What is breast cancer?

This article was last reviewed and updated on August 15, 2018.
by Jason  Wasserman, MD PhD FRCPC

Normal breast tissue

Invasive ductal carcinoma

Normal breast tissue

Adult breast tissue is made up of small structures called glands which are organized into groups called lobules. Under certain conditions, these glands can produce milk, which is transported to the nipple by a series of small channels called ducts.  


The inside of both glands and ducts is lined by specialized cells called epithelial cells which form a barrier called the epithelium. The tissue surrounding glands and ducts is called stroma and contains long, thin cells called fibroblasts.


What is breast cancer?

Cancer is a word used to describe a disease made up of abnormal cells that have the ability to increase in number faster than normal cells, damage or destroy surrounding normal tissue, and travel (metastasize) to other parts of the body. 


Most breast cancers grow as a large group of cells called a tumour. Pathologists call any tumour in the breast made up of cancer cells a malignant tumour or a malignancy. Another word for a tumour is neoplasia.

Breast cancer is a category of disease that includes any type of cancer that starts from cells normally found in the breast. Within this category there are many different types of breast cancer and the prognosis and treatment will depend on the specific type of breast cancer seen by your pathologist.

Most breast cancers develop from the epithelial cells that line the ducts and glands. These types of cancers are called carcinomas. However, some types of breast cancer develop from the cells in the stroma.

Types of breast cancer

The most common type of breast cancer is called invasive ductal carcinoma. Often, ductal carcinoma is seen with a non-invasive tumour called ductal carcinoma in situ (DCIS). The only difference between invasive ductal carcinoma and ductal carcinoma in situ is that the abnormal cells in ductal carcinoma have broken out of the ducts and glands and have entered the surrounding stroma. The movement of cancer cells from the ducts and glands into the stroma is called invasion.

Another common type of breast cancer is called invasive lobular carcinoma. Similar to ductal carcinoma, it is often seen with a precursor or precancerous tumour called lobular carcinoma in situ.

Other less common types of breast cancer include medullary carcinoma, mucinous carcinoma, tubular carcinoma, metaplastic carcinoma, and secretory carcinoma. 


Most breast cancers are diagnosed after a procedure called a biopsy is performed to remove a small sample of tissue from a lump or other abnormal area in the breast. The biopsy is then sent to a pathologist who examines it under the microscope. If cancer is seen in the biopsy, your pathologist will determine the type of cancer and the grade.


Additional tests such as immunohistochemistry may also be performed on the tissue to see if the cancer cells are producing estrogen or progesterone receptors or if they are making extra amounts of a special protein called HER2. This information is very important because it will help your doctors decide which treatment options to offer you.


Your pathology report after surgery

Most patients with breast cancer will undergo surgery to remove the tumour. Some patients will also be offered radiation therapy or chemotherapy before or after the surgery is performed. If you have surgery to remove the tumour from your body, the entire tumour will be sent to a pathologist for examination. In addition to the tumour, other tissues may also be removed and sent to pathology although the amount of additional tissue removed will depend on your diagnosis. 


Once the entire tumour has been removed, your pathologist will determine the size of the tumour and look to see if the tumour cells have spread the skin of the breast or to any tissues outside of the breast. Your pathologist will also closely examine the cut edges of the tissue (the margins) to make sure that no tumour cells were left inside your body.


If you received treatment (either chemotherapy or radiation therapy) prior to your surgery, your pathologist will also determine how well the treatment worked by looking at how much of the tumour is still alive (viable).


Finally, your pathologist will carefully examine any lymph nodes removed at the same time as the tumour to see if any of them contain cancer cells (lymph node metastases). All of these details will be in your final pathology report. 

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