Breast cancers are graded 1–3, with 1 being the most similar to normal, healthy cells, and 3 being the most different from your normal cells and most aggressive. Note that the grade of the tumor is a reflection of the pathology report and this is different than the stage of cancer.
Staging of breast cancer is determined from its size and location and whether the cancer has spread to other locations in the body. The size of a breast cancer can be measured in centimeters or millimeters. There are small and large cancers, and the size of the cancer doesn't always coincide with the aggressiveness of the cancer. Small cancers can be fast-growing, and large cancers can be slow-growing. Small cancers can be invasive, and large cancers can be noninvasive.
Sometimes cancer can occur in multiple areas of the breast (called multi-centric) or it can be in one location in the breast (multifocal).