Most breast cancers form in the milk ducts, the thin tubes that carry milk from the milk-producing glands to the nipple. About 80% of all breast cancer diagnoses begin in the lining of these ducts. The second most common starting point is the lobules, the small glands at the end of the ducts that actually produce milk. A smaller number of breast cancers arise in the connective tissue that surrounds and supports these structures.
How the Breast Is Structured
Each breast contains 15 to 20 sections called lobes, arranged in a circular pattern like the sections of an orange. Each lobe is made up of smaller sections called lobules, which end in dozens of tiny bulbs capable of producing milk. These lobes, lobules, and bulbs are all connected by a network of ducts that channel milk toward the nipple.
Surrounding this milk-producing system is a supportive framework of connective tissue called stroma, which includes fat, ligaments, and blood and lymph vessels. Cancer can start in any of these tissue types, but the vast majority originate in the cells lining the ducts and lobules.
Cancer Starting in the Milk Ducts
Ductal carcinoma is the most common form of breast cancer. It begins when cells lining the inside of a milk duct start growing abnormally. In the earliest stage, called ductal carcinoma in situ (DCIS), those abnormal cells remain contained within the walls of the duct. At this point, the cancer has not yet spread into surrounding breast tissue.
When abnormal duct cells break through the duct wall and invade the surrounding breast tissue, it becomes invasive ductal carcinoma. This is the stage at which cancer cells can reach nearby lymph vessels or blood vessels and potentially travel to other parts of the body. The first lymph node that receives drainage from the tumor, called the sentinel node, is typically the first place cancer spreads outside the breast.
Mammograms often catch ductal cancers early by detecting tiny calcium deposits called microcalcifications. These appear as small white specks, sometimes clustered together or arranged in a line. They show up in roughly half of breast cancers that have no detectable lump, and mammography screening that spots microcalcifications helps diagnose 85% to 95% of DCIS cases.
Cancer Starting in the Lobules
Lobular carcinoma begins in the lobules, the milk-producing glands themselves. It behaves differently from ductal carcinoma in a way that makes it harder to find. Instead of forming a distinct lump, lobular cancer cells typically grow in single-file strands that weave through normal breast tissue. This diffuse growth pattern barely disturbs the surrounding tissue architecture, which is why lobular cancers are more difficult to detect on both physical exams and mammograms.
This unusual growth pattern traces back to a specific genetic change. Lobular cancer cells lose a protein called E-cadherin, which normally acts like glue holding cells together in an organized structure. Without it, cells spread more loosely and can migrate more easily. Variants of lobular cancer grow in different configurations: solid clusters, honeycomb-like assemblies, or tube-like strands, but all share that tendency to infiltrate without forming an obvious mass.
Lobular cancer also spreads to different parts of the body than ductal cancer. Both types can reach the bones, lungs, brain, and liver. But lobular carcinoma has an unusual tendency to also spread to the ovaries, the gastrointestinal tract, and the tissue lining the abdomen. This distinct pattern of spread is one reason lobular cancer sometimes gets diagnosed later than ductal cancer.
Less Common Starting Points
A small number of breast cancers form outside the ducts and lobules entirely. Phyllodes tumors originate in the stroma, the connective tissue that includes fat, ligaments, and the tissue surrounding blood and lymph vessels. These tumors can contain a mix of stromal and epithelial cells and range from benign to malignant. They are rare, but they illustrate that breast cancer is not limited to the milk-producing system.
In Paget’s disease of the breast, cancer appears on the surface of the nipple and areola. Cancer cells from a tumor deeper inside the breast can travel through the milk ducts to reach the nipple, or the cancer may begin in the nipple itself. It accounts for a small fraction of breast cancers but is notable because its first symptoms, such as flaking or crusting skin on the nipple, are easy to mistake for eczema.
Inflammatory Breast Cancer and the Lymph Vessels
Inflammatory breast cancer forms when cancer cells break away from their origin point and travel into the tiny lymphatic vessels in the breast skin. The cells multiply and clog these vessels, blocking normal fluid drainage. This blockage causes the skin to swell, turn red or purple, and develop a dimpled texture sometimes described as resembling an orange peel. Unlike most breast cancers, inflammatory breast cancer rarely produces a distinct lump, which can delay diagnosis.
Breast Cancer in Men
Men have breast tissue too, including milk ducts and, in some cases, a small number of lobules. Most male breast cancers start in the milk ducts, just as in women. Lobular cancer is rare in men because male breasts contain far fewer lobules. The biology and location of the cancer are essentially the same, but male breast tissue is much smaller, which means tumors may reach the chest wall or skin sooner.