Most cancers are classified into five stages, numbered 0 through IV (4). The higher the number, the more the cancer has grown or spread. This system applies to the majority of solid tumors, though a few cancer types, including leukemias and brain tumors, use different classification methods.
The Five Stages Explained
Stage 0 is the earliest possible designation. It describes abnormal cells that are present but have not spread into nearby tissue. This is often called carcinoma in situ. The cells are confined to the layer of tissue where they first developed. Stage 0 is technically not yet invasive cancer, but it can become cancer if left untreated. Ductal carcinoma in situ (DCIS) in the breast is a common example.
Stage I means cancer is present and has begun invading surrounding tissue, but it remains small and confined to the organ where it started. There is no evidence it has reached nearby lymph nodes.
Stage II and Stage III represent larger or more locally advanced cancers. The tumor may be bigger, may have grown deeper into surrounding structures, or may have spread to nearby lymph nodes. The distinction between II and III generally comes down to how many lymph nodes are involved and how far the cancer extends beyond the original site. The specifics vary by cancer type.
Stage IV means the cancer has spread to distant parts of the body. This is also called metastatic cancer. A breast cancer that has spread to the bones or lungs, for instance, would be classified as stage IV regardless of how small the original tumor is.
How Doctors Determine the Stage
The standard framework for most solid cancers is the TNM system, maintained by the American Joint Committee on Cancer (AJCC). It evaluates three things: the size and extent of the primary tumor (T), whether cancer is in nearby lymph nodes (N), and whether it has metastasized to distant organs (M). Each letter gets a number. T1 through T4 reflects increasing tumor size or depth. N0 means no lymph node involvement, while N1 through N3 means more nodes contain cancer. M0 means no distant spread, and M1 means the cancer has reached other parts of the body.
These TNM values are then combined to assign the overall stage of 0 through IV. A small tumor with no lymph node involvement and no spread might be T1, N0, M0, which typically translates to stage I. A large tumor that has reached several lymph nodes but hasn’t spread to distant organs might be stage III. Any combination that includes M1 is stage IV.
Staging can happen at two points. Clinical staging is based on physical exams, imaging (CT scans, PET scans, MRIs), and biopsies done before treatment. Pathological staging happens after surgery, when a pathologist examines the removed tumor and lymph nodes under a microscope. Pathological staging tends to be more precise because it is based on direct tissue analysis rather than imaging alone.
Substages Add More Detail
Within each stage, you’ll often see letters that break things down further: stage IIA versus IIB, or stage IIIA versus IIIB versus IIIC. These substages reflect meaningful differences in tumor size, lymph node involvement, or how deeply the cancer has grown. A stage IIIC cancer, for example, typically carries a different outlook and treatment plan than stage IIIA, even though both fall under the broad “stage III” label.
Modern staging also increasingly incorporates biological features beyond physical size and spread. For certain cancers, factors like how quickly cells are dividing (tumor grade), hormone receptor status, or specific genetic markers can shift the assigned stage up or down. Two tumors of the same physical size can end up at different stages if their biology differs significantly. The AJCC has been rolling out updates to its staging system on a cancer-by-cancer basis, with the newest Version 9 protocols replacing older criteria for specific disease sites each year.
Cancers That Use Different Systems
Not every cancer fits neatly into the 0 through IV framework. Blood cancers like leukemia don’t form solid tumors, so a system based on tumor size doesn’t apply. Chronic lymphocytic leukemia (CLL) is staged using the Rai system in North America, which evaluates factors like whether the bone marrow is involved, whether lymph nodes are enlarged, and whether blood cell counts are abnormal. Lymphomas historically used the Ann Arbor staging system and now use the Lugano classification, which tracks which groups of lymph nodes are affected and whether the disease has spread above or below the diaphragm.
Brain and spinal cord tumors also follow a separate path. They are graded rather than staged, most commonly using the World Health Organization (WHO) system. This grading looks at how abnormal the cells appear under a microscope and how aggressively they are likely to grow, rather than whether they’ve spread to lymph nodes. Brain tumors rarely metastasize to distant organs the way other cancers do, which makes the standard TNM approach less useful.
Why Staging Matters for You
Your cancer’s stage is one of the most important pieces of information in your diagnosis. It shapes the treatment plan, influences what options are available, and provides a general framework for prognosis. Lower stages generally mean smaller, more localized disease that can often be treated with surgery alone or surgery plus a short course of additional therapy. Higher stages typically require more intensive or systemic treatment.
One important detail: staging is usually set at diagnosis and does not change, even if the cancer responds to treatment or later comes back. If a stage II cancer is treated and then recurs in a distant organ, it is described as “recurrent” rather than being restaged as stage IV. This can be confusing, but the original stage remains the reference point because it reflects the cancer at the time it was first characterized. Doctors will separately describe the current extent of disease when planning new treatment.