Understanding Genetic Testing in Cancer: What Patients Need to Know

If you or a loved one has been diagnosed with cancer, you may hear your care team mention “genetic testing.” For many patients, this can feel confusing or even overwhelming.

What does it mean? Why is it important? And how does it affect your treatment?

Let’s break it down in a simple, practical way.

What is genetic testing in cancer?

Genetic testing looks at the DNA involved in cancer to help guide treatment decisions.

There are two main types of genetic testing, and understanding the difference is key:

 Germline vs. Somatic Testing: What’s the Difference?

Germline testing (inherited genetics)

• Looks at the DNA you were born with

• Usually done with a blood or saliva test

• Helps determine if cancer may be hereditary

  
  

This information can be important for:

• Your family members

• Screening and prevention strategies

  
  

Somatic testing (tumor genetics)

• Looks at the DNA inside the cancer cells themselves

• Done on a tumor sample or through a blood test

• Helps identify specific mutations driving the cancer

  
  

This is the type of testing most often used to guide treatment decisions

 Tumor Testing vs. Liquid Biopsy

There are two main ways we can look at tumor genetics:

Tumor testing

• Uses a sample from a biopsy or surgery

• Gives a detailed look at the cancer’s DNA

Liquid biopsy

• A simple blood test

• Detects tumor DNA circulating in the bloodstream

Liquid biopsy can be especially helpful when:

• A tissue sample is hard to obtain

• We want to monitor how the cancer is evolving over time 

How does genetic testing affect treatment?

This is where genetic testing becomes especially important.

In many cases, results can help us:

• Identify targeted therapies that are more effective

• Determine if immunotherapy may be beneficial

• Avoid treatments that are less likely to work

This approach is often called precision medicine, choosing treatment based on the unique biology of your cancer.

Does everyone need genetic testing?

Not always, but many patients with cancer today will have some form of testing as part of their care.

At St. Louis Cancer Care, we consider:

• The type of cancer

• Stage of disease

• Available treatment options

Our goal is to make sure every patient receives the most personalized and effective treatment plan possible.

What this means for you

Genetic testing is one of the ways cancer care has become more advanced and more personalized than ever before.

While the terminology can feel complex, the purpose is simple:

To give you the best possible treatment, based on your specific cancer