The Types and Stages of Breast Cancer, Explained

A breast cancer diagnosis comes with a vocabulary nobody teaches you in advance. Within days of hearing the word “cancer,” patients and their families are expected to process terms like invasive ductal carcinoma, HER2-positive, stage IIA, and triple negative. These words carry enormous weight, and not understanding them makes an already overwhelming experience feel that much harder to navigate.

This post is a glossary. It breaks down the types of breast cancer, what receptor status means, and how staging works, all in plain language. It’s not a substitute for the conversation you’ll have with your oncologist, and it doesn’t cover treatment or prognosis. What it does is give you the foundation to understand what you’re hearing, ask better questions, and feel less lost in the medical language surrounding a diagnosis. Every term is defined the first time it appears.

Where Does Breast Cancer Start?

Breast tissue is made up of two main structures that produce and deliver milk: lobules and ducts. Understanding these two structures is important because the location where cancer begins determines its classification.

Lobules are the small glands that produce breast milk. There are 15 to 20 lobes in each breast, and each lobe contains clusters of lobules. Cancer that begins in the lobules is called lobular cancer.

Ducts are the thin tubes that carry milk from the lobules to the nipple. Cancer that begins in the lining of these ducts is called ductal cancer. Ductal cancers are the most common type, accounting for the majority of all breast cancer diagnoses.

The breast also contains connective tissue, fatty tissue, blood vessels, and lymph vessels. While cancer can develop in these tissues, it’s far less common. When people refer to breast cancer broadly, they’re almost always talking about cancer that started in the ducts or the lobules.

What Is the Difference Between In Situ and Invasive?

This is one of the most important distinctions in breast cancer classification, and it determines a great deal about what comes next.

In situ means “in place.” Cancer that is in situ has not spread beyond the structure where it started. The abnormal cells are contained within the duct or lobule and have not broken through the walls into surrounding breast tissue. In situ cancers are considered non-invasive or pre-invasive.

Invasive (also called infiltrating) means that the cancer cells have broken through the walls of the duct or lobule and have grown into the surrounding breast tissue. Once cancer becomes invasive, it has the potential to spread to nearby lymph nodes and, eventually, to other parts of the body through the lymphatic system or bloodstream.

The distinction matters because it affects how the cancer is treated, monitored, and classified by stage. In situ cancers are generally considered earlier and more contained. Invasive cancers require broader evaluation to determine whether they’ve spread beyond the breast.

What Are the Common Types of Breast Cancer?

Breast cancer is not a single disease. It’s a category that includes several distinct types, each with different characteristics. These are the types you’re most likely to encounter in a diagnosis or a conversation with a care team.

Ductal carcinoma in situ (DCIS)

DCIS is the most common type of non-invasive breast cancer. It means that abnormal cells have been found inside the milk ducts but have not spread into the surrounding breast tissue. DCIS is sometimes called stage 0 breast cancer.

Because DCIS is contained, it is highly treatable. However, if left untreated, some cases of DCIS can progress to invasive cancer over time. The decision about how to manage DCIS depends on several factors, including the size of the area involved, the grade of the cells (how abnormal they look under a microscope), and whether it’s found in one area or multiple areas of the breast.

Invasive ductal carcinoma (IDC)

IDC is the most common type of breast cancer overall, accounting for approximately 70% to 80% of all breast cancer diagnoses. It begins in the cells lining the milk ducts and has broken through the duct wall into the surrounding breast tissue.

IDC is classified as invasive because it has the potential to spread to the lymph nodes and other parts of the body. The extent to which it has spread at the time of diagnosis is one of the primary factors that determines staging.

Invasive lobular carcinoma (ILC)

ILC is the second most common type of breast cancer, representing about 10% to 15% of invasive breast cancers. It begins in the lobules and has spread into the surrounding tissue.

ILC can be harder to detect on a mammogram than IDC because it tends to grow in a single-file pattern rather than forming a distinct mass. This means it may not create the kind of lump that’s easy to feel during a self-exam or see clearly on standard imaging. Supplemental imaging, such as MRI, is sometimes used alongside mammography for women with ILC or those at higher risk for it.

Inflammatory breast cancer (IBC)

Inflammatory breast cancer is a rare and aggressive form of invasive breast cancer that accounts for about 1% to 5% of all cases. Unlike most breast cancers, IBC often does not present as a lump. Instead, it causes the skin of the breast to become red, swollen, warm, and dimpled, sometimes resembling the texture of an orange peel. These symptoms occur because cancer cells block the lymph vessels in the skin of the breast.

IBC tends to develop and progress quickly, which is why recognizing the symptoms matters. Because it doesn’t always show up on a mammogram and doesn’t always form a detectable mass, IBC is sometimes initially mistaken for an infection or other non-cancerous condition.

Triple-negative breast cancer (TNBC)

Triple-negative breast cancer is defined not by where it starts but by what it lacks. It tests negative for all three of the receptors commonly found in breast cancer: estrogen receptor (ER), progesterone receptor (PR), and HER2 (human epidermal growth factor receptor 2). This means that the cancer cells do not have the proteins that respond to the hormonal and targeted therapies used for other types.

TNBC accounts for about 10% to 15% of all breast cancers and is more commonly diagnosed in women under 40, women with BRCA1 mutations, and Black women. Because the most common targeted treatments are not effective against it, TNBC is typically treated with a combination of surgery, chemotherapy, and sometimes immunotherapy. Research into more targeted approaches for TNBC is an active and growing area of study.

What Does Receptor Status Mean?

When breast cancer is diagnosed, the tumor tissue is tested for specific proteins on the surface of the cancer cells. These proteins, called receptors, determine how the cancer behaves and which treatments are most likely to be effective. Receptor status is one of the most important pieces of information in a breast cancer diagnosis.

Estrogen receptor (ER)

If cancer cells have receptors for the hormone estrogen, the cancer is called ER-positive (ER+). This means that estrogen can attach to these receptors and stimulate the cancer cells to grow. Approximately 70% to 80% of all breast cancers are ER-positive. Knowing this allows the care team to consider hormonal therapies that block estrogen or reduce its production in the body.

Progesterone receptor (PR)

Cancer cells may also have receptors for progesterone, another hormone. If so, the cancer is called PR-positive (PR+). ER and PR status are often discussed together because they’re both hormone receptors, and cancers that are positive for one or both are categorized as hormone receptor-positive (HR+). The presence of these receptors signals that the cancer’s growth is fueled at least in part by hormones.

HER2 (human epidermal growth factor receptor 2)

HER2 is a protein that promotes cell growth. In some breast cancers, the gene that controls HER2 production makes too many copies of itself (called amplification), which leads to an overproduction of the HER2 protein on the cell surface. This is called HER2-positive (HER2+) breast cancer.

HER2-positive cancers tend to grow and spread more aggressively than HER2-negative cancers. However, the identification of HER2 status has been one of the most significant advances in breast cancer treatment, because it opened the door to targeted therapies designed specifically to block the HER2 protein. These therapies have significantly improved outcomes for HER2-positive patients.

Why receptor status guides treatment

Receptor status essentially tells the care team what’s driving the cancer’s growth, which determines what can be used to slow or stop it.

How receptor status influences the treatment conversation:

• HR-positive cancers may respond to hormonal therapies (such as tamoxifen or aromatase inhibitors) that block the cancer’s access to the hormones it uses for fuel
• HER2-positive cancers may respond to targeted therapies (such as trastuzumab) designed to block the HER2 protein
• Triple-negative cancers lack all three receptor targets, which means the standard hormonal and HER2-targeted therapies are not effective. Treatment relies on other approaches, including chemotherapy and emerging immunotherapy options
• Tumors that are both HR-positive and HER2-positive may benefit from a combination of hormonal and targeted treatments

Receptor status is determined through pathology testing on the tumor tissue, usually from a biopsy. It’s one of the first pieces of information the care team evaluates when planning next steps.

How Does Staging Work in Plain Terms?

Staging describes how far the cancer has spread at the time of diagnosis. It’s expressed as a number from 0 to IV, and it’s determined by evaluating three main factors, referred to as the TNM system.

T (Tumor size): How large is the primary tumor, and has it grown into nearby structures?

N (Nodes): Has the cancer spread to nearby lymph nodes? If so, how many are involved?

M (Metastasis): Has the cancer spread to distant parts of the body, such as the bones, liver, lungs, or brain?

These three factors are combined to assign an overall stage. Here’s what each stage means in general terms.

Stage 0: Non-invasive. The abnormal cells are contained within the duct or lobule (DCIS or LCIS). No spread to surrounding tissue, lymph nodes, or distant sites.

Stage I: The cancer is small (generally 2 centimeters or less) and is either limited to the breast or has only microscopically small involvement in a nearby lymph node. It has not spread to distant sites.

Stage II: The tumor is larger (up to 5 centimeters) or has spread to a small number of nearby lymph nodes, or both. It has not spread to distant parts of the body.

Stage III: The cancer is locally advanced. This may mean a larger tumor, significant lymph node involvement, or growth into the chest wall or skin of the breast. It has not spread to distant organs. Stage III is sometimes described as locally advanced breast cancer.

Stage IV: The cancer has spread to distant parts of the body. This is also called metastatic breast cancer. The most common sites of distant spread are the bones, lungs, liver, and brain.

It’s worth noting that staging has become more nuanced in recent years. The American Joint Committee on Cancer now includes additional factors beyond TNM, such as tumor grade (how abnormal the cells look), receptor status (ER, PR, HER2), and genomic test results, when assigning a prognostic stage. This means two patients with the same tumor size and lymph node status may receive different stage assignments based on the biology of their cancer.

Why Can Two People at the Same Stage Have Very Different Plans?

This is one of the most confusing aspects of breast cancer for patients and families, and it’s important to understand why it happens.

Staging tells you how far the cancer has spread. But it doesn’t tell you everything about the cancer’s biology. Two people can be diagnosed at stage II and end up on very different paths depending on their receptor status, tumor grade, genomic testing results, age, overall health, and personal preferences.

Factors that make each case unique:

• Receptor status. A stage II hormone receptor-positive cancer and a stage II triple-negative cancer are biologically different diseases that respond to different approaches.
• Tumor grade. Grade measures how abnormal the cells look under a microscope. Low-grade tumors tend to grow more slowly. High-grade tumors tend to be more aggressive. Two same-stage tumors with different grades may be managed differently.
• Genomic testing. Tests like Oncotype DX and MammaPrint analyze the genetic activity of the tumor to estimate how likely it is to recur. These results can determine whether chemotherapy is included in the plan for early-stage, hormone receptor-positive cancers.
• Lymph node involvement. The number and location of affected lymph nodes influence decisions about surgery, radiation, and systemic treatment.
• Patient factors. Age, overall health, family history, genetic mutation status, and the patient’s own values and priorities all play a role in shaping the treatment plan. Two patients facing the same clinical picture may make different decisions based on what matters most to them.

This is why comparing one person’s experience to another’s, even at the same stage, can be misleading. The plan your provider recommends is built around the specific characteristics of your cancer and your individual circumstances. It’s personalized by design.

Share This With Someone Who Needs It

Breast cancer vocabulary is dense, and nobody should have to learn it under pressure without a starting point. If you or someone in your life has recently been diagnosed, or is supporting a loved one through a diagnosis, this post is meant to give you a foundation. It won’t answer every question, but it should make the conversations with your care team easier to follow.

Save it for reference, or send it to someone who could use it right now.

At The Pink Jet, we believe that understanding is the first step toward early detection and informed action. Through aviation, we bring breast cancer awareness to audiences across the country, using a striking pink L-39 Albatros aircraft and an inspiring team of female pilots to spark conversations, support survivors, and raise funds for research and advocacy organizations.

Visit thepinkjet.org to learn more about upcoming events, explore resources, and discover how you can help take the fight against breast cancer to new heights.