Several different systems are used for ranking cancers by stage in the process of developing a treatment program. Staging cancers is important, because it allows a doctor to assess a cancer and use standardized terminology to describe it, so that the medical team can work together to develop the best course of treatment. Stages are ranked in order of severity, with slow or non-aggressive cancers at the bottom of the scale, and fast-moving, severe cancers at the top.

One very common system for staging cancers runs from zero to four, with a stage zero cancer being the least severe, while four is the most aggressive. An alternate numbered staging system is depicted in Roman numerals, with the option of I, II, III, and IV. Some physicians break the Roman numeral staging system down even further, with classifications like IIa and IIb to describe cancers which fall slightly between the stages.

Some doctors use the TNM system to stage cancers, which ranks a cancer with three separate parameters: tumor size, lymph node involvement, and metastasis. For example, someone could have a T3, N0, M1 cancer, meaning that the tumor was medium-sized, no lymph nodes are involved, and the cancer has begun to metastasize slightly.

Other oncologists refer to in situ, localized, regional, and distant cancers when they talk about cancer staging. In situ cancers are cancers which only involve a few cells, meaning that they are caught early or they develop slowly. Localized cancers affect a larger area, while regional cancers are cancers which have begun to spread to neighboring organs and lymph nodes. In a distant cancer, the cancer has spread to remove areas of the body, reflecting a widespread metastasis. This system is essentially interchangeable with the Roman numeral system of staging.

All types of cancer can be ranked under a staging system, including breast, colon, lung, and cervical cancers. Lower stages usually require less aggressive treatment, because the cancer is confined to a small area, and it may be able to be excised and eliminated. Higher stages of cancer require more serious treatment, and in some cases, a high stage cancer may be deemed untreatable as a result of its persistent and wide spread.

Doctors sometimes disagree over the staging of a particular cancer, and seeking a second opinion can sometimes generate conflicting information. If doctors do give different answers, it is a good idea to ask about why they disagree on their staging assessment, and how their treatment approaches might differ.