Types of Breast Cancer

Breast cancer is an abnormal growth of cells that normally line the ducts and the lobules. Breast cancer is classified by whether the cancer started in the ducts or lobules, whether the cells have grown or spread through the duct or lobule (invasive) or not (in situ carcinoma) and the way the cancer cells look under a microscope.  

One of the most important distinctions to understand about breast cancer is the difference between invasive cancer and carcinoma in situ. The key concepts of each are discussed below.

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 early in the course of the disease, though these traveling cancer cells do not always survive beyond the tumor.

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 originated. There are two main categories of carcinoma in situ:  ductal carcinoma in situ and lobular carcinoma in situ.

Though the word "carcinoma" is used in their titles, the cells 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.