One could be forgiven if the reason for the annual Cupid’s Undie Run, raising money for The Children’s Tumor Foundation, gets a little lost in the spectacle of scantily clad 20-somethings running around Capitol Hill.

“We put the Hilarity in Charity,” states the Cupid’s Undie Run’s Twitter header, which barely breaks through the collage of photos of said 20-somethings in skivvies ranging from arguably tasteful to arguably underwear.

But event organizers are “very careful” to keep it PG-13. “We want it to be a fun undie event versus a sexy undie event,” co-founder Brendan Hanrahan says. A quick look over this past weekend’s photos — some galleries are equipped with warnings not to open at work — suggest that this disclaimer is lost on many of the event’s most enthusiastic participants. But really that’s the point.

The same people who advise against participants stripping down to thongs and panties also encourage them to “get evidence” and tweet the best of their photos. And why not? It’s the near-nude nature of the fun run that keeps people coming back and attracts new participants.

The event, which on Saturday wraps up its fourth edition across the country, directs all of its profits to The Children’s Tumor Foundation, an organization that supports neurofibromatosis, or NF, research and assists individuals and families affected by the debilitating disorder.

According to the foundation, neurofibromatosis “encompasses a set of distinct genetic disorders that causes tumors to grow along various types of nerves.”

It’s the rarest classification of NF, schwannomatosis, that Hanrahan’s friend’s brother, Drew Leathers, was diagnosed with in high school. And it’s Drew who inspired Hanrahan and his friends to run across the Capitol grounds in their underwear.

Humble Beginnings

The scene was a bit different when Hanrahan and his friends set out on the inaugural run in 2010, amid the aftermath of Snowmageddon. There was no press. There were no spectators — or at least no premeditated spectators. “It was traffic-stopping,” Hanrahan says. “Anyone who witnessed it just happened upon it.” They raised $6,000 that day — not bad for an afternoon’s work.

Since then, Cupid’s Undie Run has raised more than $1 million, with more than half of that coming in this year alone.

Part of this year’s boom in success is the event’s expansion. There are now 17 Cupid’s Undie Runs across the United States and one in Sydney, Australia, about half of which sold out. Of course, the run is unique in that “sold out” really means the pre- and post-race bars won’t be able to fit any more people.

Another big boost this year came from corporate sponsors. Donors include life-size pro-athlete decal purveyor Fathead, Brooklyn craft brewery Sixpoint and the event’s biggest sponsor, San Francisco-based Gimbal’s Fine Candies, which also offers Web customers the option of directing 15 percent of their purchase orders to Cupid’s Undie Run.

And no charity of this size is without a few noteworthy participants. This year’s list of better-known personalities willing to strip down and strut around were comedian Christopher Titus in Los Angeles and actress Erin Cummings in Seattle.

At the flagship race in the nation’s capital, the Washington Nationals’ Racing Presidents very fittingly threw on their cupid boxers and led the pack at the start line.

And at least one notable politician also made headlines for losing his pants for a good reason this year: Orlando, Fla., Mayor Buddy Dyer ran alongside his friend, Belle Isle, Fla., Mayor Bill Brooks, whose daughter has NF.

The Gentleman From Veronica’s Secret?

Despite organizers’ efforts, it’s unlikely that a member of Congress will soon strip down and run a loop around the Capitol and back to the Pour House — for charity, at least — but they’re still holding out hope for future runs.

For all of the event’s success, Hanrahan still can’t believe how big Cupid’s Undie Run has become. But part of that is probably because he has little opportunity to look up and see how well it’s going.

Hanrahan originally came to the Washington area to pursue a doctorate degree in materials engineering from the University of Maryland. And, for the past few years, he’s been balancing the looming responsibility of producing and defending a dissertation with the ever-increasing responsibility of his charity run.

In fact, his defense is inconveniently, if not appropriately, scheduled for Friday, the day before the run’s final races. “I’ve been trying for months to get everyone in one room. There’s no rescheduling this,” he says between chuckles.

And after all the work he’s put into achieving his dream of being an engineer, he still doesn’t know for sure whether he wants to pursue it professionally or focus full time on the charity. He does know, at the very least, that he’ll never fully depart from his brainchild, even if he’s serving in a remote advisory role.

Hanrahan knows how important Cupid’s Undie Run is to The Children’s Tumor Foundation and to the kids whose lives are improved every day by the resources he has helped provide. And he also knows that all of these efforts bring the world ever closer to the ultimate goal: curing neurofibromatosis.