to stay vigilant for the cancer’s return. “If you catch cancer earlier, you should do better,” said Eltoukhy. “But there can be a challenge showing that in certain cancer types.”
The difference between the two tests’ methods is important. Foundation’s flagship test sequences the DNA from a tumor sample collected with a biopsy needle. Guardant360 analyzes a patient’s blood sample.
A tissue biopsy is like pulling a core of ice from a glacier: it yields a narrow subset of the tumor that might not capture the full picture of the tumor’s mutations. And depending on the site of the tumor, doing more than one needle biopsy might not be an option. A blood test, or “liquid biopsy,” is like a sieve to filter the floating bits of DNA that tumor cells shed into the bloodstream.
Guardant senior medical director Justin Odegaard said blood and tissue biopsies both have strengths and weaknesses. “But if both tests detect [a mutation], it’s almost surely there,” he said, and doctors should feel comfortable targeting the cancer with a drug designed to attack that mutation.
Foundation’s chief medical officer Vincent Miller said via a spokesperson that tissue tests “backed by rigorous peer-review published data, like FoundationOne, remain the gold standard.”
Practitioners who have the resources to order both types of tests say they’re complementary. “In my practice I use both and recommend both to others,” says UCSD’s Kurzrock. “At the time of diagnosis, you definitely need a tissue test because you want to look at the pathology of the cells. If you’re going to follow patients, repeat biopsies are difficult and painful. Blood [testing] lends itself to following serial samples from patients.”
That suggests taking both tests and comparing results could be a best-case scenario (though still imperfect). It’s also a luxury few could afford. Each test costs nearly $6,000 if not covered by insurance. Foundation reports that Medicare reimburses about $3,400 per test—when it agrees to reimburse at all. The company has spent years fighting to gain insurance coverage.
After years of submitting claims to Medicare that have gone unpaid, Foundation is hoping the U.S. insurance system will grant broader coverage after a long review that could finish later this year.
Meanwhile, Guardant, Foundation, and their competitors are all working on new products that test for more suspicious mutations, with plenty of updates coming this weekend at ASCO. Some want to screen for the earliest glimmers of cancer in people who are otherwise healthy, an ambitious goal that has plenty of skeptics.
But there are plenty of questions to answer first about sequencing tumors that have already been detected. Part of that process also means helping doctors in places like Maine, far from major cancer centers, ask the right questions in the first place, eventually leading to the proper use of sequencing as one part of better patient care. As Kurzrock says, “Instead of looking at different parts of the elephant, the whole elephant needs to be put together.”
Image of an immune response to a HER2+ breast cancer in a mouse model via the National Cancer Institute and Steve Seung-Young Lee/University of Chicago Comprehensive Cancer Center.
