How is genetic testing for targeted cancer therapy used?
These genetic tests are used to help guide treatment for certain cancers. They help to inform a healthcare practitioner as to whether certain targeted cancer drugs may or may not work.
Genes are the basic units of genetic material, the segments of DNA that usually code for the production of specific proteins. Alterations in DNA are called genetic variants (also polymorphisms or mutations) and occur throughout the population. Variants or mutations are largely inherited and affect all cells, but they can occur later in life, because of exposures to radiation, toxins, or for unknown reasons, and these mutations may result in cancer.
In a variety of cancers, there may be a mutation that leads to an increased amount of a particular protein present in the tumor tissue or to production of a protein that has altered activity. Tumors that have these mutations may tend to grow more aggressively, be more likely to spread (metastasize), and/or may be more resistant to standard chemotherapy. Sometimes, however, the changes in the protein also make the tumors candidates for therapy that targets the changed protein ("targeted therapy"). Genetic tests for cancer therapy detect the mutations that code for these proteins, thus identifying tumors that may be susceptible to targeted therapy.
Conversely, genetic tests may also identify tumors that will not respond to targeted therapy. Certain mutations, when present, make the cancer cells resistant to the drug and targeted therapy will not be used for treatment.
When are the tests ordered?
Testing may be ordered as part of an initial workup of particular cancers or performed on those with certain cancers that are not responding to chemotherapy. It requires a sample of the tumor tissue, and if a sample is available from a previous biopsy used for diagnosis, it can be done on that sample.
The tests are usually performed only once. However, testing may be done more than once if a patient's tumor progresses while on therapy to see if the tumor has acquired mutations that are resistant to the therapy.