While her eighth-grade peers were busy worrying about high school, Valerie Bradley had a far more pressing concern: chemotherapy, which she began when she was 14 after receiving a diagnosis of osteosarcoma, bone cancer in her left leg.
Now, Bradley is 25 years old and cancer free. On Saturday, she was one of the 5,300 cyclists in the 38th Pan-Mass Challenge, a bike-a-thon with the goal of raising $48 million for the Dana-Farber Cancer Institute.
The event kicked off in Sturbridge as the first group of riders left the starting line around 5:30 a.m under gray skies and a light rain drizzle. Not long after, the second launch of the day began in Wellesley around 7 a.m.
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Riders could choose between one-day and two-day rides continuing into Sunday. The longest ride stretches 192 miles from Sturbridge to Provincetown, which riders are expected to finish between 9 a.m. and 2 p.m. Sunday.
Founded in 1980, the event draws cyclists from 41 states and eight countries. This year, more than 6,200 cyclists and 4,000 volunteers are expected to participate in the event.

Riders pledged to raise between $500 and $7,800, and 100 percent of every dollar donated directly benefits patient care and cancer research at Dana-Farber, according to a statement from event organizers.
Like Bradley, 850 of the event’s riders and volunteers are cancer survivors or cancer patients.
“Being treated at the Jimmy Fund [clinic] made having cancer not as terrible as it could have been,” Bradley said while stopped along the route for a lunch break.
For Bradley, a Washington, D.C., resident originally from Winchester, the day of her diagnosis happened to be her 14th birthday.
“I’m sure I shocked and horrified all my friends,” she said, recalling what it was like to come home to her birthday party after the biopsy. “But I got to tell them all at once — it wasn’t a terrible day.”
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At first, Bradley said she received treatment from Massachusetts General Hospital, but then switched to the Jimmy Fund Clinic at Dana-Farber after a family friend recommended their care.
“Every single person who works there — from doctors to nurses to the people who bring you food — is an incredible human,” she said. “It’s a place that really saved my life.”
On Thursday, Bradley graduated from the Jimmy Fund clinic to the survivor clinic.

One of the worst parts about having cancer is the aftermath, Bradley said. In her case, cancer robbed her of getting to play soccer, something she loved to do before her diagnosis.
Instead, Bradley turned to biking as a way to be athletic again. In 2010, she participated in her first Pan-Mass Challenge. Saturday’s ride marked the beginning of her fourth.
Bradley opted for the most strenuous ride Saturday, the 192-mile course that will keep her cycling until Sunday, along with her mother, father, and brother.
She said she is fortunate, because a lot of children who suffer through osteosarcoma have amputations, something she never had to experience.
“Being out here reminds me of how lucky I am to be alive and to have working legs,” she said.
Billy Starr, founder of the Pan-Mass Challenge and fellow rider, said the event is on track to meet its fund-raising goal this year. If met, the event will have raised more than $595 million in its 38 years.
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“This weekend is about commitment, camaraderie, reunion — it’s awesome, it’s a great environment,” Starr said.
Starr, who was also cycling the 192-mile route to Provincetown, said the ride is a bucket-list item for cancer survivors and non-cancer survivors alike.
“It’s very rewarding, everybody has got a story,” he said.

Kiana Cole can be reached at kiana.cole@globe.com. Follow her on Twitter @kianamcole.