Cancer is big, but so are the celebrities determined to help conquer it.

Michael Douglas, Matt Damon, Tom Hanks, Julia Roberts, Gwyneth Paltrow, Sofia Vergara and Robert Pattinson were among dozens of stars who appeared on the third Stand Up to Cancer telethon in the US.

"Let's build a world where cancer is no more," a somber Tom Hanks said on the hour-long program that was carried commercial-free by the major broadcast networks and a number of cable channels.

Some of the stars who participated had seen cancer sicken or claim a family member. Others had fought the disease themselves.

"It picked a fight with the wrong guy," said Michael Douglas, who had throat cancer. "Cancer didn't bring me to my knees, it brought me to my feet. I stand tonight because I want to be part of this effort to find an end to cancer. This is possible."

The telethon included short films about people fighting the disease, including a 3-year-old boy, Justin, who distracts himself from treatment with a favourite toy.

Justin Timberlake asked the studio audience to stand up for the child, who attended the event. "My buddy Justin is a fighter. That's what you have to be as a child facing a mountain like cancer," the singer said.

Taylor Swift performed a song she wrote for a young cancer victim, a three-year-old who died last year. Swift wrote Ronan after reading the blog of the child's mother, Maya Thompson, who is credited as a co-writer. The song is available on iTunes, with proceeds to go to cancer-related charities.