Cancer doctors are increasingly turning to genetics test to help them treat their patients with advanced forms of cancer.

But as we learn more about the genetics, we're learning that a lot of it is connected to the genes we inherit, as opposed to the mutations that arise in tumors.

That's putting doctors in a difficult position, because that information can have implications on not just the cancer patient, but his or her whole family.

It started to hit Dr. David Nanus, chief of hematology and oncology at Weill Cornell Medicine and NewYork-Presbyterian.

About one in 10 of the patients he sees with a condition like advanced prostate cancer has a mutation in their DNA associated with the cancer that can be useful in treating the disease. So testing that inherited DNA is becoming a more common occurrence.

But what happens when that test comes back with a positive result for a hereditary genetic mutation?

"If I have a prostate cancer patient who's 65 years old and has a BRCA2 mutation, and advanced prostate cancer, that's fine, but what about his son who's 30 and may have inherited it or his young daughter who's now may be at risk for breast cancer?" Nanus said.

Those family members, if also carrying the mutation, may have to consider things like regular screenings, or even preventive surgical procedures. At this point, he typically refers the family out for genetic counseling.

For Nanus, the gravity of the genetic information he communicated to his patients clicked into place when thinking about treating an Ashkenazi Jewish man with big family.

"It's when you have an Ashkenazi jewish family with 14 children," Nanus said. "If you have a patient that's positive, that means statistically, seven out of his children are going to be positive. It's a one out of two chance of inheriting it. That's where it sort of hits you, it's like 'Holy cow, this is not just this guy, it could be 100 people.'"

That realization led Nanus to bring the issue up with National Cancer Institute director Dr. Norman Sharpless. At the end of a presentation about Sharpless' plans for the institute under the Trump administration, Nanus asked how best to deal with this situation.

The question was in part in response to a February update to the National Comprehensive Cancer Network's prostate cancer guidelines, recommending germline genetic testing (that is, the DNA you inherit) and counseling to men with certain forms of prostate cancer. Separately, in March, 23andMe got FDA clearance to provide reports on three BRCA1/BRCA2 gene mutations that are associated with an increased risk of certain cancers.

Nanus asked:

"So in solid tumors now, we're recognizing with the sequencing, there's a lot of genetically inherited mutations. And we're faced with this genetic counseling, germline mutation analysis — I'm just curious, what's the NCI's position on this going forward?"

It's a difficult situation. Knowing what mutations you have can be a key part of understanding your risk of getting certain types of cancer. For people diagnosed with cancer, knowing those mutations — both the ones you inherited and the ones found in the tumor itself — can be useful for treating the disease.

"It also has a whole can of worms" Sharpless said in response to the question, because knowing the hereditary mutations associated with cancer not only impact you, but the people you pass along your genetic code to: your children and grandchildren.

How to proceed

When it comes to genetic mutations, some have been clearly associated with a particular cancer. For example, risk of getting breast cancer in women goes from 7% to an average of 55-65% when you have the BRCA1 or 2 gene mutation.

But there are also a lot of genetic mutations out there that are called "variants of unknown significance" which may or may not have a link to a particular disease. That makes communicating the results even more tricky.

For now, Sharpless recommends a cautious approach.

"I think what should be done today is one should adopt an extremely cautious position about what you're going to tell the patient," Sharpless said. "Any germline event that’s going to be communicated to the patient has to be a really clear pathogenic event."

Genetic counselors, like National Society of Genetic Counselors cancer expert Joy Larsen Haidle, want to be viewed as teammates.

"This is the crossroads where our specialties are going to be merging, and working in parallel to make sure that we get the right test and the information is right," Larsen Haidle said.

For Nanus, the genetic components of cancer are already something he thinks about when treating each patient, along with their other health conditions like heart disease. Ideally, he'd like to see some guidelines put in place that could help doctors navigate the implications of a person's genetics on their cancer risk, not just for themselves but for their families.

"It is here already, I think it'll just be a growing field, and at some point the NCI will have to bring in the experts around the country and come up with some better guidance for the average doctor, internist and oncologist," Nanus said.