What is breast cancer?

What is breast cancer? 

Breast cancer is cancer found in the breasts or chest. It is an uncontrolled growth of abnormal cells in the breast. 

Almost every cell in your body has DNA - a set of instructions that tell your cells when to grow and when to stop. But occasionally, a mutation occurs. Usually, it's harmless. But when mutations accumulate and don’t get fixed properly, they can make cells grow too fast, ignoring the body’s normal rules. When lots of these uncontrolled cells start to grow, it can form a tumour. 

Just like many other cancers, that’s how breast cancer starts. 

What is secondary breast cancer? 

Secondary or metastatic breast cancer occurs when cancer cells have spread from the original breast tumour to other parts of the body, such as the bones, lungs, liver, or brain. It is also known as advanced or stage 4 cancer. While secondary breast cancer cannot currently be cured, treatment aims to control the disease, slow its progression, manage symptoms, and improve quality of life. 

Types of breast cancer. 

Not all breast cancers are the same. They are often categorised based on where they start and how far they have spread. 

Most breast cancers are invasive. Invasive breast cancer is when cancer cells have left the ducts or lobules and invaded the surrounding fatty tissue. 

The different types of invasive breast cancer are: 

• No special type (NST): The most common type, accounting for around 70% of all invasive breast cancers. Formerly called invasive ductal carcinoma, NST is where cancer cells break through ducts into surrounding breast tissue.  
• Invasive lobular breast cancer: Cancer cells originate in milk-producing glands and spread outward.  
• Inflammatory breast cancer: A rare form of breast cancer that is aggressive and causes redness, inflammation, and swelling as the breast cancer cells block lymph vessels.  
• Paget’s disease of the breast: A rare condition that causes change to the skin of the nipple and looks like eczema. People with Paget’s disease of the breast may either have invasive or non-invasive breast cancer. 

Non-invasive breast cancer is called ductal carcinoma in situ (DCIS). This is an early stage where cancer cells are still contained within the ducts or lobules and haven't spread to the surrounding tissue. If left untreated, DCIS may become invasive, though it is not currently possible to predict which cases will progress. 

DCIS is also different to lobular carcinoma in situ (LCIS). LCIS is a rare non-cancerous condition (so does not spread), although its presence is correlated with a slightly increased chance of developing breast cancer. 

Hormones and growth triggers. 

In addition to looking at the type of breast cancer, doctors will also look at how it grows to help choose the most effective treatment. Many breast cancers are fuelled by specific proteins or hormones like oestrogen. Treatments can involve lowering or blocking these signals to help slow the growth of the tumour. About 80% of breast cancers have oestrogen receptors, so they’re known as oestrogen receptor-positive (ER+). 

The most common classifications are: 

• ER+ (oestrogen receptor-positive) / PR+ (progesterone receptor-positive): Breast cancers with receptors for specific hormones are called hormone-receptor positive. When these hormones bind to the receptors, they can make the cancer cells grow and multiply. This means that the cancer’s growth is partly fuelled by these hormones. 
• HER2+ (human epidermal growth factor 2-positive): Some breast cancers have higher than normal levels of this protein which promotes tumour growth. 
• Triple-negative: Breast cancers that aren’t ER+, PR+ or HER2+ are called triple-negative. This type is often more aggressive and harder to treat. It is more common in younger women. 

A closer look at breast anatomy. 

To understand breast cancer, it helps to know the parts of the breast. 

• Lobules: Lobules are part of the glandular system; they are glands that produce breast milk. 
• Ducts: Small tubes that carry breast milk from the lobules to the nipple. 
• The nipple(s): Where breast ducts come together to allow for milk through a small opening in the skin.  
• Fat and connective tissue: The supportive components that gives the breast its shape (they surround the ducts and lobules).  
• Blood vessels: Allow for a constant supply of blood and nutrients to the breast to keep the tissue alive.   
• Lymph vessels and lymph nodes: Part of the immune system that transport and filter lymph fluid. They function to help fight any infections away from the breast. In breast cancer, cancerous cells can enter the lymph vessels and start to grow in the lymph nodes. This is known as lymph node involvement.   

Breast cancers are most often found in the ducts (ductal cancers). Some breast cancers develop in the lobules (lobular cancers). 

What causes breast cancer? 

Breast cancer doesn't have a single cause. It's a complex disease influenced by a combination of factors. Understanding these factors helps us take proactive steps toward breast cancer prevention and early detection. 

• Non-modifiable risk factors are those we cannot change. They include things like being born female, getting older, and genetics. 
• Modifiable risk factors are those we can influence. Drinking less alcohol, being more physically active, and choosing a fibre-rich diet can reduce the risk of breast cancer. 

See our page on breast cancer risk to learn more. 

In brief: 

Understanding this isn’t just about science; it can empower you to spot meaningful changes early. 

• Breast cancer happens when breast cells mutate and multiply uncontrollably. 
• It often starts in ducts or lobules and may stay local or spread. 
• Tumours are defined by their location, growth behaviour, and molecular markers. 
• Hormones and proteins often help tumours grow (but can also guide treatment). 
• Your breast cancer risk stems from many sources, from DNA to lifestyle. 

Last review: Sept-26 | Next review: Sept-27.