Cancer stages are determined by evaluating three core factors: how large the tumor is, whether it has reached nearby lymph nodes, and whether it has spread to distant parts of the body. Doctors combine information from imaging scans, biopsies, and sometimes surgery to assign a stage from 0 to IV. The process has become more sophisticated over time, with newer systems also factoring in tumor biology and genetic markers to give a more complete picture of how a cancer is likely to behave.
The TNM System
Most solid cancers are staged using a framework called TNM, maintained by the American Joint Committee on Cancer (AJCC). Each letter captures a different dimension of the disease:
- T (Tumor): The size of the primary tumor and how deeply it has grown into surrounding tissue. A T1 tumor is small and confined; a T4 tumor is large or has invaded nearby structures.
- N (Nodes): Whether cancer cells have reached nearby lymph nodes. N0 means the nodes are clear. Higher numbers indicate more nodes involved, larger deposits, or cancer that has broken through the outer wall of a node.
- M (Metastasis): Whether the cancer has spread to distant organs or lymph nodes far from the original site. M0 means no distant spread. M1 means it has traveled elsewhere in the body.
Once each letter gets a number, doctors combine them into an overall stage grouping. A small tumor with no node involvement and no distant spread might be stage I, while a tumor of any size that has metastasized to a distant organ is stage IV. The specific cutoffs for tumor size and node involvement vary by cancer type. A 3-centimeter lung tumor and a 3-centimeter breast tumor don’t necessarily receive the same T designation, because the anatomy and behavior of those cancers differ.
What Each Stage Means
Stage 0 describes abnormal cells that haven’t yet grown into nearby tissue. This is sometimes called carcinoma in situ. It isn’t technically cancer, but it can become cancer if left untreated, which is why it’s included in staging discussions.
Stages I, II, and III all indicate cancer is present. The higher the number, the larger the tumor and the more it has spread into surrounding tissues or regional lymph nodes. Stage I cancers are typically small and localized. Stage II and III cancers are progressively larger, more invasive, or involve more lymph nodes. The jump from stage II to III often reflects cancer that has moved beyond the immediate area of the organ where it started but hasn’t yet reached a distant site.
Stage IV means the cancer has spread to distant parts of the body. This could be a lung cancer that has reached the brain, or a colon cancer found in the liver. Stage IV is sometimes called metastatic cancer regardless of tumor type.
How Doctors Gather Staging Information
Staging relies on a combination of physical exams, imaging, and tissue analysis. Common imaging tools include CT scans, MRI, PET scans, ultrasound, and X-rays. Each has strengths for different situations. A CT scan might reveal whether a tumor has grown into adjacent organs, while a PET scan highlights areas of unusually active cell growth throughout the body, helping spot distant metastases that might otherwise go unnoticed. Bone scans are used when there’s concern about spread to the skeleton.
Biopsies provide the most definitive information. A pathologist examines tissue under a microscope to confirm whether cells are cancerous, how abnormal they look, and whether cancer is present in lymph nodes that were removed during surgery.
Clinical vs. Pathological Staging
There are actually two windows in which staging happens. Clinical staging occurs before any surgery, based on imaging, physical exams, and any biopsies taken through a needle or scope. Pathological staging happens after surgery, when a pathologist examines the removed tumor and lymph nodes directly. Pathological staging is generally more accurate because the tissue can be studied in detail, but not every patient has surgery, so clinical staging sometimes remains the only assessment available.
Beyond Anatomy: Tumor Biology and Genetics
Knowing the size and location of a cancer doesn’t always predict how it will behave. Two tumors that look identical on a scan can have very different outcomes depending on their biology. The AJCC recognized this by introducing “prognostic stage groups” that fold biological markers into the staging process alongside the traditional TNM anatomy.
Breast cancer is the clearest example. Staging now incorporates whether the tumor has receptors for estrogen, progesterone, or a protein called HER2. These receptors affect both how aggressive the cancer is and which treatments will work. A tumor that is hormone receptor-positive and HER2-negative, for instance, tends to grow more slowly and responds to hormone-blocking therapies. A breast cancer with a high level of the protein Ki-67, a marker of how quickly cells are dividing, is associated with more aggressive disease.
Genomic tests add another layer. One widely used test analyzes the activity of about 16 genes in breast cancer tissue and produces a recurrence score that predicts how likely the cancer is to come back. This score has been formally integrated into the AJCC staging system, meaning two patients with the same tumor size and lymph node status could end up with different stage assignments based on what their tumor’s genes reveal. A similar genomic test exists for prostate cancer, evaluating gene expression to predict recurrence risk after surgery.
For prostate cancer specifically, the Gleason score has long played a central role. Pathologists examine the pattern of cancer cells under a microscope and assign a score reflecting how abnormal they look. Higher Gleason scores indicate more aggressive cancers. PSA levels in the blood also factor in, with higher values generally pointing to more advanced disease. Genetic changes like the loss of a gene called PTEN, which normally keeps cell growth in check, are associated with higher Gleason scores and more aggressive behavior.
Cancers That Use Different Systems
Not all cancers follow the TNM framework. Lymphomas, both Hodgkin and non-Hodgkin, use the Ann Arbor staging system. Because lymphomas arise in the lymphatic system rather than a single organ, staging focuses on how many lymph node regions are involved and whether the disease has crossed the diaphragm (the muscle separating the chest from the abdomen) or reached organs outside the lymphatic system. Stage I means one lymph node region is affected. Stage IV means the disease has spread to organs like the liver or bone marrow.
Gynecologic cancers such as cervical, ovarian, and uterine cancers use the FIGO system, which is tailored to the anatomy of the reproductive tract. Blood cancers like leukemia often aren’t staged with numbered systems at all, since they’re present throughout the bloodstream from the start. Instead, they’re classified by cell type and other laboratory features.
How Staging Systems Are Updated
Cancer staging is not static. As research reveals new factors that predict outcomes, staging criteria get revised. The AJCC recently shifted from publishing a single staging manual in numbered editions to a rolling “version” system. Version 9, the most current iteration, releases updates for individual cancer types on a rolling basis rather than overhauling every cancer at once. Updated staging protocols for a given cancer type take effect on January 1 of the year following their release. All cancer types not yet updated under Version 9 continue to use the 8th Edition criteria.
This matters for patients because it means the staging system that applies to your cancer depends on when you were diagnosed and which cancer type you have. If you’re comparing your stage to someone diagnosed five or ten years ago, the criteria may have changed, which can make direct comparisons misleading even if the stage number looks the same.