In a healthy body, natural systems control the creation, growth and death of cells. Most of the time cells divide and grow in an orderly manner. But sometimes cells grow out of control. This kind of growth of cells forms a mass or lump called a tumor. Tumors are either benign or malignant. Benign tumors are not cancerous. The cells of a benign tumor do not invade nearby tissue or spread to other parts of the body. Malignant tumors are cancerous. These tumor cells can invade nearby tissue and spread to other parts of the body. A malignant tumor that develops in the breast is called breast cancer.
Tumors in the breast tend to grow slowly. By the time a lump is large enough to feel, it may have been growing for as long as 10 years. However some tumors are aggressive and grow much faster. Cells can grow out of control before any symptoms of the disease appear. That is why breast cancer screening to find early changes is so important. If breast cancer is found early, there are more treatment options and a greater chance of survival.
Between 50 and 75 percent of breast cancers begin in the milk ducts (a canal that carries milk from the lobules to a nipple opening during breastfeeding) and 10 to 15 percent begin in the lobules (spherical-shaped sacs in the breast that produce milk).
It is important to understand the differences between invasive breast cancer and non-invasive breast cancer, called ductal carcinoma in situ (DCIS). These differences affect treatment and prognosis.
Non-invasive breast cancer occurs when abnormal cells grow inside the milk ducts.
Ductal carcinoma in situ (DCIS) occurs when abnormal cells grow inside the milk ducts but have not spread to nearby tissue or beyond. The term “in situ” means “in place.” With DCIS, the abnormal cells are still “in place” inside the ducts. DCIS is a non-invasive breast cancer (you may also hear the term “pre-invasive breast carcinoma”). Although the abnormal cells have not spread to tissues outside the ducts, they can develop into invasive breast cancer.
Invasive breast cancer occurs when abnormal cells from inside the milk ducts or lobules break out into nearby breast tissue.
Cancer cells can travel from the breast to other parts of the body through the blood stream or the lymphatic system (a network of lymph nodes and vessels throughout the body). They may travel early in the process when the tumor is small or later when the tumor is large.
The lymph nodes (small clumps of immune cells that act as filters for the lymphatic system) in the underarm area (the axillary lymph nodes) are the first place breast cancer is likely to spread.
Metastatic breast cancer is breast cancer that has spread beyond the breast to other organs in the body (most often the bones, lungs, liver or brain). Although metastatic breast cancer has spread to another part of the body, it is considered and treated as breast cancer. For example, breast cancer that has spread to the bones is still breast cancer (not bone cancer) and is treated with breast cancer drugs, rather than treatments for a cancer that began in the bones.