Causes

The cause of fibroadenomas is unknown, but they might be related to reproductive hormones. Fibroadenomas occur more often during your reproductive years, can become bigger during pregnancy or with use of hormone therapy, and might shrink after menopause, when hormone levels decrease.

Types of fibroadenomas

In addition to simple fibroadenomas, there are:

• Complex fibroadenomas. These can contain changes, such as an overgrowth of cells (hyperplasia) that can grow rapidly. A pathologist makes the diagnosis of a complex fibroadenoma after reviewing the tissue from a biopsy.
• Juvenile fibroadenomas. This is the most common type of breast lump found in girls and adolescents between the ages of 10 and 18. These fibroadenomas can grow large, but most shrink over time, and some disappear.
• Giant fibroadenomas. These can grow to larger than 2 inches (5 centimeters). They might need to be removed because they can press on or replace other breast tissue.
• Phyllodes tumor. Although usually benign, some phyllodes tumors can become cancerous (malignant). Doctors usually recommend that these be removed.