What determines the stage of a cancer?

Q. What determines the stage of a cancer?

A. Cancers are categorized by stage at the time of diagnosis to describe the severity and how far it has progressed. “It gives doctors a common language to be able to talk about cancers,” says Gregory Russo, a radiation oncologist at Boston Medical Center. “If someone says [a patient] has a stage IIIb lung cancer, I know the critical issues surrounding that patient’s care.” The initial stage assigned to the cancer doesn’t change if the cancer progresses or diminishes.

The main factors that determine a cancer’s stage are the location in which it originated, the size of the tumor, whether it has spread to adjacent tissues or traveled to lymph nodes, and whether it has metastasized (created tumors in faraway parts of the body). Also important is the tumor grade, which is based on the appearance of biopsied tumor cells under a microscope. How these different factors are weighted depends on the cancer type.

The most common staging system, called TNM, assigns a number from zero to four for three different measures: the size or extent of the primary tumor (T), the degree of spread to lymph nodes (N), and the degree of metastasis (M).

Russo says that labeling cancer with a stage shouldn’t be a major concern for patients; more important, he says, is “to understand where the tumor started, where it is currently, and how that impacts the treatment and the expectations they’re going to have.”