What is Breast Cancer?

Breast cancer is a disease in which malignant (cancer) cells form in the tissues of the breast.

Cancer is a broad term for a class of diseases characterized by abnormal cells that grow and invade healthy cells in the body. The modern medical terminology for the disease we call cancer comes originally from the Greek word karkinos, meaning "crab" (later Latinized as cancer. According to ancient tradition, cancer was named after the crab because of the aggressivity or obstinacy of the crab or because of the appearance of the crab's tangled legs.

Breast cancer usually originates from the cells lining the milk ducts and glands. The cells became abnormal and grow out of control. These abnormal cells can form a tumour (lump) and invade into surrounding tissues. They may spread to nearby lymph nodes or other parts of the body (metastasis) if not detected and treated. Breast cancer can form in one or both breasts. It affects both women and men.

Early detection and advances in treatment have helped improve outcomes for many people diagnosed with the disease. Educating oneself about breast cancer is the first step toward early detection and informed decision-making.

How common is breast cancer?

Breast cancer is one of the most common cancers in Singapore and around the world. In Singapore, 1 in 12 women—or approximately 8%—will be diagnosed with breast cancer in their lifetime. While it is much less common, men can also develop breast cancer, accounting for about 1% of all breast cancer cases.

A new analysis from the Global Burden of Disease (GBD) Study projects that global breast cancer cases will rise by one-third, from 2.3 million in 2023 to more than 3.5 million in 2050. This increase in incidence is also seen in Singapore, from 41.58 per 100,000 women in 1990 to 62.33 per 100,000 women in 2023.

Although breast cancer remains the leading cause of cancer death among women in Singapore. A combination of sustained screening participation, early-stage detection, modification of lifestyle risk factors and access to treatment have enabled Singapore's breast cancer survival rates to increase.

Who can get breast cancer?

Anyone can get breast cancer. Although breast cancer is most common in women, with the median age at diagnosis being 60.4 years, it can also affect younger women, older women, and men. Furthermore, there are increasing incidence of young onset cancer in Singapore compared, with nearly 20% of diagnoses occurring in women under the age of 45.

A person's risk of developing breast cancer is influenced by many factors, including age, genetics, family history, breast density, hormone exposure, and lifestyle. However, having one or more risk factors does not mean someone will develop breast cancer, and many people diagnosed with the disease have no known risk factors. Understanding who is at risk for breast cancer can help individuals make informed decisions about screening, early detection, and breast health.

Risk Factors for Breast Cancer

Cancer grows when a cell's DNA is damaged, but why or how that DNA becomes damaged is still unknown. The damage could be caused by genetic or environmental and lifestyle factors, or in most cases, a combination of the two. Most patients will never know exactly what caused their cancer. However, there are certain established risk factors that are associated with breast cancer.

Risk factors for breast cancer are broadly classified into genetic and environmental risk factors. Genetic risk factors are inherited, or passed down, from parent to child through the genes on both the mother's and father's side. These risk factors cannot be changed (non-modifiable) because they are built into your DNA from birth. Environmental and lifestyle risk factors are avoidable risk factors that are typically under an individual's control (modifiable). These risk factors can be reduced by changing elements within your environment or making alterations to your lifestyle.

Genetic (non-modifiable) risk factors include:

Environmental and lifestyle (modifiable) risk factors include:

Although it is important to identify the risks factors for breast cancer, one need to understand that approximately 60-70% of people with breast cancer have no connection to these risk factors at all, and other people with risk factors will never develop cancer.

Understanding Your Diagnosis: Subtype, Grade and Stage

Breast cancer is not a single disease—it has different forms, each with unique biological traits that affect how the cancer grows and responds to treatment. There are four main molecular subtypes of breast cancer which help predict how aggressive a tumour may be and which treatments are likely to be most effective.

When a patient is diagnosed with breast cancer, doctors not only determine the size, stage, and grade of their tumour, but they generally classify it based on hormone receptor (estrogen and progesterone) and HER2 protein status as well. Each piece of information is important when making treatment decisions and plans.

But breast tumours can also be classified by molecular subtype, which provides additional information about their biology and helps to create a fuller picture of the cancer beyond its size, grade, stage, and receptor status. Depending on the tumour, a specific gene within its cells may be more or less active compared to the same gene in other cells. Examining these gene expression patterns allows researchers to paint a "molecular portrait" of each tumour, which can be classified into four distinct breast cancer molecular subtypes: luminal A, luminal B, HER2-enriched, and basal-like.

The Four Molecular Subtypes of Breast Cancer

Luminal A breast cancers are characterized by higher levels of estrogen receptor (ER) and lower levels of Ki-67, a gene indicative of cellular proliferation (how quickly cells grow and divide to make new cells). They tend to be progesterone receptor (PR)--positive and HER2-negative, and are more likely to be lower grade, meaning that the tumour cells look more like normal breast cells.

Because luminal A tumours typically respond well to hormone therapy, they tend to have the best prognosis of the four subtypes. They grow more slowly and have a lower risk of returning. As such, patients with this subtype have the highest rate of survival. Luminal A breast cancer is the most commonly observed subtype, accounting for about 50 to 60 percent of breast cancers.

Luminal B breast cancers are also characterized by high levels of ER, but are more likely to be PR-negative, HER2-positive, and have higher levels of Ki-67. They therefore tend to grow faster and are a higher grade, meaning that the tumour cells look very different from normal cells. Luminal B breast cancers have a worse prognosis than luminal A tumours.

Luminal B breast cancer may not respond as well to hormone therapy as luminal A breast cancer. However, they are more likely to respond to chemotherapy. Approximately 15 to 20 percent of tumours are classified as luminal B.

Basal-like breast cancers are often triple-negative (lacking ER, PR, and HER2), though not always. They tend to have higher rates of proliferation, are aggressive tumours that are more likely to spread and recur. Basal-like breast cancer is seen in about 10-15 percent of cases and is more likely to be diagnosed in younger patients and those with BRCA1 mutations.

While basal-like breast cancers have a poorer prognosis than luminal A and B tumours, they tend to be very responsive to chemotherapy. For basal-like tumours with BRCA mutations, PARP inhibitors may also be an option.

HER2-enriched breast cancers often have high levels of the HER2 protein and tend to be ER-negative. Similar to basal-like tumours, they are often aggressive, fast-growing, and a higher tumour grade.

When left untreated, the prognosis for the HER2-enriched breast cancer is poor. Tumours of this subtype are more likely to be lymph node-positive and spread to the brain. However, prognosis has significantly improved since the discovery of HER2-targeting drugs. HER2-enriched tumours are typically responsive to trastuzumab (Herceptin®) and pertuzumab (Perjeta®), in combination with chemotherapy or other targeted drugs including kinase inhibitors like tucatinib (Tukysa®) or antibody-drug conjugates like ado-trastuzumab emtansine (T-DM1/Kadcyla®) and fam-trastuzumab deruxtecan-nxki (Enhertu®). HER2-enriched breast cancer is seen in about 10 to 15 percent of cases.

Ductal carcinoma in situ (DCIS), also known as stage 0 breast cancer, can also be categorized into similar molecular subtypes, enabling researchers to further examine what molecular drivers cause this precursor to breast cancer to become invasive.

Advancements in genomic technologies and artificial intelligence expanding the potential of molecular subtyping even further. Molecular insights into breast cancer are more and more important to emerging treatments such as immunotherapies and targeted agents, offering hope for improved survival and quality of life. As research continues to uncover the biological differences among breast cancers, treatment strategies are becoming more precise. For patients, that means growing opportunities for therapies that are better matched to the unique features of their disease.

Frequently Asked Questions About Breast Cancer

Is breast cancer painful?

Contrary to popular belief, breast cancer usually does not cause pain — it is typically painless. Most patients with breast cancer do not have pain, and most patients with breast pain do not have cancer. This is important, because some people delay seeing a doctor about a lump simply because it does not hurt. If you notice a new lump or change in your breast, see a doctor for assessment — whether or not it is painful.

Can breast cancer look like a breast infection?

Rarely, yes. Most breast infections and abscesses are not cancer — they are usually caused by mastitis or blocked ducts (often during breastfeeding), or by skin infections in people with lower immunity such as diabetics and smokers. But in uncommon cases, breast cancer can present in a way that resembles an abscess or infection — this is called inflammatory breast cancer. Any breast infection that does not settle with treatment should be reviewed by a specialist.

Does a family history determine my risk of breast cancer?

Family history is an important risk factor, but it is not the whole picture. In fact, about 80% of patients diagnosed with breast cancer do NOT have a family history. So while a family history of breast cancer — or an inherited genetic mutation such as BRCA1 or BRCA2 — does raise your risk and is worth discussing with your doctor, the absence of family history does not mean you are safe. Regular screening and breast awareness matter for everyone.

Can breast cancer affect younger or older women?

Yes. Although the peak incidence of breast cancer in Singapore is in women aged 50 to 59, there is a rising trend in younger women (under 40). At the other end, a common misconception is that women are "safe" after 70 because they are no longer invited for subsidised screening — but older women can develop breast cancer too, and risk increases with age. Breast cancer is not limited to middle age. Discuss the screening approach that is right for your age and risk with your doctor.

How can I reduce my risk of breast cancer?

Some risk factors cannot be changed — age, being female, family history, genetic mutations, and dense breast tissue. But several lifestyle factors are within your control. You can lower your risk by maintaining a healthy weight (especially after menopause), staying physically active, limiting alcohol, eating a balanced diet rich in fruits and vegetables, breastfeeding if you are able to, and discussing the risks of long-term hormone replacement therapy with your doctor. Not all breast cancers are preventable, but healthy habits can meaningfully lower your risk.

Do soy products increase breast cancer risk?

No. Eating soybeans or soy-containing foods does not raise a person's risk of breast cancer. In fact, across populations, research suggests soy foods may lower the risk. Earlier confusion came from animal studies in mice and rats, which process soy isoflavones very differently from humans and used much higher doses than people consume. Human studies show soy either has no effect or may reduce breast cancer risk — possibly because soy isoflavones can block more potent natural estrogens in the body.

Do oral contraceptives (birth control pills) increase breast cancer risk?

The evidence suggests only a small increase. Most research comes from observational studies, which cannot definitively prove that oral contraceptives cause breast cancer, because women who take them may differ in other ways from those who don't. Meta-analyses have shown a slight increase in breast cancer risk (in the range of 8–24%), but the overall risk remains low — because the baseline risk in the premenopausal women who typically use these pills is already low, so the actual difference between users and non-users is small.

Does hormone replacement therapy (HRT) increase breast cancer risk?

It depends on the type. In women over 50 with no history of breast cancer, combination HRT (estrogen plus progestogen) taken for five or more years slightly increases breast cancer risk, and higher doses increase it more. This increased risk is generally not seen with estrogen-only HRT — though estrogen-only therapy carries other considerations for women who still have a uterus. Low-dose vaginal estrogen does not increase breast cancer risk, because most of the hormone stays in the vaginal tissue. For many women, the benefits of HRT can outweigh the risks — this is a decision to make with your doctor based on your circumstances.

Can stress cause breast cancer?

Short-term stress is a normal part of life and is not a concern. Chronic stress that lingers for weeks or months, however, can affect how the body functions — it can weaken the immune system and increase inflammation. A healthy immune system helps identify and destroy abnormal cells before they become cancer, and persistent inflammation can create an environment where cancer cells grow more easily. So while chronic stress does not directly cause breast cancer, it may shape a biological environment that makes cancer more likely to develop. Managing long-term stress is part of overall health.

Does breast size affect breast cancer risk?

No — there is no connection between breast size and breast cancer risk, although larger breasts can sometimes be more challenging to examine. What does increase risk are factors like obesity and breast density, along with family history and lifestyle factors. Whatever your breast size, regular breast self-examination, routine check-ups, and mammograms (from age 40, or as advised) remain important.

Do underwire bras, mobile phones, or antiperspirants cause breast cancer?

No. There is no scientifically proven link between developing breast cancer and wearing underwire bras, using deodorants or antiperspirants, or using a mobile phone. These are common myths not supported by evidence.