What is Breast Cancer?

This is the first article in the Understanding Breast Cancer Guide. This guide was developed by the HealthTree Education Team and was last updated and reviewed on May 18, 2026

Cancer begins when cells in the body start to grow out of control. Breast cancer starts in the cells of the breast, most often in the milk ducts or the lobules (the glands that make milk). 

When you have breast cancer, the cells in breast tissue do not follow the normal cell cycle. Healthy cells grow, copy themselves, and then die. Cancer cells change, grow faster than normal, and do not die the way they should.

What is the breast?

The breast is made up of several types of tissue. The breast contains lobules, which are glands that produce milk, and ducts, which are thin tubes that carry milk from the lobules to the nipple. The breast also contains fatty and connective tissue, blood vessels, and lymph vessels.

Both men and women have breast tissue, and both can develop breast cancer, though breast cancer is much more common in women.

How breast cancer starts

Most breast cancers begin in the cells lining the milk ducts (ductal carcinoma) or the lobules (lobular carcinoma). Cancer can start as a non-invasive growth that stays within the duct or lobule, or it can be invasive, meaning it has grown into the surrounding breast tissue.

Non-invasive breast cancers include:

• Ductal carcinoma in situ (DCIS). Abnormal cells that are confined to the milk duct and have not spread into surrounding breast tissue. DCIS is considered a pre-cancer or early cancer.
• Lobular carcinoma in situ (LCIS). Abnormal cells found in the lobules. LCIS is not cancer itself but indicates an increased risk of developing breast cancer in the future.

Invasive breast cancers have grown beyond the duct or lobule into the surrounding breast tissue and have the potential to spread to lymph nodes or other parts of the body.

How does breast cancer affect the body?

As breast cancer grows, it can affect many parts of the body:

How breast cancer affects the breast and lymph nodes:

A tumor in the breast can cause a lump, skin changes, nipple discharge, or changes in the shape or size of the breast. Cancer cells may also travel to nearby lymph nodes, particularly those in the armpit (axillary lymph nodes).

How breast cancer affects the blood:

Breast cancer that has spread or cancer treatment can cause changes in blood counts, leading to anemia, increased infection risk, or bleeding issues.

How breast cancer affects other organs:

When cancer spreads to other parts of the body, it has metastasized. When this happens, it is called metastatic or advanced breast cancer. Breast cancer most often spreads to the bones, liver, lungs, and brain. This can affect how these organs work and may cause additional symptoms.

Learn more about the signs and symptoms of breast cancer.

Types of breast cancer

There are many types of breast cancer, classified by where they start and by their molecular features:

By where the cancer starts (histologic type):

• Invasive ductal carcinoma (IDC). The most common type of breast cancer, accounting for about 70% to 80% of all diagnoses. It starts in the milk ducts and grows into surrounding breast tissue.
• Invasive lobular carcinoma (ILC). Starts in the lobules and spreads into surrounding tissue. It is the second most common type.
• Ductal carcinoma in situ (DCIS). A non-invasive cancer confined to the milk ducts.
• Inflammatory breast cancer (IBC). A rare but aggressive form of breast cancer that causes the breast to look red and swollen. It often does not cause a lump.

By molecular subtype (receptor status):

• Triple-negative breast cancer (TNBC). A subtype that lacks estrogen receptors, progesterone receptors, and HER2 protein. It tends to grow faster and has fewer targeted treatment options.
• HER2-positive (HER2+) breast cancer. A subtype with high levels of the HER2 protein, which promotes cancer cell growth. It can be treated with HER2-targeted therapies.
• Hormone receptor-positive (HR+) breast cancer. The most common subtype, driven by the hormones estrogen and/or progesterone. It is treated with hormone therapy.

Less common types:

• Paget disease of the breast. A rare cancer that starts in the nipple
• Phyllodes tumors. Tumors that form in the connective tissue of the breast
• Angiosarcoma. A rare cancer that starts in the blood or lymph vessels of the breast

Related cancers

Breast cancer belongs to a broader group of cancers related to hormones and reproductive tissue. Some people with breast cancer have hereditary conditions like BRCA1 or BRCA2 gene mutations that can increase the risk of ovarian cancer, pancreatic cancer, and other cancers.