Tumors often become resistant (stop responding) to drugs used to treat metastatic breast cancer.
Some metastatic breast cancer cells need specific proteins or cell pathways to grow. Drugs that target the proteins or pathways can slow or stop the growth of these cancer cells for a period of time.
You can think of the proteins as traffic signs and the pathways as roads. Breast cancer cells must pass through the signs to continue along the road.
If the cancer cell hits a roadblock (such as a drug that targets the protein), it can’t continue down that pathway.
At some point however, the cancer cell finds a detour around the roadblock and uses another pathway to continue to grow.
If you have metastatic breast cancer, you’ll be monitored every few months to see if the cancer is responding to treatment. This is called “restaging.” Tests may include a physical exam, blood tests and/or imaging tests (such as an X-ray, CT scan, PET scan or bone scan).
Because metastatic breast cancers often develop resistance to drugs, it’s common to change therapies multiple times.
You usually start a drug therapy and see whether:
The treatment controls the growth of the cancer
The side effects of the treatment can be managed
If the treatment is working (and the side effects aren’t too bad) at the time of restaging, then the treatment is typically continued.
If the treatment is no longer working or if you are having a lot of side effects, you may switch to a different drug.
Blood tests for tumor markers
In some cases, blood tests for tumor markers may be used to help monitor metastatic breast cancer.
For example, you may be tested every few months for cancer antigen 15-3 (CA15-3) or cancer antigen 27.29 (CA27.29) . These tests are similar. Health care providers usually check one, but not both of these tests.
There is no test score that means the tumor has spread (the cancer has gotten worse).
Rather, whether your personal test score rises or falls over time may give some information on tumor response to a drug or tumor spread.
Tumor marker tests are not helpful in every case. Some people with rising tumor marker levels don’t have tumor growth and some people with tumor growth have normal or unchanged tumor marker levels.
Providers don’t make treatment decisions based upon tumor marker testing alone.
They may combine findings from a tumor marker test with information on symptoms and findings from imaging tests (such as bone scans). This combined information can help your providers understand if treatments are helping control tumor growth.
Talk with your provider about whether tumor marker testing is right for you.