Breast Cancer Treatment: How Subtype Decides Everything

"Breast cancer" describes several diseases that happen to start in the same organ. They behave differently, respond to different drugs, and carry different outlooks. The subtype on your pathology report is what determines treatment.

The three questions that define subtype

Every breast cancer pathology report answers three things:

• Oestrogen receptor (ER) — positive or negative
• Progesterone receptor (PR) — positive or negative
• HER2 — positive or negative

From those come the main groups: hormone receptor-positive/HER2-negative (roughly 70% of cases), HER2-positive (15-20%), and triple-negative (10-15%). A proliferation marker, Ki-67, is often reported alongside.

Hormone receptor-positive, HER2-negative

This is the most common group, and the one where treatment has changed most in the past decade.

The foundation is endocrine therapy — blocking oestrogen's effect on the tumour. Which agent depends largely on menopausal status: tamoxifen blocks the receptor directly, while aromatase inhibitors such as letrozole, anastrozole and exemestane reduce oestrogen production and are used after menopause.

For advanced disease, endocrine therapy is now routinely combined with a CDK4/6 inhibitor — palbociclib, ribociclib or abemaciclib. These block proteins the cell needs to divide, and adding one roughly doubles the time before the disease progresses compared with endocrine therapy alone.

Where the cancer carries a PIK3CA mutation, alpelisib is an option; where it carries a BRCA mutation, PARP inhibitors such as olaparib or talazoparib are.

HER2-positive

HER2-positive disease was historically the most aggressive subtype. Anti-HER2 antibodies changed that outright.

Trastuzumab was the first, and remains the backbone. Pertuzumab is commonly added to it. For disease that progresses, antibody-drug conjugates — trastuzumab emtansine, and more recently trastuzumab deruxtecan — deliver chemotherapy directly to HER2-expressing cells, and have produced some of the most striking results in modern oncology. Oral tyrosine kinase inhibitors such as tucatinib, lapatinib and neratinib add further options, and tucatinib in particular is used where disease has reached the brain.

Cardiac function is monitored throughout anti-HER2 treatment, as these agents can affect the heart's pumping strength. It is usually reversible when caught early, which is why the monitoring exists.

Triple-negative breast cancer

Triple-negative disease lacks all three targets, so endocrine and anti-HER2 therapies do not apply. Chemotherapy remains central, but it is no longer the only option:

• Immunotherapy — pembrolizumab, where the tumour expresses PD-L1
• PARP inhibitors — where there is a germline BRCA mutation
• Sacituzumab govitecan — an antibody-drug conjugate targeting Trop-2

Genetic counselling and BRCA testing are particularly relevant here, both for treatment choice and for family members.

Why the same diagnosis produces different plans

Two people with the same stage can be offered entirely different treatment because their subtypes differ. This is not inconsistency — it is the system working as intended. It also explains why comparing your treatment with someone else's is rarely informative.

Treatment order varies too. Neoadjuvant therapy — drugs before surgery — is standard in HER2-positive and triple-negative disease, partly because how the tumour responds provides information that guides what comes next.

Living with long-term therapy

Endocrine therapy is typically taken for five to ten years. Adherence over that span is genuinely difficult, and side effects — joint pain, hot flushes, bone density loss — are the usual reason people stop. Most are manageable if raised early. Stopping silently is the worst outcome, because the protection is real and cumulative.

Browse our breast cancer range, or the full oncology catalogue.

This article is general information, not medical advice. Treatment decisions belong with a qualified clinician who knows your history. Every medicine mentioned is prescription-only and requires a valid prescription.