A boy and his turtle make a CHOP social worker run the Boston Marathon | Mike Newall

Amy Mack, a CHOP social worker, is running in the Boston Marathon on Monday to raise money for pediatric cancer and to honor the memory of Jonathan Smyth, a 7-year-old whose story of a rescued turtle became the subject of a children’s book.

When Amy Mack, a clinical social worker at Children’s Hospital of Philadelphia, would travel back to Massachusetts to visit her mother’s grave, one headstone always hurt her heart. The one just a few yards away, shaped like a Lego. The one that bore the name Jonathan Smyth, who died in 1998. He was 7.

She thought about him over the years, wondered who he was. The boy she never met with a grave the shape of a child’s toy became a familiar marker in the landscape of her own grief. Her mother, Patty Shields, originally of Yeadon, died of uterine cancer at 47 in 1994. She was the grease that kept the family’s wheels going. She and Amy’s father, Chuck Shields, who grew up in Olney, moved north and raised their three kids in Sudbury, Mass., just outside Boston.

Camera icon courtesy of Amy Mack
Patty Shields, originally of Yeadon, died of uterine cancer at 47 in 1994.

“When she died,” Mack said of her mother, “no one knew what to do.”

Mack, who moved to Philly after Gettysburg College, searched for ways to ease the grief, channel it. To find a way to memorialize her mother. She found it in the Dana-Farber Marathon Challenge, where hundreds run the famous marathon on the third Monday in April to raise money for cancer research. It’s the Boston equivalent of running to raise money for CHOP, where Mack, 45, works as a therapist for kids with eating disorders.

You can run in honor of kids struggling with the disease, or in memory of those lost to it. Mack had been running the marathon for a couple of years when, at one of the pre-race pasta dinners, a photo of a young boy flashed on a screen during the memorial video. It was Jonathan. The little boy whose name she had only seen on a tombstone.

Camera icon courtesy of Amy Mack
Jonathan Smyth was 7 when he died from cancer in 1998.

He died from neuroblastoma, a pediatric cancer of the adrenal glands.

It was a sign Mack couldn’t ignore. She asked his mother if she could start running in memory of the boy. That first year she was a half-mile from the finish line when the bombs went off. She and Marie Smyth, Jonathan’s mother, agreed that Jonathan and Patty had kept Mack safe.

When she finally met Marie before the next race, they hugged, a hug that seemed to carry everything — Jonathan, Patty. In that way, they became like family.

That year, Marie Smyth told Mack about the book. The one she could not bring herself to look at for so long. It was a manuscript for a children’s book that a parent at Jonathan’s school had written. It’s about the turtle Jonathan rescued from a Dunkin’ Donuts parking lot and called Little Dude. It’s the story of a sick boy who reached out to care for something as others were caring for him. And as the turtle grew, Johnny worsened, and when summer break came, and the end was near, Johnny and his friends and family gathered by the pond to let Little Dude go free.

A page from the children’s book “Goodbye Little Dude,” illustrated by Steve Dansereau.

The book, written by Rebecca Trotsky, is called Goodbye Little Dude. It’s beautiful. And Mack read it, and thought of something else she could do for Jonathan Smyth. Though she knew nothing about publishing, she knew people who did. She started making calls. The wheels started to spin. The book got published — and the story behind it — one of fate and happenstance and the power of human connection — went viral last week in a powerful front-page story in the Boston Globe.

“This story about a sick boy and his turtle will renew your faith in humanity,” the headline read.

And now a book born of grief and loss — one meant to be a personal solace — will help many. The book is selling out on Amazon. You should read it. All of the profits will be donated to children’s cancer research.

Camera icon Jessica Griffin
“Goodbye Little Dude” is selling out on Amazon.

For her part, Mack is preparing to run another race — she has raised nearly $100,000 over the years — and has marshaled her colleagues to use the book in their work.

Max Greenspan, a trauma-focused clinical social worker at CHOP, said he plans to use the book with the children he treats who are fighting terminal diseases — or who just lost a sibling or parent.

“It paints a really clear picture of the experience of families and kids around illness and loss,” he said. “Of the life cycle.”

Back in Sudbury, Marie Smyth leaves a single sprig of a silk flower at the graves of her son and Patty Shields when she visits. Something small, like the small difference she hopes the story about her son can make in someone else’s life. She leaves the flowers next to the stone turtles Mack’s family left there not too long ago. Marie likes to think Patty is holding Johnny’s hand.