During my third year of medical school, I had the privilege to spend a few weeks working at the Cross Cancer Institute in Edmonton, under the brilliant medical oncologists practicing there. I spoke to many patients in that period, some of whom had been living with cancer for years -- maybe even decades -- and some who were receiving information on their diagnosis for the very first time. In my brief experience, one of the most common questions I was asked, particularly by new patients, was: "What stage is my cancer?" Many people have a general idea of what cancer staging represents. Hearing "Late stage" or "Stage 4" has negative connotations to most people, whether they really understand what it means or not. On this page, we will discuss how cancer staging is performed, and the basics of what it means for a patient and their cancer experience.
Before I started medical school, cancer staging was much a mystery. I knew it spit out a number, or a stage, at the end, but I didn't have the slightest idea what information was used or needed to get there. I imagine this is the level of understanding that many people have, which can make hearing a cancer stage both confusing and frightening. In reality, the criteria behind a cancer's stage is far from a mystery. In fact, there are very clear guidelines that dictate how to stage a cancer, and they are specific to the type of cancer that one is considering. That being said, there are some over-arching categories that are considered for most cancer types. This is known as the TNM system.
T Category
We will start with T. The T category is focused on the size of the primary tumor, or the area of cancer where the cancer first started. This can be determined clinically, based on scans and imaging of the body, or pathologically, after the tumor is removed from the body and exact measurements can be made. A number is generally placed after the T (for example, T1 or T3) which is a representation of how large the tumor is. The higher the number, the larger the tumor. Which exact measurements corelate to which T stage is dependent on the type of cancer. For example, a 3cm tumor may be staged differently for a lung cancer versus a breast cancer. Which tissue the tumor invades may also affect its T stage, including whether it has spread locally to other structures. In some cases, the area where the cancer started, or the primary cancer, cannot be identified. In this case, you may see T0 reported.
N Category
The next category that must be considered is written as N. This category considers whether the cancer has spread to the lymph nodes near the primary tumor. I like to think of lymph nodes as train stations, and the greater lymphatic system as a series of tracks that connects the entire body. Often, cancer cells will have to pass through these "stations" before they can spread elsewhere. Knowing whether cancer is present in the lymph nodes is therefore a helpful tool in understanding the amount or spread of cancer that is present. The N stage is again written as N, followed by a number, similar to the T stage. If no cancer is identified in the relevant lymph nodes, N0 is written. Whether the N staging is N1, N2, or N3, can depend on the number of lymph nodes affected, as well as their location and size. Again, the exact criteria depends on the specific type of cancer. Often, lymph nodes will be removed from the body during surgical resection of a cancer, so that a pathologist can determine whether there is cancer present within them. This is the most exact way to determine the N stage. However, spread of cancer to the lymph nodes can sometimes also be identified or estimated clinically using imaging, such as with a PET scan.
M Category
The final category in TNM staging is M. This category is the simplest of the three. When a cancer is staged, it is either M0 or M1. An M0 cancer is localized to the area which it started. In other words, there is no evidence of spread to other parts of the body, or metastasis. When a cancer is staged as M1, this means that cells from the original source have broken off, and spread through either the lymph or the blood to another location. I think of this like seeds blowing from a dandelion: they can be carried by the wind to entirely new locations and start a new growth there. I represented this in the artwork featured on this page. This process can drastically impact the treatment offered to a patient, and their prognosis. Often, once a cancer metastasizes, it is considered incurable, and treatment focuses on slowing or shrinking its spread, or managing symptoms. However, there are exceptions to this rule, and this may change further in the future as new treatments and guidelines are developed.
While the above categories are the backbone of cancer staging, the process can get more complicated. For example T1N0M0 seems straight-forward enough, but ypT1aN2M0 starts to look complicated. These additional letters are used to further subcategorize a stage, so that it is as specific and as accurate as possible. "p" or "c" can also be written before a stage to note a pathologic or clinical stage respectively. In other words, it's an efficient way to cram a lot of information into a small amount of space. Generally, there are guidelines in place for each type of cancer that determine how the TNM stage is funneled into a single final stage. For example, T1N0M0 may equal stage I, whereas T2N3M1 may equal stage IV. But even this isn't everything. A cancer's grade (or how different the cells are from normal cells), location, cell type, or even the patient's age can be factored in when determining a final stage. Some cancers, such as prostate cancer, may also be staged based on the level of cancer markers in the blood. This is important because all these things can vastly impact how a cancer behaves, how it affects a person, and how it is managed. Physicians will use all this information together to determine which treatment options are best, as well as when and how they should be administered.
References:
1. https://www.cancer.org/cancer/diagnosis-staging/staging.html
2. Edition, S., Edge, S., & Byrd, D. (2017). AJCC cancer staging manual. AJCC cancer staging manual.
3. Gress, D. M., Edge, S. B., Greene, F. L., Washington, M. K., Asare, E. A., Brierley, J. D., ... & Gershenwald, J. E. (2017). Principles of cancer staging. AJCC cancer staging manual, 8, 3-30.
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