What is tumor grade?
Tumor grade is the description of a tumor based on how abnormal the tumor cells and the tumor tissue look under a microscope. It is an indicator of how quickly a tumor is likely to grow and spread. If the cells of the tumor and the organization of the tumor’s tissue are close to those of normal cells and tissue, the tumor is called “well-differentiated.” These tumors tend to grow and spread at a slower rate than tumors that are “undifferentiated” or “poorly differentiated,” which have abnormal-looking cells and may lack normal tissue structures. Based on these and other differences in microscopic appearance, doctors assign a numerical “grade” to most cancers. The factors used to determine tumor grade can vary between different types of cancer.
Tumor grade is not the same as the stage of a cancer. Cancer stage refers to the size and/or extent (reach) of the original (primary) tumor and whether or not cancer cells have spread in the body. Cancer stage is based on factors such as the location of the primary tumor, tumor size, regional lymph node involvement (the spread of cancer to nearby lymph nodes), and the number of tumors present. More information about staging is in the NCI fact sheet Cancer Staging.
How is tumor grade determined?
If a tumor is suspected to be malignant, a doctor removes all or part of it during a procedure called a biopsy. A pathologist (a doctor who identifies diseases by studying cells and tissues under a microscope) then examines the biopsied tissue to determine whether the tumor is benign or malignant. The pathologist also determines the tumor’s grade and identifies other characteristics of the tumor. The NCI fact sheet Pathology Reports describes the type of information that can be found in a pathologist’s report about the visual and microscopic examination of tissue removed during a biopsy or other surgery.
How are tumor grades classified?
Grading systems differ depending on the type of cancer. In general, tumors are graded as 1, 2, 3, or 4, depending on the amount of abnormality. In Grade 1 tumors, the tumor cells and the organization of the tumor tissue appear close to normal. These tumors tend to grow and spread slowly. In contrast, the cells and tissue of Grade 3 and Grade 4 tumors do not look like normal cells and tissue. Grade 3 and Grade 4 tumors tend to grow rapidly and spread faster than tumors with a lower grade.
If a grading system for a tumor type is not specified, the following system is generally used (1):
- GX: Grade cannot be assessed (undetermined grade)
- G1: Well differentiated (low grade)
- G2: Moderately differentiated (intermediate grade)
- G3: Poorly differentiated (high grade)
- G4: Undifferentiated (high grade)