Pediatric brain cancer is rare. A CHOP doctor explains why some kids are more vulnerable than others.

The odds that a doctor will diagnose your child with pediatric cancer are slim, and the chances of a brain cancer diagnosis are even smaller.

Each year, roughly 4,000 children — out of about 73 million in the United States — will be diagnosed with a brain tumor. Of those, roughly half will have one of several types of a malignant, aggressive tumor. For instance, medulloblastoma is among the most common type of malignant brain tumor in children: about 500 new cases are diagnosed each year. That’s 0.0007% of all children nationwide, or 1 in almost 150,000.

Pediatric brain cancer is so rare that doctors and researchers are still learning why it occurs, though some children have genetic disorders, such as neurofibromatosis type 1 (NF1), that predisposes them to a brain tumor. The Inquirer spoke with Garrett Brodeur, a pediatrician and director of the Cancer Predisposition Program at Children’s Hospital of Philadelphia, about what causes cancers in kids, and why some may be more vulnerable. The interview has been edited for length and clarity.

We don’t know for sure. I would say we have a good suspicion that it has to do with mistakes made as cells are multiplying during development, either prenatally or postnatally. When a person is conceived, they’re one cell — one fertilized egg. And when they’re born, they’re 10 trillion cells.

Each time one cell becomes two, it has to copy six billion base pairs of DNA exactly. It’s a very complicated process.

» READ MORE: A medical mystery: Why two Philly-area cousins, just miles apart, got rare brain cancers

When mistakes happen, most of the time, those cells end up dying. They end up not being able to survive — either because the mistakes may cause the cells to die, or there are other protective mechanisms that recognize that something’s wrong and internally activate a self-destruct mechanism, or cellular suicide.

But it doesn’t always happen. A cell gets some growth advantage, and it doesn’t die. And that’s probably what causes most childhood cancers of the brain or elsewhere.

We know that some patients are genetically predisposed to get their cancer, because they have a mutation in a gene that puts them at increased risk. Sometimes that increased risk is very high compared to the general population. There are several syndromes that increase the risk of getting certain brain tumors. There is something called neurofibromatosis (N1F), where you get tumors of the peripheral nervous systems as well as in the central nervous system, or in the brain.

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There are really just two ways. One way is there’s a family history of a certain cancer, or group of cancers, that tend to go together related to a syndrome. The second way is they have physical features of that syndrome. For instance, there’s a syndrome where you get thyroid cancer and a lot of those patients — but not all — will have a very large head.

We biopsy tumors and see what it’s like under the microscope to make the diagnosis, but then we genetically analyze the tumor itself. So virtually every tumor has some, or in many cases, a lot of genetic changes. When we look at the changes in the tumor, sometimes we’ll see a change in a gene that we know — if it were in all the cells of the body — it would have predisposed them to get that cancer.

The main thing we can offer right now is surveillance. So if we know what type of tumor they’re likely to get, we can do tests, whether it’s a physical exam, blood tests, ultrasounds, MRIs, X-rays, or whatever. We can do tests to try and identify the tumor when it’s small and when it’s easy to remove surgically, and they usually don’t need radiation. They need very little chemotherapy. It’s usually not spread throughout the body and they have a very good prognosis. So if we know someone has an increased risk for a brain tumor, we can do MRIs of the brain every six months or a year.

The general signs are headache, vomiting, and abnormal eye movements caused by paralysis or impairment of some of the cranial nerves that control the eye muscles.

Read about two cousins battling cancer together at inquirer.com/pediatriccancer.