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October is Breast Cancer Awareness Month, so it’s a good time to learn about the types of breast cancer and what your type means.

According to the American Cancer Society, breast cancer accounts for 1 in 3 of all new cancers diagnosed in women in the United States each year. It is estimated that more than 43,000 women in the United States will die of breast cancer this year. After skin cancerBreast cancer is the most common cancer diagnosed in women in the United States

However, not all breast cancers are the same. And it’s important to understand what kind of breast cancer you have and how it differs from other types of breast cancer.

What started breast cancer?

The type of tissue where breast cancer starts determines how the cancer behaves and what treatments are most effective.

The parts of the breast where cancer starts include:

  • Mammary ducts: Ductal cancer is the most common type of breast cancer. This type of cancer forms in the lining of the milk duct in the breast. Ducts carry breast milk from the lobules where it is produced to the nipple. Ductal carcinoma can remain inside the duct as a non-invasive cancer, called ductal carcinoma in situ, or it can break out of the duct, called invasive ductal carcinoma.
  • Lobes that produce milk: Lobular carcinoma begins in the lobules of the breast where breast milk is produced. If it breaks out of the lobules, it is considered invasive lobular carcinoma. The lobes are connected to the ducts that carry breast milk to the nipple.
  • Connective tissue: Rarely, breast cancer can start in the connective tissue which consists of muscle, fat and blood vessels. Cancer that starts in the connective tissue is called a sarcoma. Examples of sarcomas that can arise in the breast include phyllodes tumor and angiosarcoma.

What do cancer cells look like under a microscope?

When your breast sample is examined under a microscope, the pathologist looks for the following:

  • Cancer cells with a unique appearance: Certain subtypes of breast cancer are named for how they look under the microscope. Subtypes include tubular, mucinous, medullary, and papillary. Your subtype gives your healthcare team some clues about your prognosis and how your cells may respond to treatment.
  • The degree of difference between Art cancer cells and normal cells: How different your cancer cells are normal cells called the stage of your cancer. Breast cancer is graded on a scale of 1 to 3, with stage 3 cancer being the most different in appearance and considered the most aggressive.

Are your cancer cells fueled by hormones?

Some types of breast cancer are sensitive to the natural female hormones estrogen and progesterone. Breast cancer cells have receptors on the outside of their walls that can pick up certain hormones circulating in your body. Knowing that your breast cancer is sensitive to hormones gives your healthcare team a better understanding of how to best treat the cancer or prevent the cancer from coming back.

Hormonal status of breast cancer includes:

  • Estrogen receptor (ER) positive: Cells in this type of breast cancer have receptors that allow them to use the hormone estrogen to grow. Treatment with anti-estrogen hormone therapy can block the growth of cancer cells.
  • Progesterone receptor (PR) positive: This type of breast cancer is sensitive to progesterone and the cells have receptors that allow them to use this hormone for growth. Treatment with endocrine therapy blocks the growth of cancer cells.
  • Hormone receptor (HR) negative: This type of cancer has no hormone receptors, so it will not be affected by endocrine treatments that aim to block hormones in the body.

What is the genetic makeup of breast cancer cells?

Medical professionals are increasingly using genetic information about breast cancer cells to classify breast cancer. These groups help make decisions about which treatments are best.

Groups of breast cancer include:

  • Group 1 (lumina A): This group includes tumors that are ER and PR positive but HER2 negative. Lumen A breast cancer is likely to benefit from hormone therapy as well as chemotherapy.
  • Group 2 (luminal B): This type includes tumors that are ER-positive, PR-negative, and HER2-positive. Luminal B breast cancer is likely to benefit from chemotherapy and may benefit hormone HER2-targeted therapies and treatments.
  • Group 3 (HER2-positive): This type includes tumors that are ER-negative and PR-negative but HER2-positive. HER2 breast cancer is most likely to benefit from HER2-targeted chemotherapy and treatment.
  • Group 4 (Basal-like): This type is also called triple negative breast cancer, includes tumors that are ER negative, PR negative, and HER2 negative. Basal-like breast cancer is likely to benefit from chemotherapy.

Breast cancer with low HER2 protein is not a distinct subtype, study shows

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