Grade 1 breast cancer is the lowest grade of breast cancer, meaning the cancer cells look and behave most like normal breast tissue. On a pathology report, it may also be called “well-differentiated” or “low grade.” Of the three possible grades, grade 1 carries the least aggressive potential and generally has the best prognosis.
If you’re reading this, you or someone close to you likely just received a pathology report with this term on it. Here’s what it means in practical terms.
What “Grade” Actually Measures
Grade describes how abnormal the cancer cells look under a microscope. It’s separate from stage, which measures how large the tumor is and whether it has spread to lymph nodes or other parts of the body. Two people can have the same stage of breast cancer but different grades, and vice versa. Grade reflects the tumor’s “aggressive potential,” while stage reflects how far it has already progressed.
Pathologists determine grade using the Nottingham scoring system, which evaluates three features of the tumor tissue:
- Gland formation: How much the cancer cells are still organizing themselves into the tube-like glands found in normal breast tissue. More gland formation means the cells are closer to normal.
- Nuclear appearance: How uniform the cell nuclei look in size and shape compared to healthy cells. In grade 1 tumors, the nuclei show minimal variation and are less than 1.5 times the size of normal breast cell nuclei.
- Mitotic count: How many cells are actively dividing. Fewer dividing cells means the tumor is growing more slowly.
Each feature is scored from 1 to 3, and the scores are added together. A total of 3 to 5 points results in a grade 1 classification. A score of 6 or 7 is grade 2, and 8 or 9 is grade 3.
How Grade 1 Cells Differ From Higher Grades
Under a microscope, grade 1 breast cancer cells are still trying to do what normal breast cells do. They form small glands, their nuclei are fairly uniform in size, and they divide relatively slowly. A pathologist looking at a grade 1 invasive ductal carcinoma (the most common type) typically sees small, organized glands with regular-looking nuclei. The tissue hasn’t lost its basic architecture the way higher-grade cancers have.
Grade 2 and grade 3 tumors progressively lose this organization. By grade 3, cells look very different from normal breast tissue, have large irregular nuclei, and are dividing rapidly. This is why higher-grade cancers tend to grow and spread faster.
Grade 1 and Hormone Receptor Status
Grade 1 breast cancers are very commonly hormone receptor positive, meaning the cancer cells have receptors for estrogen, progesterone, or both. This matters because hormone receptor positive cancers respond to treatments that block or lower these hormones. The strong association between low grade and hormone receptor positivity is one reason grade 1 cancers tend to respond well to treatment and have favorable outcomes.
Grade 1 tumors are unlikely to be triple-negative (lacking estrogen receptors, progesterone receptors, and the HER2 protein), which is a more aggressive subtype typically seen in higher-grade cancers.
How Grade 1 Breast Cancer Is Treated
Treatment for grade 1 breast cancer typically starts with surgery to remove the tumor, either a lumpectomy (removing the tumor and a margin of surrounding tissue) or a mastectomy. The choice depends on tumor size, location, and personal preference rather than grade alone.
After surgery, the next steps depend on the specifics of your case:
- Radiation therapy: Usually recommended after a lumpectomy to reduce the chance of cancer returning in the breast. After a mastectomy, radiation may not be needed.
- Hormone therapy: Because grade 1 tumors are so frequently hormone receptor positive, hormone therapy is a common part of the treatment plan. This involves taking medication for several years that either blocks estrogen or reduces its production in the body.
- Chemotherapy: This is where grade matters most in treatment decisions. You are more likely to receive chemotherapy if the tumor is high grade, has spread to lymph nodes, is HER2 positive, or is triple negative. Grade 1 tumors that are hormone receptor positive and node negative are among the least likely to need chemotherapy.
Genomic tests that analyze the activity of specific genes in the tumor tissue can further refine whether chemotherapy would provide meaningful benefit. For many grade 1 cancers, these tests confirm that hormone therapy alone (after surgery and possibly radiation) is sufficient.
What Grade 1 Means for Prognosis
Grade 1 breast cancers tend to be less aggressive, grow more slowly, and have a better prognosis than grade 2 or grade 3 tumors. The cells are still well-organized and dividing at a relatively low rate, which translates to lower recurrence rates and higher long-term survival compared to higher grades.
That said, grade is just one piece of the picture. Stage, hormone receptor status, HER2 status, lymph node involvement, and tumor size all factor into the overall outlook. A grade 1 cancer that was caught early and is hormone receptor positive has an excellent prognosis. The combination of low grade with early stage is one of the most favorable scenarios in breast cancer.
Reading Your Pathology Report
Your pathology report may describe the grade using several interchangeable terms. “Grade 1,” “grade I,” “low grade,” “well-differentiated,” and a Nottingham score of 3, 4, or 5 all mean the same thing. You might also see “Scarff-Bloom-Richardson” or “Elston-Ellis” mentioned, which are names for the grading system used.
The report will list the individual scores for gland formation, nuclear appearance, and mitotic count, then the total. If your total is between 3 and 5, that’s how the grade 1 designation was reached. The report will also include your tumor’s hormone receptor status, HER2 status, tumor size, and margin status (whether cancer cells were found at the edge of the removed tissue), all of which your care team uses alongside grade to plan treatment.