Types of Breast Cancer Explained

Invasive ductal carcinoma (IDC)

The most common type, accounting for around 75% of all breast cancers. It starts in the milk ducts and has grown (invaded) into the surrounding breast tissue. "Invasive" means the cancer has broken through the duct wall — it does not necessarily mean it has spread to other parts of the body.

Invasive lobular carcinoma (ILC)

Starts in the milk-producing lobules. Accounts for about 15% of invasive breast cancers. ILC can be harder to detect on mammogram because it often grows in a scattered pattern rather than forming a distinct lump.

Ductal carcinoma in situ (DCIS)

A non-invasive (pre-cancerous) condition where abnormal cells are found inside the milk ducts but have not grown through the duct wall. DCIS is classified as Stage 0. It is almost always detected by mammogram rather than symptoms. Left untreated, DCIS may become invasive breast cancer in some women — which is why treatment is recommended.

Triple-negative breast cancer (TNBC)

Lacks three receptors common to most breast cancers: oestrogen receptor (ER), progesterone receptor (PR) and HER2. This means hormone therapies and targeted drugs that work for other types do not work for TNBC. It tends to grow faster and be more aggressive. Treatment relies on chemotherapy, and increasingly on immunotherapy.

HER2-positive breast cancer

Has an excess of a protein called HER2 (human epidermal growth factor receptor 2) that promotes cancer cell growth. These cancers tend to grow faster, but there are highly effective targeted therapies available, including trastuzumab (Herceptin).

HER2-low breast cancer

A recently defined category (since 2022) for breast cancers that express small amounts of HER2 — not enough to qualify as HER2-positive, but detectable. HER2-low accounts for approximately 55–60% of all breast cancers. This distinction matters because a new drug class — antibody-drug conjugates, specifically trastuzumab deruxtecan (Enhertu) — targets HER2-low tumours very effectively, significantly improving outcomes for patients with metastatic HER2-low breast cancer.

Inflammatory breast cancer (IBC)

A rare but aggressive form that often does not cause a lump. Instead, the breast becomes red, swollen and warm — which can be mistaken for an infection. IBC is a medical emergency and should be investigated urgently if suspected.

Secondary (metastatic) breast cancer

Also called Stage 4 or advanced breast cancer. This is breast cancer that has spread from the original tumour to other parts of the body — most commonly the bones, liver, lungs or brain. It is not curable, but modern treatments can control it for months or years and maintain quality of life.