Breast cancer occurs when abnormal cells grow out of control in one or both breasts. These cells can invade nearby tissues and form a mass, called a malignant tumour. The cancer cells can spread (metastasize) to the lymph nodes and other parts of the body.

The first treatment for early-stage breast cancer usually includes surgery and sometimes radiation. Your doctor may also talk to you about added treatment, such as chemotherapy ("chemo") and hormone therapy, that may help keep cancer from coming back.

It isn't possible for all women to know for sure who will benefit from added treatment. But if you have early-stage, estrogen receptor positive (ER+) breast cancer with no cancer in the lymph nodes, you may have a gene test. Gene tests, such as the Oncotype DX, may be done on the cancerous tissue that was removed to look for tumour markers. These tests can give your doctor important information about whether chemotherapy will help you.

The type of added treatment you have depends on the stage and classification of your breast cancer:

• Stage: How far the cancer has spread in your breast and whether it has spread to nearby tissues or other organs is called the stage. This is one of the most important things in choosing the treatment option that is right for you. If breast cancer cells have spread to your lymph nodes, there is a greater risk that they may also have spread to other places in your body. Added treatment, including chemo, may help destroy cancer cells in your body.
• Classification: Breast cancer is classified according to what the cancer cells look like under a microscope. This tells your doctor how fast the cancer cells are growing and dividing and where they may have started in the breast tissue. If your cancer cells are growing and dividing quickly, then chemo may help you. When cancer is the kind that grows quickly, doctors call it "aggressive" or "poorly differentiated." Breast cancers are also classified by types, such as hormone-receptor-positive or triple-negative.

Different chemotherapy medicines tend to cause different side effects. Many women do not have problems with these side effects, while other women are bothered a lot. There are other medicines you can take to treat the side effects of chemo.

Talk to your doctor about the type of chemotherapy medicine that he or she is planning to give you. Ask about any side effects that the chemo may cause.

Short-term side effects can include:

• Nausea and vomiting.
• Loss of appetite.
• Fatigue.
• Hair thinning or hair loss.
• Mouth sores.
• Diarrhea.
• Increased chance of bruising, bleeding, and infection.
• Memory and concentration problems.

Long-term side effects of chemotherapy can include:

• Early menopause, which means not being able to have children anymore. It also can include symptoms like hot flashes, vaginal dryness, and thinning bones (osteoporosis).
• Concentration problems that may last for many months after your treatments are finished.
• In rare cases, heart damage and a higher risk of other types of cancers, such as leukemia.

Your doctor might recommend chemotherapy ("chemo") after surgery if:

• You are younger than 35. Women younger than 35 usually have a more aggressive type of breast cancer.
• Your breast cancer was bigger than a pea. Breast cancers that are at least 1 cm (0.4 in.) are more likely to come back later.
• Your breast cancer has spread to the lymph nodes under your arm. If that's the case, there is a bigger chance that the cancer may also have spread to other places in your body.
• Your breast cancer is HER-2 positive or triple-negative. These types of cancer tend to grow faster and spread more quickly.

Your doctor may use a genetic test to find your risk for having your cancer come back. This information can help you and your doctor decide about chemotherapy.