Breast Anatomy and Cancer Development: A Timeline

Each breast contains 15-20 glands called lobes, where milk is produced. Each lobe consists of a branching structure made up of lobules and acini (alveoli). The acini are lined by a single layer of milk-secreting epithelial cells. In turn, each acinus is surrounded by an interwoven pattern of contractile myoepithelial cells. (myoepithelial cells can contract and expel the secretion of exocrine glands i.e. milk.) The lumens of the acini connect to collecting intra-lobular ducts, which empty into the main 15 to 20 lobular collecting ducts. Subsequently, each lobe drains into the nipple which is surrounded by the pigmented areola.

The glandular tissue is embedded in fat, which account for most of the mass of the breast. The breast and armpit also contain lymph nodes and vessels carrying lymph fluid, which are part of the immune system. Breast cancer can sometimes spread to other areas of the body through this lymph system or blood vessels.

Breast DiagramBreast Diagram

Breast cancers can start from different parts of the breast. Most breast cancers begin in the ducts that carry milk to the nipple (ductal cancers). Some start in the glands that make breast milk (lobular cancers). There are also other types of breast cancer that are less common.

Breast cancers take many years to grow to a size large enough to be clinically detectable by mammography or palpation. The doubling time of tumors is approximately 200 days in postmenopausal women and 160 days in premenopausal women.1The mammographic threshold for clinical detection varies as a function of breast density and age but averages approximately 1.16 cm for 50 to 80-year-old women. Palpation thresholds are even higher estimated to be about 2.0 cm. De novo(new) tumors must undergo 30 doubling to reach the detection threshold, on average this would require 16.4 years. Only 15% of de novotumors have rapid doubling of 50 days or less and reach the detection threshold over a period of 5 to 7 years. Accordingly, most newly diagnosed breast cancer is present for many years prior to its detection.1

  1. Santen R.J.Y.W.H.D. Modeling of growth kinetics of occult breast tumors: role in interpretation of studies of prevention and menopausal hormone therapy. Cancer Epidemiology, Biomarkers, and Prevention. EPI 12-0043, abstract, 2012.