The specific cellular events that trigger the transformation of a normal cell into a cancerous cell are still unknown. Cancer is caused by a genetic change, sometimes influenced by heredity and sometimes due to environmental influences, or both. Breast cancer does not appear overnight. It is thought to be a gradual process in which certain cells lining the ducts (the epithelial cells) change from normal cells showing an abnormal amount of growth (hyperplasia) to cells that are noticeably different from normal breast cells (atypical) but are not cancerous by definition. These atypical cells eventually begin to regenerate themselves (autonomous growth), an uncontrolled growth that can extend through the cells lining the breast ducts.

Breast cancer begins when a change in a breast duct cell gives that cell a growth advantage over other breast duct cells. The advantage may be a faster rate of cell division or a lower probability of cell death.

Cancer cells tend to be less well developed than normal cells and do not function normally. Their nuclei appear irregular and enlarged and they grow in a disorderly fashion with no regard to spatial patterns typical of healthy tissue.

Cancers similar to normal tissue are well differentiated and tend to be less aggressive. Cancers very different from normal tissues are poorly differentiated and tend to be more aggressive.

Cells that form invasive cancer are characterized by uncontrolled growth. Cancer cells do not necessarily reproduce more rapidly than normal cells. Cancer cells, however, fail to respond to normal signals to stop reproducing. They also do not respect tissue boundaries and invade or infiltrate adjacent tissues via the circulatory or lymphatic systems. The cancer cells can spread within the breast (local invasion) or they can metastasize.

The defining characteristic of invasive cancers is their ability to metastasize, break off from the tumor and travel through the bloodstream or lymph vessels to establish a new growth at a different and sometimes distant site.

Not all abnormal growths are cancerous. Benign tumors do not metastasize.

Types of Breast Cancer

Breast cancers are classified as either noninvasive or as invasive. Noninvasive carcinomas are confined to the ducts or lobules of the breast and invasive carcinomas have grown through the borders of the ducts and invaded surrounding fatty or fibrous structures.

Noninvasive Breast Cancer

The earliest tissue changes that a pathologist can identify as unmistakably malignant are the noninvasive cancers, either ductal carcinoma in situ (DCIS) or lobular carcinoma in situ (LCIS). Noninvasive tumors are often seen with invasive forms and, in these situations, they are grouped with the more serious invasive types and treated accordingly. Carcinoma in situ DCIS and LCIS represent 15% to 20% of breast cancers diagnosed today.

Paget's disease of the nipple is a unique type of ductal cancer characterized by a rash, itching, scaliness or redness on the nipple and an underlying ductal carcinoma that is frequently noninvasive. It represents 3% of all breast cancers.

Invasive (Infiltrating) Breast Cancer

Infiltrating ductal carcinoma arises from the milk ducts and is by far the most common type of breast cancer representing 70% to 75% of all breast malignancies.

Lobular carcinoma arises from the small end ducts of the breast and occurs in both invasive and noninvasive forms. The invasive form is thought to develop from, or be associated with, lobular carcinoma in situ (LCIS). Lobular carcinoma accounts for approximately 5% to 10% of all breast cancers and has a tendency toward bilateral involvement.

Inflammatory carcinoma is characterized by red (inflamed) breast skin. The breast skin may exhibit a pitted appearance known as peau d'orange, or orange peel. Symptoms may mimic those seen with mastitis. Neither fever nor other signs of infection are present, however. Representing between 1% and 4% of all breast cancers, inflammatory carcinoma is associated with a poor prognosis. Recent research demonstrates that long-term remission is possible for patients treated aggressively for this type of breast cancer.

Three of the more common special types of invasive breast cancer are: Medullary cancer which carries with it a good prognosis; Tubular breast cancers which are slow-growing and, if small, rarely involve the lymph nodes and Mucinous (also called colloid) breast cancer that is characterized by the mucin gathered around the tumor cells. The more mucinous the tumor, the more slow growing it is.