What Is Breast Cancer?
A plain-language overview of breast cancer, where it starts, and how it is found and treated, based on National Cancer Institute resources.
Source: National Cancer Institute · NCI reviewed 2025-12-02 · Verified 2026-07-02
The 30-second version
Breast cancer is cancer that starts in the breast, when cells there grow without control and form a tumor. It mostly affects women aged 45 and older, but anyone with breasts can get it. Mammograms can find it early, and there are many types depending on where it begins and how far it has spread.
Key takeaways
- Breast cancer starts when cells in the breast grow without control and form a tumor that may spread.
- It can start in the milk ducts, the milk-making lobules, the nipple, or other breast tissue.
- Most breast cancers begin in the ducts and are called ductal cancers.
- Breast cancer mostly affects women aged 45 and older, but anyone with breasts can get it; it is rare in children and men.
- Mammograms can detect breast cancer early, possibly before it has spread.
- Cancer that stays within the ducts or lobules is called carcinoma in situ; invasive cancers have spread into nearby breast tissue.
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The full explanation.
The simple version
Breast cancer is cancer that starts in the breast. It can start in one or both breasts. Breast cancer happens when cells in the breast grow without control and create a mass called a tumor. That tumor may spread to other parts of the body.
Breast cancer mostly affects women aged 45 and older. But anyone with breasts can get breast cancer. It is rare in children and in men.
In short: breast cancer begins when breast cells grow out of control and form a tumor.
Where breast cancer starts
The breast is made of several kinds of tissue, and breast cancer can begin in different places:
- Glandular tissue. This includes the milk glands, milk ducts, and lobules. Lobules are tiny glands that make milk. After a baby is born, breast milk flows from the lobules through thin tubes called ducts to the nipple. Cancers that start in the ducts are called ductal cancers. Cancers that start in the lobules are called lobular cancers. Most breast cancers are ductal cancers.
- Fibrous and fatty tissue (stroma). This tissue fills the spaces between the lobules and ducts and holds the breast in place. It gives breasts their shape and size. A tumor called a phyllodes tumor can start here.
- The nipple. This is the small, raised area in the center of the breast. A type of breast cancer called Paget disease of the breast can start in the nipple.
- Blood vessels and lymph vessels. Inflammatory breast cancer happens when cancer cells block lymph vessels in the skin of the breast. A cancer called angiosarcoma can start in the cells that line blood and lymph vessels.
In place or spread
There are many types of breast cancer, depending on where it begins and how far it has spread.
- When the abnormal cells are still within the lobules or ducts and have not spread to other tissue in the breast, it is called carcinoma in situ.
- Invasive cancers have spread into the surrounding breast tissue. They can spread to nearby lymph nodes or to other organs in the body.
Most breast cancers are invasive.
In short: doctors describe breast cancer by where it started and whether it has stayed in place or spread.
Finding breast cancer early
Mammograms can detect breast cancer early, possibly before it has spread. Finding cancer early may make it easier to treat. Your healthcare team can explain when screening is right for you and what your results mean.
Everyone's situation is different. Your care team is the best source of information about your own breast health and any next steps.
Watch instead
Animated lessons are in production. Here’s the planned video slate for this topic — each one will be based on the same NCI-sourced explanation you’re reading.
What Is Breast Cancer: the quick overview
A one-breath explanation you can watch before an appointment.
Coming soonWhat Is Breast Cancer, explained simply
The core ideas with friendly animation and plain language.
Coming soonUnderstanding what is breast cancer — full lesson
A deeper walkthrough covering the key takeaways and common questions.
Coming soonVideo transcript▾
A full, readable transcript will appear here when the video is published — so the lesson is accessible whether you prefer to watch, listen, or read. For now, the article above is the complete text version.
Suggested animation storyboard▾
- 1Open on a calm title card: "What Is Breast Cancer?" with the Cancer Explained mark.
- 2Narrator reads the 30-second summary while a soft animated diagram builds on screen: "Breast cancer is cancer that starts in the breast, when cells there grow without control and form a tumor. It mostly affects women aged 45 and older, but anyone with breasts can get it. Mammograms can find it early, and there are many types depending on where it begins and how far it has spread."
- 3Scene 2: illustrate the idea — "Breast cancer starts when cells in the breast grow without control and form a tumor that may spread."
- 4Scene 3: illustrate the idea — "It can start in the milk ducts, the milk-making lobules, the nipple, or other breast tissue."
- 5Scene 4: illustrate the idea — "Most breast cancers begin in the ducts and are called ductal cancers."
- 6Close on a reminder card: this is educational only; talk with your healthcare team, and a link to the NCI source.
Words to know
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Quick knowledge check
According to this article, what happens when breast cancer begins?
Frequently asked questions
▸Where does breast cancer start?
Breast cancer starts in the breast, and it can begin in one or both breasts. It can form in glandular tissue (the milk ducts and lobules), in the fibrous and fatty tissue that gives the breast its shape, in the nipple, or in the blood and lymph vessels of the breast. Most breast cancers start in the ducts.
▸Who can get breast cancer?
Breast cancer mostly affects females aged 45 and older, but anyone with breasts can get it. It is rare in children and in men.
▸What is the difference between carcinoma in situ and invasive breast cancer?
When the abnormal cells stay within the ducts or lobules and have not spread to other breast tissue, it is called carcinoma in situ. Invasive cancers have spread into the surrounding breast tissue and can spread to nearby lymph nodes or other organs. Most breast cancers are invasive.
▸Are there different types of breast cancer?
Yes. There are many types, depending on where the cancer begins and how far it has spread. Cancers that start in the ducts are called ductal cancers, and cancers that start in the lobules are called lobular cancers. Less common types include inflammatory breast cancer, Paget disease of the breast, and phyllodes tumor.
▸Can breast cancer be found early?
Yes. Mammograms can detect breast cancer early, possibly before it has spread. Finding cancer early can make it easier to treat.
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Questions to ask your healthcare team
Consider bringing these questions to your next appointment.
- What type of breast cancer do I have, and where did it start?
- Has the cancer spread beyond the breast, or is it still in place?
- What tests do I need to learn more about my breast cancer?
- When should I be screened for breast cancer, and how often?
- What do my results mean for my treatment options?
- Where can I find reliable information and support?
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