Breast cancer development

The earliest breast cancers are called "in situ" cancers. In situ means the tumor has stayed inside the place where it first formed and has not spread to nearby tissues.
The most common form of in situ cancer is called "ductal carcinoma in situ," or DCIS. DCIS forms inside the ducts of the breast, which are tubes that carry milk to the nipple when a woman is breastfeeding.
Lobular carcinoma in situ, or LCIS has a similar name but is not considered a true cancer. Instead, it is considered a risk factor for developing cancer in the future. LCIS refers to abnormal cells in the lobules of the breast, which is where milk is made.
When breast cancers spread beyond the ducts or lobules of the breast into the nearby tissue, they are called "locally invasive cancers."
When tumors spread to areas outside the breast through the blood and lymph vessels, they are called "metastatic cancers."