What is tumor grade?

Tumor grade is a description of how abnormal the cancer cells look under a microscope, based on the pathologist's examination of stained tissue. It is part of the microscopic description in a pathology report.

Grade is different from stage. Stage describes the extent of the cancer in the body (such as tumor size and whether it has spread), while grade describes how the cells themselves appear. Both are used, along with other findings, to understand a cancer and plan treatment.

What the pathologist is judging is resemblance. Normal cells in any organ have a familiar look and a tidy arrangement. As cells turn cancerous, they can lose that. Nuclei grow larger and darker, shapes vary from cell to cell, and the tissue's normal structures fall apart. The medical word for how closely cancer cells still resemble the tissue they came from is differentiation. Cells that still look like their home tissue are called well differentiated. Cells that have lost the resemblance are called poorly differentiated or undifferentiated.

Appearance turns out to predict behavior. Cells that look close to normal usually grow more slowly. Cells that look very abnormal tend to grow and spread faster. That is the whole reason grade is worth recording.

The general scale runs like this:

• Grade X — grade cannot be assessed
• Grade 1 — well differentiated (low grade)
• Grade 2 — moderately differentiated (intermediate grade)
• Grade 3 — poorly differentiated (high grade)
• Grade 4 — undifferentiated (high grade)

Not every cancer uses that scale, and this trips people up. Several cancers have their own grading systems, and the same number can mean different things in different systems. Prostate cancer is the clearest example. A pathologist finds the two most common cell patterns in the tissue, grades each from 3 to 5, and adds them to get a Gleason score between 6 and 10. A score of 6 is low grade, 7 is medium grade, and 8, 9, or 10 is high grade. Those same results are now also reported as Grade Groups. Grade Group 1 is a Gleason score of 6 or less. Grade Groups 2 and 3 both cover a score of 7. Grade Group 4 is a score of 8, and Grade Group 5 is a score of 9 or 10. So "grade 3" means nothing on its own until you know which system produced it.

Your doctor uses tumor grade along with other factors to build a treatment plan, and in some cases to estimate prognosis. A high-grade cancer may call for more aggressive treatment right away. Grade can also help decide whether to add chemotherapy after surgery, or whether a small, slow-growing cancer can be watched rather than treated immediately. It never works alone. Stage, surgical margins, and molecular test results all feed into the same decision.

One thing to expect: the grade can change. A needle biopsy samples a small piece of a tumor, and tumors are not uniform. When the whole tumor is removed later, the pathologist sees more of it, and the grade on the surgical report sometimes differs from the grade on the biopsy. A second pathologist reviewing the same slides can also reach a different grade, which is one reason second opinions on pathology are common and reasonable.

Your healthcare team is the best source of information about your tumor's grade and what it means for your care. When you get the report, ask which grading system was used and where your result falls inside it. "Grade 2 of 3" or "Grade Group 2" will serve you much better later than a bare number.