Lily Walker, 17, was in eighth grade when she suddenly lost weight and suffered headaches so severe she’d throw up. Her mom thought it was stress, or maybe a virus. After the double vision set in, a doctor told the family to head to an emergency room.

Her cousin, Peter Cellucci, then 7, stopped eating two years later. He suddenly needed to grip the banister with both hands to walk down the stairs at school. When he could no longer step into the bathtub without losing his balance, a doctor also told his parents to take him to a hospital ER.

Lily and Peter’s mothers are second cousins living 17 miles apart in the Philadelphia suburbs. Since December 2020, their families have shared a journey that has tested the limits of medical understanding, as well as their shared Catholic faith.

Each morning, Lily swallows a chemo pill to tame an inoperable brain tumor. Every three months, Peter gets an MRI scan of his head and spine to make sure the lime-sized brain tumor hasn’t grown back.

Pediatric cancer is rare. Malignant brain tumors in kids are even rarer. The cousins are simultaneously in treatment for two different, though equally aggressive, types of brain cancer at Children’s Hospital of Philadelphia.

The day after Peter’s diagnosis, his mom said she found a quiet space in the hospital and commiserated with Lily’s mom over the phone.

“I remember saying, ‘What the hell is happening?’” recalled Tricia Cellucci, whose mom is a first cousin to Megan Walker’s dad. “‘How is this even possible?’”

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

Doctors are unable to explain why this happened to one family. Only one in roughly 182,500 children in the United States are diagnosed each year with Lily’s tumor type and grade and one in about 150,000 for Peter’s.

That means that out of all 850,000 school-aged kids in the eight-county Philadelphia region, only about 10 will be diagnosed with either Lily’s or Peter’s type of brain cancer in any given year.

And the odds of it happening twice in the same family circle are unfathomable.

“Some people describe it as lightning striking, where it’s just wrong place, wrong time,” said Kavita Desai, Peter’s oncologist at CHOP. “That’s the hardest thing for parents to come to terms with.”

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Lily’s parents had expected to watch their daughter run track and cross country through her Haverford High School years.

Then a biopsy of her brain revealed a fast-growing tumor, known as a high-grade anaplastic astrocytoma, in her thalamus, which is essentially the body’s relay station. The thalamus processes sensory and motor signals and regulates consciousness.

The tumor can’t be removed, Lily said, without “disrupting matter in my brain, which would make me not be able to walk, not be able to talk.”

After the pandemic shutdowns, Peter’s parents had envisioned returning to the sidelines of a Malvern soccer field to cheer on their nimble son.

Instead, they had to find a way to explain to a first grader that a tumor called a medulloblastoma was growing inside his head.

“They said there was a doughnut in my head. A jelly doughnut,” Peter said, recalling the conversation. A doctor would do a “medullo-blast-the-doughnut” to take it out, and if there were any “munchkins” left, they’d shoot it with lasers. Medication would get any crumbs.

So far, there’s no known link between Lily and Peter’s cancers, though the science is ever-evolving.

Lily’s tests revealed a genetic disorder, called neurofibromatosis type 1 (NF1), that predisposed her to a brain tumor. Her parents don’t have NF1, meaning she didn’t inherit the disorder.

Peter doesn’t have NF1, which doctors can sometimes identify early by spotting coffee-colored splotches, known as “cafe au lait macules,” on an infant’s skin, according to Michael Fisher, director of CHOP’s Neurofibromatosis Program. He also doesn’t have Gorlin syndrome, another genetic disorder that can cause medulloblastoma. So the “why” in his case remains a mystery.

Where medical science cannot provide answers, the Walker and Cellucci families look to God.

The Walkers were always faithful but have become more religious. They pray each morning at 11:15, Lily’s birth date.

Before Dan Cellucci knew his son had cancer, his oldest daughter, Annie, went through confirmation at St. Patrick Catholic Church. Annie, then 13, picked Lily as her sponsor, inspired by her cousin’s strength and grace during her cancer treatments. Peter’s father thought during the ceremony, “God’s going to do something with this friendship.”

A week and one day later, the Celluccis learned Peter had a brain tumor. Dan Cellucci said he doesn’t believe in accidents, and Lily has helped Annie cope with her brother’s illness.

“When Peter was diagnosed, it was obviously special in a terrible way to be on the journey together with them,” he said.

Lily’s mom cried when she heard about Peter. “I just felt so bad for them because I know what it does to the child, the parents, the family,” she said.

Her mind flashed back to the moment she learned that Lily, then 14, had a large mass in her brain’s center in March 2019. A neurologist ushered Megan and her husband, Matt Walker, into a hospital conference room. It was so austere; she only remembers a big bottle of Purell on the table.

Their daughter, the doctor told them, required surgery to biopsy the mass. Megan Walker grabbed the bottle and hurled it.

It smacked against a wall. “I was just so angry and so sad for Lil,” she said.

Lily is the oldest of four, followed by Annie, 15, Elizabeth, 14, and 10-year-old Henry. “We were totally fine. A family of six. Everything’s normal and then they were dropping a bomb on us,” recalled Megan Walker during an interview earlier this month in the family’s Havertown home.

When Peter’s symptoms started, his parents knew something was wrong “but never did we think it was going to be a malignant brain tumor,” Tricia Cellucci said. They initially thought he was suffering anxiety due to the pandemic shutdowns.

In Peter’s case, only one parent was allowed in the hospital because of COVID. On Dec. 11, 2020, Peter’s dad, Dan Cellucci, took his son to Nemours Children’s Hospital in Wilmington. The staff “did a million and one tests” over eight hours, he recalled, the last of which was an MRI at about 5 p.m.

A nurse escorted Peter’s dad “into the terrible room.” A doctor told him what the MRI showed: “A beast of a tumor.” A medulloblastoma was growing in his son’s cerebellum, at the back of his head, near his spine.

The cerebellum controls appetite, balance, and emotions. That explained Peter’s uncharacteristic meltdowns, like when he thought he’d be late for school and his oldest sister had to cover the clock radio in the family’s car to keep him calm.

When the doctor finished explaining, he asked, “Is there somebody you want to call?”

“And then I lost it,” Dan Cellucci said, tearing up. “I kept saying, ‘I knew something was wrong’ over and over again.”

He called home, where it was Friday movie night for the family. Peter’s three sisters had sat down to watch The Grinch That Stole Christmas. Their mom put Tater Tots in the oven for dinner.

Dan Cellucci asked his wife to step into their office at home in Malvern, out of earshot from their girls: Annie, who just turned 13, Katie, 10, and 4-year-old Norah.

The doctor spoke in details. Tricia Cellucci took notes, scribbling “significant tumor,” “cerebellum,” and “medulloblastoma.” Still, she asked, “How do you know?”

Annie and Katie sensed something was wrong. They came to the office door and walked to their mom’s side. Their dad and the doctor told them. They knew what a brain tumor meant because of their cousin, Lily. Katie let out a “guttural, animalistic scream” — one her mom can’t forget. Annie sobbed.

Lily, Peter, and their families have spent countless hours at CHOP, enduring a whirlwind of treatments: physical therapy, platelet transfusions, blood draws, MRI scans, and chemotherapy. Sometimes all in a 12-hour day. Sometimes together. The families share hugs, advice, and support.

Turning around in a hospital and seeing family was a joy that mitigated the loneliness of the first visit, Tricia Cellucci said.

“It’s the very best and the very worst at the exact same moment,” she said.

Prior to her tumor biopsy, Lily had never had surgery before. The procedure lasted a nerve-racking eight hours. She emerged “thank goodness, with all her faculties,” her mom said.

She began 60 days of chemo, and simultaneously, 33 days of proton radiation therapy.

During proton-beam radiation, Lily lay down on a table, and hospital staff fitted a plastic mask over her face, clamping it down so her head couldn’t move.

Her family waited in a room outside. They knew when the 30-minute treatment started because a fire-engine red sign lit up and white letters glowed, “beam on.”

Lily listened to a “radiation playlist” on her iPhone with songs by Lizzo, Zac Efron, and Fergie.

The tumor shrunk after Lily completed treatment, but it grew soon afterward.

In the summer of 2019, Lily stopped talking. Her mom peppered her with questions. She just sat and shrugged. Lily’s friends visited. She said nothing.

“Why won’t you talk to me?” her mom pleaded. She thought she was depressed, but an antidepressant didn’t work. “It was awful. That summer was the worst time of my life.”

Lily had hydrocephalus, a buildup of fluid on her brain that cut off her ability to talk. A surgeon implanted a drainage shunt in her brain. After the procedure, her speech came back. Among the first words she spoke: “Good morning, Daddy. I love you.”

When the tumor started to grow back, her doctor, Michael Fisher, decided to try a new oral chemo medication. It was “a little bit out of the box,” and he wasn’t sure it would work, he said.

Remarkably, Fisher said, the pill “stopped the tumor in its tracks.”

In Peter’s case, doctors could operate. The tumor in Peter’s head measured five-by-five centimeters, roughly the size of a lime or lemon. Medulloblastomas are grade four tumors. They grow rapidly. Peter’s tumor likely got that big within a few weeks, his doctor said.

Before Peter’s arrival in an operating room, the neurosurgeon stood in the hospital hallway with the boy’s parents. He told them everything that could go wrong, from paralysis to loss of speech.

Dan Cellucci, suddenly clammy, slid down the wall. “They had to help me up off the floor,” he said.

Peter spent 19 days, including Christmas, at the hospital. He had to relearn how to swallow, stand, walk, and hold a toothbrush. A neurosurgeon at Nemours hospital removed the tumor. Then Peter switched to CHOP for chemo and radiation.

Unlike Lily, Peter was too young to go through radiation fully awake. Doctors couldn’t risk a single millimeter of movement. He had to be sedated each time, with a breathing tube threaded down his throat — first for 17 days of radiation on his brain and spine, then 13 days of just the brain.

“He was super sick during radiation. He threw up all the time. Fatigue like nobody else. Burn spots on his head,” Tricia Cellucci said. “But he was a trooper.”

CHOP named Lily and Peter as “ambassadors” at the 2022 Parkway Run & Walk, which raises money to fight pediatric cancer. Both cousins were well enough to participate in Sunday’s event on the Benjamin Franklin Parkway.

Lily was too sick to do the 2K walk in September 2019, the last time the event was in person before COVID. She was determined to walk this year, even though the chemo causes her feet to burn and peel.

Lily, now a senior at Haverford High, has gained back the 35 pounds she lost at her sickest. She does well in school, though requires accommodations, like more time to complete her work.

With treatment, Lily’s tumor has remained stable over the past two years.

She travels to area schools and talks to students at assemblies about living life with cancer. She gives out green rubber bracelets that read, “Lily Strong.” NFL Quarterback Carson Wentz, who met Lily when he played for the Eagles, has worn it during games.

“I love to talk to people and spread my message, ‘Be happy. Just be happy,’” she said.

After graduation, she said she plans to take a gap year and travel around Europe.

“If you want to go somewhere, we’ll figure it out,” her mom interjected. “We’re going to take it one day at a time. I don’t know what next year will hold.”

Peter is in third grade at St. Patrick’s School in Malvern. He loves to learn about ancient Rome and play with Legos. He bounds down the steps at his family home and entertains visitors by reciting whole passages from his favorite book series, Dog Man, by Dav Pilkey.

Fortunately, Peter’s tumor didn’t spread down his spine, which can happen with medulloblastoma. Today, there’s no sign of disease, said Peter’s oncologist, Kavita Desai.

“I’m hopeful that this doesn’t come back for him,” said Desai, noting that his condition requires close monitoring. “We never say ‘never.’”

He still struggles to eat and relies on a feeding tube. On a recent September morning, he had a single grape for breakfast.

That day, his brown bangs formed a spike atop his head — a style, he explained, that the girl he likes at school finds attractive. “It does. It attracts her. We’ve got a whole relationship,” he said. He has a full head of brown hair for the first time in a long while.

Tricia Cellucci said she reminds herself that this is only “a chapter in the story of his life.” He’ll have more healthy days than sick ones.

“And today,” she said, “is a good day.”

Inquirer staff writer Aseem Shukla contributed to this article.