Once you've been diagnosed with breast cancer, your doctor works to find out the specifics of your tumor. Using a tissue sample from your breast biopsy or using your tumor if you've already undergone surgery, your medical team determines your breast cancer type. This information helps your doctor decide which treatment options are most appropriate for you.
Here's what's used to determine your breast cancer type.
Is your cancer invasive or noninvasive?
Whether your cancer is invasive or noninvasive helps your doctor determine whether your cancer may have spread beyond your breast, which treatments are more appropriate for you, and your risk of developing cancer in the same breast or your other breast.
- Noninvasive (in situ) breast cancer. In situ breast cancer refers to cancer in which the cells have remained within their place of origin — they haven't spread to breast tissue around the duct or lobule. One type of noninvasive cancer called ductal carcinoma in situ (DCIS) is considered a precancerous lesion. This means that if it were left in the body, DCIS could eventually develop into an invasive cancer. Another type of noninvasive cancer called lobular carcinoma in situ (LCIS) isn't considered precancerous because it won't eventually evolve into invasive cancer. LCIS does, however, increase the risk of cancer in both breasts.
- Invasive breast cancer. Invasive (infiltrating) breast cancers spread outside the membrane that lines a duct or lobule, invading the surrounding tissues. The cancer cells can then travel to other parts of your body, such as the lymph nodes. If your breast cancer is stage I, II, III or IV, you have invasive breast cancer.
In what part of the breast did your cancer begin?
The type of tissue where your breast cancer arises determines how the cancer behaves and what treatments are most effective. Parts of the breast where cancer begins include:
- Milk ducts. Ductal carcinoma is the most common type of breast cancer. This type of cancer forms in the lining of a milk duct within your breast. The ducts carry breast milk from the lobules, where it's made, to the nipple.
- Milk-producing lobules. Lobular carcinoma starts in the lobules of the breast, where breast milk is produced. The lobules are connected to the ducts, which carry breast milk to the nipple.
- Connective tissues. Rarely breast cancer can begin in the connective tissue that's made up of muscles, fat and blood vessels. Cancer that begins in the connective tissue is called sarcoma. Examples of sarcomas that can occur in the breast include phyllodes tumor and angiosarcoma.
How do your cancer cells appear under a microscope?
When a sample of your breast cancer is examined under a microscope, here's what the pathologist looks for:
- Cancer cells with unique appearances. When viewed under a microscope, invasive ductal carcinoma cells look different from healthy cells. Subtypes of invasive ductal carcinoma that describe how the cells appear under a microscope include tubular, mucinous, medullary and papillary. Your subtype gives your doctor some clues about your prognosis and how your cells may respond to treatment.
- The degree of difference between the cancer cells and normal cells. How different your cancer cells look from normal cells is called your cancer's grade. Breast cancers are graded on a 1 to 3 scale, with grade 3 cancers being the most different looking and considered the most aggressive.
Are your cancer cells fueled by hormones?
Some breast cancers are sensitive to your body's naturally occurring female hormones — estrogen and progesterone. The breast cancer cells have receptors on the outside of their walls that can catch specific hormones that circulate through your body. Knowing your breast cancer is sensitive to hormones gives your doctor a better idea of how best to treat the cancer or prevent cancer from recurring.
Hormone status of breast cancers includes:
- Estrogen receptor (ER) positive. This type of breast cancer is sensitive to estrogen.
- Progesterone receptor (PR) positive. This type of breast cancer is sensitive to progesterone.
- Hormone receptor (HR) negative. This type of cancer doesn't have hormone receptors, so it won't be affected by treatments aimed at blocking hormones in the body.
With ER positive or PR positive breast cancer, hormone-blocking medications, such as tamoxifen, may be an option to slow the cancer's growth. HR negative cancers don't respond to hormone-based therapy.
What is the genetic makeup of your breast cancer cells?
Doctors are just beginning to understand how the individual DNA changes within cancer cells might one day be used to determine treatment options. A cell's DNA is full of instructions (genes) that tell it how to behave. By analyzing the genes, doctors hope to find ways to target specific aspects of the cancer cells to kill them.
Laboratory testing can reveal certain genes in your cancer cells, such as:
- HER-2 gene. Cancer cells that have too many copies of the HER-2 gene produce too much of the growth-promoting protein called HER-2. Medications are available to shut down the HER-2 protein, thus slowing the growth and killing these cancer cells.
- Other genes. Researchers are studying ways to interpret the genetic makeup of tumor cells. Doctors hope this information can be used to predict which cancers will spread and which may need aggressive treatments. That way, women with relatively low-risk breast cancers may avoid aggressive treatments. Tests that analyze the genetic makeup of breast cancers are available, but aren't recommended in all situations. Ask your doctor whether this type of test might be helpful in your case.
Using genetic information about cancer, doctors are now able to categorize breast cancers into groups. Categorizing cancers based on their genetic makeup helps guide decisions about which treatments are best. Breast cancer groups include:
- Group 1 (luminal A). This group includes tumors that are ER positive and PR positive, but negative for HER-2. Luminal A breast cancers are likely to benefit from hormone therapy and may also benefit from chemotherapy.
- Group 2 (luminal B). This type includes tumors that are ER positive, PR negative and HER-2 positive. Luminal B breast cancers are likely to benefit from chemotherapy and may benefit from hormone therapy and treatment targeted to HER-2.
- Group 3 (HER-2 positive). This type includes tumors that are ER negative and PR negative, but HER-2 positive. HER-2 breast cancers are likely to benefit from chemotherapy and treatment targeted to HER-2.
- Group 4 (basal-like). This type, which is also called triple-negative breast cancer, includes tumors that are ER negative, PR negative and HER-2 negative. Basal-like breast cancers are likely to benefit from chemotherapy.
Understanding more about the chemical and genetic makeup of your cancer may help doctors choose the most effective treatment for your specific cancer.