My tumor was classified as Grade 3, which is aggressive. Here is a little bit more information about grading cell growth and necrosis from http://www.breastcancer.org/:
"Grade" of cancer cell growth: Patterns of cell growth are rated on a scale from 1 to 3 (also referred to as low, medium, and high instead of 1, 2 or 3). Calm, well-organized growth with few cells reproducing is considered grade 1. Disorganized, irregular growth patterns in which many cells are in the process of making new cells is called grade 3. The lower the grade, the more favorable the expected outcome. At the same time, the higher the grade, the more vulnerable the cancer is to treatments such as chemotherapy and radiation. Therefore, women with a "high" or "grade 3" breast cancer can also feel hopeful about treatment.
Dead cells within the tumor: It's tempting to think that the only good cancer cell is a dead cancer cell. However, necrosis (or dead tumor cells) is one of several signs of excessive tumor growth. It means that a tumor is growing so fast that some tumor cells wither and die because there's not enough blood supply to feed all of them. This is a small feature of the cancer but an unfavorable one, because the growth is at such a high rate.
This is interesting and explains to me why so many women have regular mammograms and don't get callbacks, then suddenly they discover a relatively large lump a few months later.
I don't think it is an indicator of the likelihood of a recurrence. Even though my tumor was high-grade, it had not spread to the nodes, which is where it's "supposed" to infiltrate first if the sentinel nodes are doing their jobs.
The lesson learned is definitely to do your self-exams along with getting your annual mammograms. Know your breasts, your lumps and bumps, and if you think there is something "different" going on, get it checked out. Because yes, it could be nothing; or it could be an angry tumor.