Most cancers are graded on a scale of 1 to 4, based on how abnormal the cells look under a microscope compared to healthy tissue. Some cancer types use their own grading systems with different scales, so the number of grades depends on where in the body the cancer is found.
The Standard 4-Grade System
The most widely used grading system, defined by the National Cancer Institute, assigns one of four grades:
- Grade 1 (well differentiated): Cancer cells look close to normal and are organized in patterns similar to healthy tissue. These tend to grow slowly.
- Grade 2 (moderately differentiated): Cells look somewhat abnormal and fall between low and high grade in terms of growth speed.
- Grade 3 (poorly differentiated): Cells look clearly abnormal, with little resemblance to normal tissue. These cancers tend to grow and spread more quickly.
- Grade 4 (undifferentiated): Cells look so abnormal that pathologists sometimes can’t identify what type of normal cell they originally came from. These are the most aggressive.
You may also see “GX” on a pathology report, which means the grade couldn’t be assessed. This is sometimes called an undetermined grade, and it usually means the tissue sample wasn’t adequate for grading.
What “Differentiation” Actually Means
The word “differentiated” refers to how much cancer cells still resemble the normal cells they came from. A well-differentiated tumor (grade 1) still forms structures that look like healthy tissue. An undifferentiated tumor (grade 4) has lost those features almost entirely. Think of it as a spectrum: the less a cancer cell looks and behaves like its original cell type, the higher the grade and the more aggressively it tends to grow.
This matters because grade reflects the biology of the cancer, not its size or location. Two tumors can be the same size and in the same organ but behave very differently depending on their grade. A low-grade tumor may grow slowly for years, while a high-grade tumor of the same size could spread within months.
Grade Is Not the Same as Stage
Grade and stage are easily confused, but they measure different things. Grade describes what the cancer cells look like at the microscopic level. Stage describes how large the tumor is and whether it has spread to lymph nodes or other parts of the body. A cancer can be low grade but high stage (slow-growing cells that have already spread) or high grade but low stage (aggressive-looking cells caught early before spreading). Both grade and stage factor into treatment decisions.
Prostate Cancer Uses 5 Grade Groups
Prostate cancer has its own grading system based on the Gleason score, which evaluates how the glandular tissue is organized. Pathologists assign a score based on the two most common patterns seen in a biopsy, producing a combined score. In practice, the lowest score assigned today is 6 on a scale of 2 to 10.
Because Gleason scores can be confusing (a 6 sounds alarming on a 10-point scale, even though it’s actually the lowest grade assigned), the system was reorganized into five Grade Groups:
- Grade Group 1 (Gleason score 6 or lower): well-formed glands only, the least aggressive
- Grade Group 2 (Gleason score 3+4=7): mostly well-formed glands with some poorly formed areas
- Grade Group 3 (Gleason score 4+3=7): mostly poorly formed glands with some well-formed areas
- Grade Group 4 (Gleason score 8): predominantly poorly formed or absent gland structures
- Grade Group 5 (Gleason score 9-10): almost no recognizable gland formation, the most aggressive
The distinction between Grade Groups 2 and 3 is worth noting. Both correspond to a Gleason score of 7, but Group 3 has a worse outlook because the dominant pattern is more abnormal. If your pathology report shows a Gleason 7, the order of the two numbers (3+4 versus 4+3) matters significantly.
Breast Cancer Grading
Breast cancer uses a three-grade system called the Nottingham grading system. Instead of evaluating just one feature, pathologists score three characteristics: how well the cells form tube-like structures (which normal breast tissue does), how abnormal the cell nuclei look, and how quickly the cells are dividing. Each characteristic gets a score of 1 to 3, and the totals are added up to assign an overall grade of 1, 2, or 3. This composite approach gives a more detailed picture than looking at any single feature alone.
Brain Tumor Grading
Brain and spinal cord tumors follow a World Health Organization system that assigns grades 1 through 4, labeled as “CNS WHO grade” to distinguish them from the standard system used elsewhere in the body. The 2021 classification switched from Roman numerals (I, II, III, IV) to Arabic numerals (1, 2, 3, 4) and dropped older terms like “anaplastic” in favor of grade numbers alone. Importantly, brain tumors are graded within their specific type, meaning a grade 2 of one tumor type is not directly comparable to a grade 2 of a different type.
How Grade Affects Treatment
Low-grade cancers (grade 1 or 2) tend to grow slowly and may need less aggressive treatment. In some cases, particularly with very low-grade prostate cancer, doctors may recommend active surveillance rather than immediate treatment, monitoring the cancer over time with regular testing.
High-grade cancers (grade 3 or 4) grow faster and are more likely to spread, so they typically call for more intensive treatment. The specific combination of grade, stage, and other factors like genetic markers determines the full treatment plan. Grade alone doesn’t dictate what happens next, but it’s one of the most important pieces of the puzzle. When you receive a pathology report, the grade tells you something fundamental about how the cancer is likely to behave, which is why it’s worth understanding what your specific number means in the context of your cancer type.