Breast Cancer ·

01 — What Is Breast Cancer?

BREAST CANCER SERIES

Article 01 — What Is Breast Cancer?

A scientific introduction to the biology, anatomy, classification and progression of breast cancer

Breast cancer is a group of malignant diseases that originate from cells within breast tissue. At its most fundamental level, cancer develops when cells acquire genetic and epigenetic abnormalities that disrupt the normal controls governing cell division, differentiation, survival and tissue organization. In breast cancer, these abnormal cells can proliferate locally, invade surrounding tissue and, in some cases, enter lymphatic or blood vessels and establish tumors at distant sites. (Cancer.gov)

Illustrations: breast anatomy and the biological progression from normal breast epithelium toward invasive carcinoma. The NCI's breast-anatomy illustration is public domain and may be reused with credit to the National Cancer Institute.(National Cancer Institute Visuals)

The breast is a living tissue system

The breast is not simply fatty tissue. It is a complex organ composed of glandular epithelium, connective tissue, adipose tissue, blood vessels, lymphatic vessels and supporting extracellular matrix.

Its milk-producing structures are organized hierarchically:

Lobes → lobules → milk-producing epithelial cells → ducts → nipple

The lobules contain the epithelial cells capable of producing milk, while the ductal system transports milk toward the nipple. The surrounding stromal compartment provides structural and biochemical support. Lymphatic vessels drain breast tissue toward regional lymph nodes, particularly those in the axilla. (Cancer.gov)

This organization is important because breast cancers are classified partly according to which cells and anatomical structures give rise to the tumor.

How normal cells become cancerous

Normal breast cells continuously receive molecular signals telling them when to divide, differentiate, repair damage or undergo programmed cell death (apoptosis).

Cancer arises when accumulated molecular alterations interfere with these regulatory systems.

A simplified model is:

Normal cell
↓
Genetic/epigenetic alterations
↓
Abnormal proliferation
↓
Pre-invasive lesion
↓
Invasive carcinoma
↓
Lymphatic or blood-vessel invasion
↓
Metastatic disease

These changes can involve genes responsible for DNA repair, cell-cycle regulation, growth signaling and apoptosis. Some alterations are inherited, while many arise during a person's lifetime. Importantly, having a risk factor does not mean that cancer will necessarily develop, and many people diagnosed with breast cancer have no identifiable major risk factor. (Cancer.gov)

In situ versus invasive breast cancer

One of the most important distinctions in breast pathology is whether malignant cells remain confined to their original structure.

Carcinoma in situ

In carcinoma in situ, abnormal malignant cells remain within the duct or lobular structure in which they developed and have not invaded the surrounding breast tissue.

The best-known example is ductal carcinoma in situ (DCIS). It is considered an early form of breast cancer and is often detected through screening mammography, sometimes before a person develops a palpable abnormality. (Cancer.gov)

Invasive carcinoma

An invasive breast carcinoma has breached the normal tissue boundary surrounding its site of origin and entered the surrounding breast tissue.

Once cancer cells acquire invasive capacity, they can potentially access lymphatic or blood vessels. This creates the biological possibility of metastasis, in which cancer cells establish tumors in organs elsewhere in the body. (World Health Organization)

Invasion is therefore a biological transition—not simply an increase in tumor size.

The major types

Most breast cancers are carcinomas, meaning they originate from epithelial cells.

The two major anatomical categories are:

Invasive ductal carcinoma (IDC)
Begins in cells lining the milk ducts and invades surrounding breast tissue. It is the most common breast cancer diagnosis.

Invasive lobular carcinoma (ILC)
Begins in the epithelial cells of the lobules and subsequently invades surrounding tissue. (Cancer.gov)

There are also less common forms, including Paget disease of the breast, inflammatory breast cancer, metaplastic carcinomas and tumors arising from stromal or vascular tissues. (Cancer.gov)

Breast cancer is also classified by molecular biology

Anatomical classification tells us where the cancer began. Molecular classification tells us what biological machinery the cancer is using.

Tumor cells are routinely evaluated for biomarkers including:

Estrogen receptor (ER)
Progesterone receptor (PR)
HER2
Other molecular and genomic characteristics where clinically appropriate.

These characteristics help define biologically distinct groups of breast cancer and influence treatment selection. For example, a tumor expressing estrogen receptors may respond to endocrine therapies, whereas triple-negative breast cancer lacks expression of ER, PR and HER2 and therefore requires different therapeutic strategies. (World Health Organization)

Thus, “breast cancer” is not one single biological disease. It is an umbrella term encompassing tumors with substantially different molecular behaviors.

Why breast cancer can become life-threatening

A localized breast tumor may remain confined to the breast and be highly treatable. The major biological danger arises when malignant cells acquire the capacity to invade, survive outside their original tissue, circulate and colonize distant organs.

Breast cancer commonly spreads first through regional lymphatic pathways, particularly toward axillary lymph nodes, although lymph-node involvement is not synonymous with distant metastasis. Cancer cells can subsequently spread through the bloodstream to organs including the bone, liver, lungs and brain. (World Health Organization)

Distant metastatic breast cancer is classified as stage IV disease.

How breast cancer is confirmed

Imaging can identify suspicious abnormalities, but imaging alone cannot establish that a lesion is cancer.

A definitive diagnosis generally requires biopsy, in which cells or tissue are removed and examined microscopically by a pathologist. The pathological examination determines characteristics such as:

tumor type;
invasive versus in situ status;
histological grade;
receptor status;
HER2 status; and
other clinically relevant biomarkers.

The extent of disease is then assessed through staging, which considers factors including tumor characteristics, lymph-node involvement and distant spread. (Cancer.gov)

The global picture

Breast cancer is a major global health problem. The World Health Organization reports that approximately 2.4 million women were diagnosed with breast cancer and approximately 694,000 died from the disease in 2024. Breast cancer occurs in every country and can occur at any age after puberty, although incidence increases substantially with age. Approximately 0.5–1% of breast cancers occur in men. (World Health Organization)

Importantly, approximately 80% of breast cancers occur in women without a specific risk factor other than sex and age. This is one reason breast cancer cannot accurately be reduced to lifestyle, family history or a single environmental exposure. (World Health Organization)

Conclusion

Breast cancer is fundamentally a disease of abnormal cellular regulation within a complex biological tissue. It begins when breast cells acquire changes that allow them to proliferate beyond normal physiological control. Some tumors remain confined to ducts or lobules; others acquire invasive and metastatic capabilities.

Understanding breast cancer therefore requires more than identifying a lump. It requires understanding breast anatomy, epithelial biology, genetics, tumor microenvironment, molecular signaling, invasion and metastasis.

That biological framework is essential because two tumors that both carry the label breast cancer can behave very differently at the cellular and molecular levels—and consequently require very different approaches to treatment.

References

National Cancer Institute (NCI). What Is Breast Cancer? National Institutes of Health. NCI — What Is Breast Cancer? (Cancer.gov)
World Health Organization (WHO). Breast Cancer. 3 July 2026. WHO — Breast Cancer Fact Sheet (World Health Organization)
National Cancer Institute (NCI). Types of Breast Cancer. NCI — Types of Breast Cancer (Cancer.gov)
National Cancer Institute (NCI). Breast Cancer Causes and Risk Factors. NCI — Causes and Risk Factors(Cancer.gov)
National Cancer Institute (NCI). How Is Breast Cancer Diagnosed? NCI — Diagnosis of Breast Cancer(Cancer.gov)
American Cancer Society. Breast Cancer. American Cancer Society — Breast Cancer (American Cancer Society)
National Cancer Institute Visuals Online. Breast Illustration. Public-domain medical illustration. (National Cancer Institute Visuals)

URIBHO Breast Cancer Series — Article 01
The biology of breast cancer begins with understanding the tissue itself.