On Aug. 1, a group of 6,000 cyclists — cancer survivors, their families, friends and allies, including Massachusetts Gov. Charlie Baker and Boston Mayor Marty Walsh — gathered to ride hundreds of miles across Massachusetts to raise money for cancer care and research at the Dana-Farber Cancer Institute. The event raised $41 million last year; once all the donations are tallied for this year, the total amount raised since the event began in 1980 is expected to hit half a billion dollars.

Among those riding was Christopher Sweeney, a medical oncologist at Dana-Farber who treats and researches testicular and prostate cancers. This was the second year that Sweeney has ridden in the event. He was inspired by a patient, Chris Ryan, who had begun riding several years ago with a friend on Team Shawmut, sponsored by Shawmut Design and Construction, a Boston-based construction management firm. At Ryan’s urging, Team Shawmut had begun raising money directly for Sweeney’s research.

At the 2013 Pan-Mass Challenge, Sweeney went to a breakfast send-off for the riders to thank them. “Halfway through the speech,” he recalls, “I realized that although I was a not terribly fit 44-year-old, I really should do the ride.”

The money raised enables him to conduct research that others might not support. “We come up with ideas, and sometimes those ideas are very germinal, very innovative, and need to be supported somehow. It was a very dicey proposition to get funding from the National Institutes of Health or other funding bodies in those early days,” he says. “Seed funding from the Pan-Mass Challenge really helps those ideas to become projects.”

Sweeney’s research has focused on several areas. He’s developed a unique database of specimens from 1,200 patients with testicular cancer, which he and his team use to study why testicular cancer is generally quite curable, as well as why a small percentage of patients fail to be cured.

He’s also the lead author of a study published in the New England Journal of Medicine that demonstrates the superiority of treating prostate cancer by combining chemotherapy with hormonal therapy, rather than starting with just hormonal therapy, and then adding chemo when the patient becomes resistant. “The study shows one of the largest improvements in survival that we’ve seen in the context of prostate cancer, if not the largest,” says Phillip Kantoff, M.D., leader of the Dana-Farber/Harvard Cancer Center Prostate Cancer Program.

In addition, Sweeney is working on developing a drug to block a key protein that causes resistance to chemotherapy, hormonal therapy and radiation. When given with radiation, it makes that treatment much more effective. It has applications not only for prostate cancer, but also for lung cancer and certain other malignancies. The drug is derived from a common perennial plant called feverfew, and Sweeney has been working on making it more suitable for clinical use, finding the most effective ways to get it into the bloodstream and to the cancer cells. “At this point, we’re refining where it could best be placed in clinical trials,” Sweeney explains.

Riding in the Pan-Mass Challenge, Sweeney says, isn’t just about raising funds; it’s a way to build rapport and bond with patients. “As a physician riding side by side with patients, we’re taking the fight against cancer outside the clinic, working together to raise not only money, but also awareness. It’s great because so many people who are riding are patients who [are cancer survivors]. And they’re working together to help patients who are now going through cancer treatment.”

“Chris is a superstar,” says Kantoff. “He’s brilliant, he’s energetic and he is extremely elegant in the way he deals with people.” And, Kantoff adds, “Whatever he gets into, he gets into in a big way. This is true for the Pan-Mass Challenge.”