Most breast cancer starts in the milk ducts, the thin tubes that carry milk to the nipple. Over 80% of breast cancers are ductal in origin, making this the most common type by a wide margin. The remaining cases begin in other breast structures, and understanding exactly where cancer develops within the breast helps explain why different types behave differently and show up in different ways.
Inside the Breast: Ducts and Lobules
The breast is made up of fatty tissue, connective tissue, and a network of milk-producing glands called lobules, connected by ducts that funnel toward the nipple. These two structures, ducts and lobules, are where the vast majority of breast cancers begin.
Invasive ductal carcinoma accounts for over 80% of all breast cancer diagnoses in the United States each year. It starts in the cells lining the milk ducts and, if not caught early, grows through the duct wall into surrounding breast tissue. Before it becomes invasive, it can exist as ductal carcinoma in situ (DCIS), a stage where abnormal cells are still contained within the duct lining.
Invasive lobular carcinoma is the second most common type, representing 10% to 15% of new diagnoses. It begins in the lobules, the small glands that produce milk. Lobular cancers tend to grow in a single-file pattern through breast tissue rather than forming a distinct lump, which can make them harder to detect on mammograms. They’re more likely to appear in both breasts compared to ductal cancers.
Which Part of the Breast Is Most Affected
Breast tissue isn’t limited to the rounded shape you see from the outside. It extends from the collarbone down to the lower ribs, and from the breastbone out toward the armpit. The upper outer quadrant of the breast, closest to the armpit, contains the most glandular tissue and is where breast cancer most frequently develops.
A small extension of breast tissue called the axillary tail (sometimes called the tail of Spence) reaches into the armpit area itself. Cancer arising in this tail is extremely rare, with a reported incidence of just 0.3%, but it’s worth knowing about because a lump near the armpit can be breast cancer rather than a swollen lymph node.
Cancer on the Skin and Nipple
Not all breast cancers originate deep inside the breast. Paget disease of the breast is a rare type that involves the nipple and usually the areola, the darker circle of skin surrounding it. It typically causes redness, itching, flaking or crusting skin on or around the nipple, and sometimes a flattened nipple or yellowish or bloody discharge. Because these symptoms resemble eczema or a skin infection, Paget disease is often misdiagnosed at first. Most people with Paget disease also have a tumor deeper in the breast.
Inflammatory breast cancer affects the skin across a broader area. Rather than forming a lump, it causes the breast to become red, swollen, and warm. The skin takes on a dimpled, orange-peel texture (called peau d’orange in medical terms) because cancer cells block tiny lymph vessels in the skin. The breast may look distorted or enlarged, and the skin feels thick to the touch. This type is aggressive and accounts for a small fraction of cases, but its unusual presentation means it can be mistaken for an infection.
Connective Tissue and Blood Vessels
A small number of breast cancers don’t start in the ducts or lobules at all. Angiosarcoma of the breast, for example, originates in the cells lining blood vessels within breast tissue. It accounts for about 1% of soft tissue tumors in the breast. It can arise on its own (primary angiosarcoma) or develop years after radiation therapy for a previous breast cancer (secondary angiosarcoma). These cancers behave differently from the more common ductal and lobular types and require different treatment approaches.
Breast Cancer in Men
Men have a small amount of breast tissue, mostly ducts with very little lobular development. The most common type of breast cancer in men is invasive ductal carcinoma, the same type that predominates in women. Because men have less tissue overall, tumors are often closer to the nipple and may be noticed as a lump beneath or around the areola. Male breast cancer is uncommon but not as rare as many people assume.
Where Breast Cancer Spreads Locally
As a breast tumor grows, it can extend beyond the breast tissue itself. The lymph nodes in the armpit (axillary lymph nodes) are the most common first stop. These nodes are organized in three levels based on their position relative to a chest muscle called the pectoralis minor: level I nodes sit to the side of the muscle, level II nodes are behind it, and level III nodes are deeper toward the collarbone. Cancer typically spreads through these levels in order, which is why surgeons check the level I nodes first.
A tumor can also grow into the chest wall, which includes the ribs, the muscles between the ribs, and a muscle along the side of the ribcage called the serratus anterior. Invasion into the chest wall represents a more advanced stage than involvement of the pectoral muscle alone, which sits between the breast and the chest wall. This distinction matters for staging and treatment planning.
Where Breast Cancer Spreads in the Body
When breast cancer becomes metastatic, meaning it spreads beyond the breast and nearby lymph nodes, it has a clear pattern of preferred destinations. Bone is by far the most common first site of distant spread, accounting for 51% of cases. This often shows up as pain in the spine, hips, or ribs that doesn’t improve with rest and gradually worsens.
The lungs are the next most common site at 17%, followed closely by the brain at 16%. Liver metastases account for about 6% of first distant spread. These four organs, bone, lungs, brain, and liver, are the sites oncologists monitor most closely in people with a high risk of recurrence. The specific subtype of breast cancer influences where it’s most likely to spread: lobular cancers, for instance, have an unusual tendency to metastasize to the lining of the abdomen and the digestive tract, locations rarely affected by ductal cancers.
Breast cancer cells reach these distant sites through the bloodstream and lymphatic system. A tumor doesn’t need to be large to send cells traveling. Even small, early-stage cancers can occasionally shed cells into circulation, which is why systemic treatments like chemotherapy or hormone therapy are sometimes recommended even after a tumor has been completely removed surgically.