Most phyllodes tumors are benign, but in rare cases they can be malignant (cancerous). In addition, their behavior is unpredictable. In rare cases, a benign tumor can recur (grow back) as a cancer. So they’re a little different from the other benign breast conditions linked to a slight increase in breast cancer risk.
Phyllodes tumors develop in the breast’s connective tissue, which is called the stroma. The stroma includes the fatty tissue and ligaments that surround the ducts, lobules, and blood and lymph vessels in the breast. The tumor can contain both connective tissue and glandular tissue (the kind that makes up the ducts and lobules).
Phyllodes tumors are rare, but they are usually benign. They tend to grow quickly, so they’re usually felt as a lump first. It can be difficult to tell the difference between a benign phyllodes tumor and a fibroadenoma, which is the most common type of benign breast tumor.
Fibroadenomas tend to occur in younger women (under age 30), while phyllodes tumors are more common for women in their 40s. Your doctor would need to remove the tumor and have it examined under a microscope to make the correct diagnosis. A key feature is an obvious overgrowth of connective tissue.
Even when phyllodes tumors are benign — as opposed to malignant or “borderline,” meaning that the cells have some abnormal features — they have a tendency to recur. If any cells were left behind, in rare cases they can change into malignant cells. This is why even benign tumors are treated by removing the entire growth and a margin of normal breast tissue all around the tumor. A benign phyllodes tumor treated in this way is not associated with any future increased risk of breast cancer.
Learn more about phyllodes tumors of the breast.