Inside a woman's breast are 15 to 20 sections, or lobes. Each lobe is made of many smaller sections called lobules. Fibrous tissue and fat fill the spaces between the lobules and ducts (thin tubes that connect the lobes and nipples). Breast cancer occurs when cells in the breast grow out of control and form a growth or tumor. Tumors may be cancerous (malignant) or not cancerous (benign).
SEER Cancer Stat Facts: Female Breast Cancer. National Cancer Institute. Bethesda, MD, http://seer.cancer.gov/statfacts/html/breast.html