ABC News' Anna Wild reports:

Running, gymnastics and karate were just some of the activities that filled the life of Caitlin Stewart of Albuquerque, N.M.

That changed when, at the age of 12, Caitlin began to feel pains in her right knee.

The pains turned into a shocking diagnosis for the young girl, osteosarcoma, an aggressive form of bone cancer.

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"And that's when her world, all of our worlds, changed overnight," said Caitlin's father, Shawn Stewart. "When you're worried about your child, it's hard to explain unless you go through it. It never leaves your mind."

Caitlin, now 14, underwent chemotherapy to fight the cancer, suffering nausea and losing her hair in the process. She couldn't walk for months after she underwent surgery to replace her knee.

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"I would bottle up my tears," Caitlin said. "And so I would just break down sometimes because there was just … it was hard."

Courtesy Stewart family

What kept Caitlin going throughout her treatment was a life-size cutout of her favorite actor, Josh Hutcherson, star of "The Hunger Games."

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"He just helped me through the thing 'cause he just kept me happy," she said.

Caitlin, with her life-size Hutcherson by her side, fought through chemotherapy and saw her cancer go into remission.

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When the Make-A-Wish Foundation asked Caitlin what her biggest dream was, she said it would be to meet Hutcherson in person.

That's where " Good Morning America" stepped in to help make Caitlin's wish come true.

Under the guise that Caitlin had won a behind-the-scenes tour of "GMA" through a sweepstakes, the Stewart family flew to New York from Albuquerque this week.

They stopped by the Times Square studio of "GMA" on Tuesday, the same day that Hutcherson happened to appear on the show to promote the new "Hunger Games" sequel, "Catching Fire."

"Is there a Caitlin in here," Hutcherson asked the crowd.

"Oh my God," a shocked Caitlin responded.

Just seeing her dream actor in person was not Caitlin's only surprise.

Hutcherson also invited her entire family to see "Catching Fire" later that day, three days before its U.S. opening.

"I can't breathe," Caitlin said in response.

Hutcherson also gave her a poster signed by the whole "Catching Fire" cast, and another gift with a very personal meaning for Caitlin.

"There is a cardboard cutout of myself," Hutcherson said to his fan. "I heard you had one, so I got you an updated one."

The magical surprise ended with a hug from Hutcherson.

"I mean, it just made my whole life," she said.