How do doctors determine what stage of cancer you have? - Hyunsoo Joshua No and Trudy Wu

Each year, approximately 20 million people across the world receive a cancer diagnosis. At this overwhelming, and often scary time, a patient usually learns their cancer’s stage, which is typically a number, ranging from 1 to 4. While staging is designed in part to help patients better understand what they’re facing, extracting this information from a simple number can be confusing and less than straightforward.

So, what do cancer stages actually mean? To understand stage numbers, we first need to unpack the three variables that inform it. Doctors utilize a system which uses the letters T, N, and M to describe a tumor’s size, its presence in the immune system’s lymph nodes, and whether it has metastasized, or spread, to other organs. Arriving at this letter staging takes thorough investigation—physicians will consider a person’s symptoms and overall health, and may sample, or biopsy, cancerous tissue, order medical scans, and analyze blood tests.

The T designation is usually a number between 1 to 4, and is, in most cases, based on tumor size. But each type of cancer has its own T staging criteria. Five-centimeter-wide tumors are labeled as T3 in oral cancers, but T2 in breast cancers. And some cancers use other staging criteria, like esophageal cancers, which are staged based on how deeply the tumor invades the layers of tissue.

To assign an N stage, doctors evaluate the lymph nodes through biopsies and imaging. Cancer cells tend to break off from the initial tumor and spread. They often travel through the lymphatic system—a network of vessels and nodes, which filter waste and harbor cells that help fight infection. Cancers that spread to larger, more distant, or a greater number of lymph nodes typically file into higher N stages.

M staging involves a more threatening category of cancers’ spread—when diseased cells scatter and then settle on other organs or on bones. Historically, this stage has been a matter of just “yes” or “no,” because once a cancer has metastasized, it’s considered to be much more lethal. But advances in treatment have recently prompted the medical community to rethink the M stage as a continuum. Doctors now consider the number of organs in which the cancer has spread, as well as the abundance and characteristics of the metastatic tumors.

All sorts of combinations of T, N, and M are possible, and one letter doesn't always follow the other. For example, some head and neck cancers will test positive in the lymph nodes N1 with no clear initial tumor, or T0. So how do these three variables inform a cancer's stage number? Each TNM combination correlates to a different overall stage, ordered by how difficult the cancer is to treat. This sorting is rigidly defined for each type of cancer, based on generations of research looking at how cancers with different spreads and characteristics tend to behave.

Importantly, what a certain overall stage means varies from cancer to cancer. For example, a T3N1M0 combination for a breast cancer is considered stage 3 and carries an 85% five-year survival rate. A pancreatic cancer with this same TNM combination, however, is sorted to stage 2, and yet is more difficult to treat with a 15% survival rate. The system is intricate—and ever-changing.

For instance, someone with a stage 4 throat tumor in 2017 might be considered stage 1 just one year later. The cancer didn’t improve; the staging system did. Experts realized that a subset of these advanced cancers responded to existing treatment better than others, so their staging was downgraded. Similar discoveries and advancements in the genetic testing of tumors are refining staging in breast, prostate, and gynecological cancers.

Meanwhile, breakthroughs in therapies can change things seemingly overnight. Many cancers one considered near impossible to treat are now met with high rates of remission. And thanks to improvements in screenings, more and more cancers are being discovered at early stages.

So while many will deal with the reality of cancer, either for themselves or through the diagnosis of a loved one, these advances offer better treatments, more targeted cures, and greater hope for the years to come.