Duncan Avery thought his son was imitating his big sister when he said his head hurt.

Avery’s other child, 6-year-old Kalea, had been diagnosed with a brain tumor a week earlier. She had just had surgery to remove the mass.

Now Noah, 4, was pointing to a spot between his eyebrows, the same place Kalea had complained of pain.

Then Noah vomited. He started walking oddly, his small body tilted to the right.

On June 21, an MRI revealed that Noah too had brain cancer . A tumor that looked just like his sister’s, in the same part of the brain.

“We broke down in tears,” said Avery, who lives in Torrance. “How could two kids in 14 days have the exact same tumor? How does that happen?”

There appears to be no precedent for the twin diagnoses. Doctors say they’re stunned that both children developed symptoms within days of each other.

There are rare reports of siblings developing brain cancer, even the same kind of brain cancer, but not at the same time.

“None of us have probably seen that,” said Dr. Sonia Partap, a professor at Stanford University who studies pediatric brain tumors.

In late May, Kalea threw up on a Sunday morning. She seemed fine otherwise, so her parents wrote it off as an upset stomach.

It was Memorial Day weekend. She played with her cousins at a family barbecue. She strapped on her yellow helmet and went skateboarding with her dad, skillfully guiding around a U-shaped halfpipe.

But later that week, Kalea began saying her head hurt. Her mom, Nohea, took her to the emergency room.

There, doctors discovered a mass in the girl’s brain.

Duncan and Nohea Avery with their children Kalea and Noah outside their home in Torrance in April. Avery family

Physicians explained it was a medulloblastoma, an aggressive tumor in the back of the brain. Roughly 500 children in the U.S. are diagnosed with one each year, though it’s unclear what causes them.

The diagnosis was heartbreaking.

Treatment typically involves surgery to remove the tumor, followed by radiation and chemotherapy to make sure the cancer doesn’t return. Every step of the treatment carries serious risks.

Kalea underwent a four-hour surgery on June 11. In just a few weeks, “we go from having a healthy baby girl who’s a skateboarder and a soccer player, who’s just loving life, to having a tumor removed from her brain,” said Duncan, 36.

Duncan, who is a surf coach at Redondo Union High School, and Nohea, a nurse practitioner, soon also started to worry about Noah.

Just two years apart, Noah and Kalea often played together. They are both shy, with the same wide foreheads and rounded cheeks.

Their parents noticed that while Kalea was in the hospital, Noah started taking naps every day, pointing to his head and walking a little strangely.

Duncan figured Noah was depressed. He understood; he was sad too. His oldest child, whom he’d started to teach to surf last summer, was sick. He couldn’t say her name without tearing up.

But Duncan took Noah to the pediatrician, just in case.

Dr. Lauren Nguyen examined Noah. She already knew what was going on with Kalea.

She also knew that medulloblastomas grow near the cerebellum, the part of the brain that controls balance and movement. Though Kalea hadn’t had such symptoms, a wobbly gait can be a sign of that kind of tumor.

“When I watched Noah walk down the hall, my heart dropped,” Nguyen said. “But, of course, could lightning strike twice?”

Tests revealed that Noah’s medulloblastoma was even larger than his sister’s.

How common is brain cancer in kids? 16 out of every 100,000 children ages 0 to 14 years are diagnosed with some kind of cancer each year in the U.S.

Brain tumors are the second most common kind of cancer among children, second only to leukemias, which are cancers of the bone marrow and blood.

About 3,000 kids in the U.S. are diagnosed with a brain tumor each year, a fifth of whom have medulloblastomas. Source: National Cancer Institute

“It’s very unusual,” Nguyen said. “It could be random, but probably not.”

Doctors say the siblings probably have something in their genes that makes them susceptible to this kind of cancer. It’s unlikely the trigger was from the environment.

“It’s not in the water in L.A.,” Partap said.

Physicians are testing the family for some hereditary syndromes that make people more likely to develop brain cancers.

It’s also possible that they have a genetic mutation that predisposes children to medulloblastomas that hasn’t been discovered. Scientists have identified mutations in several genes that increase the risk of various cancers, the most well known of which are in the BRCA gene and are linked to breast cancer .

“Maybe the reason we’re put on this Earth is so we can find the gene that causes medulloblastoma,” Duncan said.

The timing of the Avery children’s symptoms is remarkable, experts say.

In scientific literature, there are about 10 reported cases of medulloblastoma among siblings. A 1990 paper detailed a brother and sister who developed the tumors within 12 months of each other, but most examples tell of tumors that showed up years apart.

Dr. Ramin Javahery, the medical director of pediatric neurosurgery at Miller Children’s & Women’s Hospital in Long Beach, already had operated on Kalea when he got a call about a new brain tumor patient.

“I thought, ‘Huh, same last name, how odd,’ ” Javahery said. “It was just so not within my thought processes that you could have her sibling coming in. I assumed it was someone else. … Then I was told by the oncologist about what was going on, and I’m like, ‘Oh, my God.’ ”

Javahery operated on Noah on Monday. He was able to fully remove Noah’s and Kalea’s tumors. Neither of the masses appears to have spread to other parts of the children’s brains or bodies.

The chance of surviving five years for both kids is about 80%, he said. After they clear the five-year mark, it’s unlikely the cancer will return, he said.

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However, the radiation and chemotherapy included in the treatment could harm their brains. Radiation isn’t great for anyone, but especially not for growing brains.

“With radiating young children, you can essentially affect every part of the brain, but the way it manifests is most importantly cognitive. It severely delays cognitive development,” said Dr. Anthony Wang, who studies pediatric brain tumors at UCLA.

Kalea and Noah could need years of speech therapy, occupational therapy and physical therapy, experts say.

“It’s not just that you’re talking about your child’s life. You’re also talking about what their future holds. So, yes, will my child survive? But how will they survive?” Javahery said.

Duncan said he knows his family has a long road ahead.

Soon, they are planning to transfer to Children’s Hospital Los Angeles, he said. He wants to move the kids together, so they’re waiting a few days for Noah to recover from his surgery.

“Everyone says their kids are best friends, but our kids are 100% best friends,” Duncan said. “We’ll go to check on them and they’ll say, ‘Go away, Mom, go away, Dad, we’re just playing.’ ”

When Noah was admitted to the hospital last week, he was sent to the same ward as his sister. It was a floor for cancer patients. Their rooms were next to each other.

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soumya.karlamangla@latimes.com

Twitter: @skarlamangla