Understanding Different Types of Breast Cancer

Breast cancer is the most common cancer among women worldwide, but not all breast cancers are the same.

If you’re navigating a breast cancer diagnosis, understanding the type of breast cancer may help you feel more informed and prepared.

What determines the type of breast cancer?

Doctors determine the specific type of breast cancer based on several factors, including:

• Where the cancer originally starts in the breast
• Whether the cancer has stayed where it first formed (non-invasive) or has grown into surrounding breast tissue (invasive)
• What features the cancer cells have
• How the cancer cells look under a microscope

Where breast cancer starts

Most breast cancers begin in either the:

• Lobules (glands that make milk). Breast cancer that starts here is called lobular carcinoma.
• Ducts (small tubes that carry milk to the nipple). Breast cancer that starts here is called ductal carcinoma.

Non-invasive vs. invasive breast cancer

Breast cancer is also classified as either non-invasive or invasive:

• Non-invasive (in situ): This means the cancer hasn’t grown beyond where it originally formed in the breast and has remained inside the duct or lobule. This is the earliest stage of breast cancer, sometimes referred to as stage 0. About 20% of breast cancer cases are non-invasive (in situ).
• Invasive: This means the cancer has grown outside the duct or lobule into nearby breast tissue. About 80% of breast cancers are invasive, which is about 4 out of 5 breast cancer cases.

Invasive breast cancer is not the same as metastatic breast cancer. While invasive breast cancer has grown beyond the duct or lobule in the breast, metastatic breast cancer has spread beyond the breast to other parts of the body.

Read more: What Does a Metastatic Breast Cancer Diagnosis Mean?

Breast cancer biomarkers

All breast cancers are tested to see if the cancer cells have certain features, called biomarkers.

These biomarkers give doctors more information about how the cancer may grow and behave and which treatments may be most effective.

The three key biomarkers tested for are:

• Estrogen receptor (ER): If the cancer is ER-positive (ER+), estrogen helps fuel the cancer's growth. If the cancer is ER-negative (ER-), estrogen does not help fuel the cancer's growth.
• Progesterone receptor (PR): If the cancer is PR-positive (PR+), progesterone helps fuel the cancer's growth. If the cancer is PR-negative (PR-), progesterone does not help fuel the cancer's growth.
• Human epidermal growth factor receptor 2 (HER2): If the cancer is HER2-positive (HER2+), the cancer cells have high levels of HER2, a protein that can cause the cancer to grow more quickly. If the cancer is HER2-negative (HER2-), the cancer cells do not have high levels of HER2.

Breast cancer cells can have all three of these receptors, none of the receptors, or a combination of these receptors.

If breast cancer is ER-positive, PR-positive, or both, it’s considered hormone receptor-positive (HR+) and is more likely to respond to hormone therapy.

How the cells look under a microscope

Breast cancer cells are also looked at under a microscope to determine the cancer’s grade. The grade describes how different the cells look from normal breast cells and gives doctors an idea of how quickly the cancer may grow.

Breast cancer is typically given a grade from 1 to 3:

• Grade 1 (low grade): The cancer cells look more like normal breast cells and tend to grow more slowly.
• Grade 2 (intermediate grade): The cancer cells look somewhat different from normal breast cells and tend to grow at a moderate rate.
• Grade 3 (high grade): The cancer cells look very different from normal breast cells and tend to grow more quickly.

Grade is different from stage.

Grade describes the appearance and likely behavior of the cancer cells, while stage refers to the tumor size and how advanced the cancer is.

Most common types of breast cancer

Invasive ductal carcinoma (IDC): This breast cancer starts in the milk ducts and has grown into nearby breast tissue. IDC is the most common type of breast cancer, accounting for up to 80% of invasive breast cancer cases.

Invasive lobular carcinoma (ILC): This breast cancer starts in the lobules and has grown into nearby breast tissue. ILC is the second most common type of breast cancer, accounting for up to 10-15% of invasive breast cancer cases.

Some breast cancers may also be a mix of both ductal and lobular carcinoma.

Less common types of breast cancer

Triple-negative breast cancer (TNBC): This subtype of breast cancer doesn’t have estrogen receptors, progesterone receptors, or high amounts of HER2 receptors. It tends to grow and spread more quickly than other types of breast cancer.

HER2-positive breast cancer (HER2+): This subtype of breast cancer has high amounts of HER2 receptors. It tends to grow more quickly but may respond well to targeted therapies.

Inflammatory breast cancer (IBC): This rare, fast-growing type of breast cancer occurs when cancer cells block lymph vessels in the skin of the breast. This can cause redness, swelling, warmth, and thick or dimpled skin that may look like the peel of an orange. IBC accounts for about 1% to 5% of all breast cancers.

Paget disease of the breast: This rare type of breast cancer affects the nipple and may spread to the areola. It accounts for fewer than 1-4% of all breast cancer cases.

Understanding your breast cancer diagnosis

A breast cancer diagnosis can bring stress, uncertainty, and many questions. The medical terms may feel unfamiliar, but understanding your specific type of breast cancer can help you feel more confident navigating the path ahead and help your care team develop a personalized treatment plan.

If you have questions about your diagnosis, your healthcare provider can help you answer any questions and guide you through your next steps.