow-up care, and outcome predictions. If you are not attuned to the use of staging classifications, you will hopefully learn about them primarily from your oncologist and, secondarily, from other reputable sources. I hope this article will serve as an opportunity to learn about one of the most important tools we have for classifying and staging solid tumors: the TNM classification system, where TNM stands for Tumor, Node, and Metastasis.
The TNM Classification System
Staging provides a framework for discussion for everyone on the cancer care team, including you. Staging is designed to place cancers with similar features into groups that will likely have similar outcomes. To achieve this, a common classification system that is valid for all solid tumors was created: the TNM classification system.
The TNM classification system was developed by Pierre Denoix, a surgeon working in Paris in the 1940s and 1950s. It is considered an “anatomical” system based on measurable characteristics of the cancer.
T Is for Tumor
“T” represents the size or thickness of the tumor. A solid tumor first grows locally in overall size and may extend into surrounding tissues. At some sites, such as the stomach, colon, or rectum, the size of the tumor is measured by the depth of penetration through the organ wall. At other sites, such as breast, lung, liver, thyroid, or melanoma, the diameter, or thickness, of the tumor is measured in millimeters or centimeters. This measurement determines the T category.
N Is for Node
“N” represents the presence of cancer cells in the lymph nodes. Here, we are looking at the spread of cancer cells to regional lymph nodes, or those lymph nodes that are close to the original tumor site. The presence of cancer cells in the lymph nodes usually occurs before the spread of cancer cells throughout the body.
M Is for Metastasis
“M” represents the presence of cancer cells in other organs throughout the body, such as the liver, lung, or brain. This spread is also known as metastasis, or metastases if the cancer has spread to more than 1 organ.
As explained by the National Cancer Institute,1 when cancer is described by the TNM system, numbers are included after each letter. These numbers give more details about the cancer. For example, T3N1M0 indicates a stage III primary tumor with cancer present in at least 1 lymph node but no metastasis. The following may help explain this part of the TNM staging system:
The primary tumor (T)
- TX: Main tumor cannot be measured
- T0: Main tumor cannot be found
- T1, T2, T3, T4: The numbers refer to the size and/or extent of the main tumor. The higher the number after the T, the larger the tumor or the more it has grown into nearby tissues
Regional lymph nodes (N)
- NX: Cancer in nearby lymph nodes cannot be measured
- N0: There is no cancer in nearby lymph nodes
- N1, N2, N3: These numbers refer to the number and location of lymph nodes that contain cancer. The higher the number after the N, the more lymph nodes that contain cancer
Distant metastasis (M)
- M0: Cancer has not spread to other parts of the body
- M1: Cancer has spread to other parts of the body
Clinical vs Pathological Staging
Determining the TNM stage can occur in one of two methods: clinical or pathological. Clinical staging is determined prior to beginning treatment and is based on physical examination and imaging studies (scans) that can determine the extent of a cancer.
The second method gives us the pathological stage of the cancer. The pathological stage is determined if or when the tumor and/or lymph nodes and any other sites of the cancer are surgically removed and examined by a pathologist. The pathologist describes the tumor’s extent based on features of the removed tumor and lymph nodes.
Selection of appropriate chemotherapy or radiation treatment before surgery is based on knowledge of the clinical or pathological stage of the cancer.
General Staging Terms
Your oncology team may use a different staging system. For many cancers, the many combinations of the TNM system are explained in 5 general, less-detailed stages (Box).
General Staging Terms
Stage 0: Some abnormal cells are present, but they have not spread to nearby tissue. Also called
carcinoma in situ, or CIS. CIS is not cancer, but it may become cancer
Stage I, stage II, and stage III (or stage 1, stage 2, and stage 3): Cancer is present. The higher the number, the larger the cancer tumor and the more it has spread into nearby tissues
Stage IV (or stage 4): The cancer has spread (metastasized) to distant parts of the body
It’s important to know that cancer is always referred to by the stage it was given at diagnosis. The cancer itself may change, but the staging will always remain the same. Determining stage is a critically important step in your care that enables your healthcare team to assess the extent of the cancer and determine the best possible treatment plan for you.
- National Cancer Institute. Cancer Staging. National Cancer Institute website. www.cancer.gov/about-cancer/diagnosis-staging/staging. Accessed April 24, 2023.
About the Author
Dr. Greene is the Medical Director, Cancer Data Services at the Levine Cancer Institute in Charlotte, NC. He received his medical degree from the University of Virginia and completed a surgical residency at Yale University School of Medicine. Dr. Greene is a US Navy veteran, was surgeon on the USS Nimitz, and has held leadership positions in the American College of Surgeons and the Commission on Cancer.
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