What is Breast Cancer?

Breast cancer is a type of cancer where cells in the breast tissue divide and grow without the normal control. About 85 percent of breast cancers originate in the mammary ducts, while about 15 percent arise in the lobules [5]. Cancerous tumors in the breast usually grow very slowly so that by the time one is large enough to be felt as a lump, it may have been growing for as long as ten years.

One of the most important distinctions to understand about breast cancer is the difference between invasive cancer and carcinoma in situ (kar-sin-O-ma in SY-too). The key concepts of each are discussed below, with greater detail provided in the Diagnosis section.
Invasive Cancer

The more serious of the two, invasive breast cancer, develops when abnormal cells from inside the lobules or ducts break out into the surrounding breast tissue. This provides an opportunity for cancer to spread to the lymph nodes and, in advanced stages, to organs like the liver, lungs and bones.

In the past, breast cancer was thought to grow in an orderly progression from a tiny tumor in the breast tissue to a larger one, sequentially traveling out to the nearby lymph nodes, then distant ones, and finally metastasizing in other parts of the body. Now, however, it is thought that cancer cells are capable of traveling from the breast through the blood and lymphatic system very early in the course of the disease, though these traveling cancer cells do not always survive beyond the tumor [1].
Carcinoma In Situ

When abnormal cells grow inside the lobules or milk ducts but have not spread to the surrounding tissue or beyond, the condition is called carcinoma in situ. The term "in situ" means "in place" and is used to describe this condition because the abnormal cells are still "in place" inside the lobules or ducts where they first developed. There are two main categories of carcinoma in situ: ductal carcinoma in situ and lobular carcinoma in situ.

Although the word carcinoma is used in their titles, the cells involved in the different carcinomas in situ are not fully cancerous because they have not developed the ability to invade tissues outside of the ducts or lobules and metastasize. They are often referred to as precancerous conditions because they can either develop into or raise the risk of invasive cancer.