Anatomy of the Breast
Each breast contains 15 to 20 lobes, each made up of several smaller sections called lobules (NCCN 2016; PDQ Adult Treatment Editorial Board 2017). Breast milk is produced in the lobules and travels through the ducts to the nipple (Figure 1).
The tissue around the lobes and ducts is called stroma. Within the stroma, vessels carry a clear fluid called lymph. Lymph delivers immune cells, water, and nutrients to the breast tissue and drains to nearby lymph nodes (NCCN 2016). Healthy breasts also contain fat tissue, connective tissue, nerves, and blood vessels.
Figure 1. Breast Anatomy (OpenStax College 2013)
The hormones estrogen and progesterone are important for the development and function of the breast during puberty and pregnancy (Hilton 2017). Estrogen is primarily produced by the ovaries before menopause, but some is produced by the adrenal glands and, in smaller amounts, in fat tissue and the liver (McNamara 2016). Estrogen can also be produced in the breast tissue itself by a process called aromatization (Yaghjyan 2011). There are two main types of estrogen receptors: estrogen receptor alpha and estrogen receptor beta (McNamara 2016). Activation of estrogen receptors plays a role in breast growth and in many cases of breast cancer.
Non-Cancerous Breast Conditions
The breasts can develop several non-cancerous conditions, some of which may cause symptoms similar to breast cancer. Cysts, fibroadenomas, and calcifications are common benign or non-cancerous breast diseases (Neal 2014; Mayo Clinic 2018). Another condition, hyperplasia, is a condition in which the cells in the duct or lobe are dividing too frequently (Davidson 2016). Women with so called atypical hyperplasias are at increased risk for developing breast cancer (Dion 2016; Farshid 2017).
Breast cancer can develop from cells in either the ducts or lobes, but ductal tumors are more common (NCCN 2016; PDQ Adult Treatment Editorial Board 2017). The cells acquire mutations that cause them to divide too rapidly or survive too long. One mutation in breast cancer leads to increased signaling through the HER2 growth factor receptor pathway (Davidson 2016). Cells with extra copies of the HER2 gene can grow more quickly (Petrelli 2017). Some people inherit or develop a mutation in the BRCA1 or BRCA2 genes, which are involved in DNA repair (Takaoka 2017; Davidson 2016). Presence of certain BRCA1 and BRCA2 mutations increase breast and ovarian cancer risk (Toland 2017).
As breast cells acquire more mutations, they begin to look less normal under a microscope. They can divide quickly and are less likely to die when normal cells would (NCCN 2016). Over time, they form a mass or tumor.
Tumors that are small and confined to the lobular or ductal tissue are called non-invasive (Davidson 2016; Posner 1992). Ductal carcinoma in situ (DCIS) is a non-invasive tumor that may become invasive (Rakovitch 2012). DCIS is typically detected by a mammogram (PDQ Screening Prevention Editorial Board 2017b). While some studies found that many DCIS lesions would not progress even without treatment, some of them may become invasive tumors over time. DCIS is normally treated with surgery and radiation, and the prognosis is excellent (NCCN 2017b; PDQ Adult Treatment Editorial Board 2017; Lee 2012; Jones 2006; Vatovec 2014).
Lobular carcinoma in situ (LCIS) is less common than DCIS and less likely to become an invasive tumor (Venkitaraman 2010; Frykberg 1999). LCIS does not normally require standard breast cancer treatments, although these are sometimes used, depending on the patient’s characteristics, but all women diagnosed with LCIS should be carefully monitored and make lifestyle changes to reduce their risk of developing breast cancer (see, “Dietary and Lifestyle Considerations”) (NCCN 2017b; Davidson 2016; Cutuli 2015).
Tumors that have grown into the nearby stromal tissue in the breast are called invasive (NCCN 2016). In some cases, tumor cells are able to move away from the original tumor and invade nearby lymph nodes (Davidson 2016; NCCN 2016). Beyond the lymph nodes, breast cancer may spread to distant sites such as the bones, lungs, liver, or brain (Rostami 2016; Chen 2017).