Questions to ask about genomic tumor testing
By Kate Reed
Consider Theo and Justine. Theo has recently been diagnosed with non-small cell lung cancer, stage 2. Justine has been found to have a recurrence of her breast cancer, for which she had surgery three years ago. Is genomic tumor testing appropriate for either Theo or Justine?
There’s no single right answer to the question. The goal of genomic tumor testing is to identify genomic changes in the cancer cells that are driving cancer growth for the purpose of using targeted treatments. With the increasing number of available tests, knowing when and what to order can be challenging. Here are a few questions to consider when you are weighing the option of genomic tumor testing.
For which patients is genomic tumor testing appropriate?
In theory, genomic tumor testing is appropriate for all patients, with any type of cancer, because its goal is to identify genomic changes that may be targetable with specific therapies. In practice, though, having such information is only as useful as your ability to incorporate it into a treatment plan.
There can be multiple reasons the results from genomic testing may not be actionable for a particular patient. For example, if the patient’s cancer can be managed without a systemic therapy, such as a slow-growing prostate cancer, having information about possible targeted therapies is unlikely to change the treatment plan. The patient’s health status and interest may make treatment options only available through clinical trials or associated with significant side effects inaccessible or undesirable. So, like any treatment plan, discussing the potential benefits and limitations of genomic testing with the patient helps to inform whether such testing is “right”.
What is the right size panel to order?
A variety of genomic tumor testing panels are available. It is possible to test for the presence of a single variant or a few variants associated with a particular cancer type. Other tests (panels) assess cancer cells for variants in hundreds of genes simultaneously.
Which genes and variants are included on a panel determines what types of results you may get. Smaller, targeted panels typically will provide information about FDA-approved therapies for that cancer type and variant and, in some cases, approved therapies for that variant in other cancer types. Larger panels typically provide information about both of those categories as well as information about clinical trials testing therapies associated with specific variants. Either large or small panels may be appropriate; it depends on your patient’s interest and ability to consider broader treatment options, as well as your confidence to interpret and prioritize the results.
Are there limitations to larger panels?
While larger genomic tumor panels have the advantage of testing more genes and, therefore, potentially identifying more treatment options, they do have some limitations including:
- Lower sensitivity, meaning decreased ability to detect variants that are present at low frequency;
- Biopsy sample quality and quantity may limit test performance.
- Variants of uncertain significance are likely to be detected;
- Evidence for clinical actionability of some variants may be lacking or contradictory;
- Access to a recommended therapeutic may be limited by eligibility for a clinical trial, off-label use, or lack of insurance coverage; and
What kind of sample is required for genomic tumor testing?
Currently, many laboratories offer testing only on solid tumors, or in the case of hematologic cancers, blood samples. The lab must have enough tumor tissue to extract sufficient genomic material for testing. It is important that the sample has a high enough percentage of tumor (versus normal) cells for testing. The laboratory should have information on their website about the specific specimen that they require for testing.
Liquid biopsy, acquiring cancer cells from the blood, is getting increasing attention and study. One main advantage of such testing is that it is non-invasive, requiring only a blood draw rather than a biopsy. Currently, these tests assess fewer variants and genes than those using tumor samples. Liquid biopsy is likely to be increasing in availability and use in the near future.
When is the right time in my patient’s cancer journey to do genomic tumor testing?
There are no guidelines available to direct the appropriate timing for genomic testing. Currently, many oncologists consider large panel genomic testing only when a patient has been diagnosed with a late-stage or aggressive cancer or when multiple treatment options have failed. There are some oncologists who order large panels when the patient is first diagnosed or early in the treatment plan. This approach allows the provider and patient to have information about possible targeted therapies from the beginning, though most will start with standard of care options.
A challenge to genomic testing at any point is that the sample tested represents only a subset of the cancer. Cancer cells acquire additional variants over time, which is why first-line treatments sometimes fail. Once a cancer has metastasized, the cancer cells have probably acquired mutations that were not present in the primary tumor. It is unlikely that any testing will identify all genomic variants present.
Returning to the question of the right approach to genomic testing for Theo and Justine, as you can see there are multiple factors to consider. With few guidelines currently available and a rapidly moving technology, clear communication with patients about benefits and limitations of testing, along with collaborating with laboratories and molecular tumor boards can be particularly valuable.
For practice assessing appropriateness of large panel genomic tumor testing for different patient scenarios, enroll in a free, short course Exploring Somatic Cancer Panel testing for CME or nursing CE credit.
Interpreting Somatic Cancer Panel Test Results is a new module that will be available early in 2017 (also free and available for credit). Be notified of its release by joining our clinical education mailing list.