What is Breast Cancer?

Breast cancer happens when cells in the breast grow in an uncontrolled way. These cells become abnormal and stop working like healthy cells.

Normally, breast cells grow, divide, and die when they get old or damaged. But cancer cells keep growing and dividing. They don’t die when they should and can spread to other parts of the body. Over time, they may form a lump or mass called a tumour.

Breast cancer usually starts in one of two places:

• Lobules: The glands that make milk. Cancer that starts here is called lobular carcinoma.
• Ducts: The tubes that carry milk to the nipple. Cancer that starts here is called ductal carcinoma.

Types of Breast Cancer
Breast cancers are grouped into two main types:

• Non-invasive (in-situ): The cancer cells stay in the place where they started. They have not spread to nearby tissues.
• Invasive: The cancer has spread into the surrounding healthy tissue.

While ductal and lobular carcinomas are the most common types, less common forms include:

• Inflammatory Breast Cancer
• Paget Disease of the Breast
• Triple Negative Breast Cancer (TNBC)

In rare cases, other types of cancer can start in the breast, such as non-Hodgkin lymphoma or soft tissue sarcoma.

Not all breast changes are cancer. Some conditions—like atypical hyperplasia, cysts, or intraductal papillomas—are non-cancerous (benign). These may still need follow-up, as they can affect your future risk of breast cancer.

Medical Review by Roochi Arora, MD, FRCPC, August 2025

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