(CNN) It was a cold morning in early December 2016 when Melissa Unger received a phone call at work that changed her family's life forever.
She heard a physician on the other end say "the words that no parent ever wants to hear, that your child has a mass on her femur and you have an appointment that afternoon with a pediatric oncologist," Unger said.
For the Ungers and their 12-year-old daughter, Delaney, a dedicated dancer since the age of 3, the news was devastating. Delaney had a rare and aggressive bone cancer called osteosarcoma of the knee, which would require chemotherapy and amputation of her left knee.
Her future as a dancer seemed over.
But this brave girl took an unusual course. Today, Delaney appears to show no signs of cancer. She keeps a contagious smile on her face and even has resumed her training as a lyrical, hip-hop and jazz dancer -- despite having a left leg that now faces backward.
Osteosarcoma affects fewer than 1,000 people a year in the United States, and about half are children and teens, according to the American Cancer Society.
The cancer can grow anywhere but normally attacks a child's rapidly growing knee, said Dr. Fazel Khan, an orthopedic surgeon at Stony Brook Medicine in New York who treated Delaney.
More than 90% of patients, Khan said, get a massive artificial knee replacement, which in a growing child is unstable and limits the ability to do any intensive activity such as dance or sports.
Yet because of the location of Delaney's cancer, the Ungers had another option: a rare procedure called a rotationplasty.
"Her cancer really was in the knee and nowhere below the knee," Khan explained. "Her ankle, her foot, the bottom part of her calf, all of those muscles, nerves and even the ankle joint were fully intact."
Instead of an above-the-knee amputation, Khan said, they would cut below the knee, and "rather than throw out the good ankle, leg, foot and some of the muscles in the bottom part of the calf, we actually take the ankle, the calf, the foot, and we use that to make a new knee."
In other words, the ankle, turned 180 degrees, functions as the new knee. Her ankle sits in the location of where her knee would be, since her lower leg was reattached to her thighbone.
Doctors say they keep the foot because the toes provide important sensory feedback to the brain.
Delaney's father, Noah Unger, said he was told that by having a natural joint at the knee, instead of a prosthetic joint, Delaney would be able to do "the leaps, the jumps, the hops" that dancing requires.
"So that's the reason for the rotation," he explained. "You're using a natural joint in the direction it's supposed to go."
Delaney would then have an entire foot where her old knee had been, pointing backward. A lower-leg prosthesis would fit over the backward foot, giving her an artificial leg and foot.
The family knew that it would be a startling sight, a foot facing the opposite way. Mom Melissa was apprehensive. After all, Delaney, who lives in Selden, New York, would soon be a teenager, going to parties, meeting people who would not know her story.
They debated the options in a family huddle, Noah said, until Delaney spoke up.
"She looked at Melissa and said, 'I would rather have a chance to try and fail then not have a chance at all,' " Noah remembered. "And this surgery was the only chance she had at ever doing what she wanted to do."
The 13-hour surgery occurred at Stony Brook Children's Hospital in New York in April 2017, followed by chemotherapy and a prosthetic below her new knee -- which the family has affectionately dubbed the "knankle."
Recovery was tough. "She had to learn to flex and things like that in the opposite way ... so there definitely is a learning curve," Melissa said.
"You really have to rewire your brain," Noah said.
But Delaney had a goal: to resume dancing and try out for the school kickline team, something she always wanted to do.
"I wanted to be normal again," she said. "I wanted to make sure I could do most of the stuff that all my friends were doing and I could keep up with them."
For Khan and Dr. Jason Ganz, another surgeon on the 13-hour operation, Delaney's spirit has been inspiring.
"I've never met someone that had such a clear vision of what she needed," Ganz said, adding that Delaney had a smile on her face constantly.
"Every time she was in the hospital, every time I'd see her, she had that same grin, which is incredible."
Delaney's positive attitude was an important part of her journey, they said, and has contributed to her rapid recovery.
"She is blowing us all away with how fast she is progressing with dance and walking," Khan said. "We have videos of her walking, and when she has pants on, it's almost impossible to tell that she had any surgery to begin with."
When the doctors first saw videos of Delaney in recovery and later dancing, they say, they both choked up.
"Literally, there were tears in both of our eyes," Khan said. "I'm so happy to see her free from her cancer, so happy to see her actually get back to the thing that she wanted to do."
Ganz added, "I have a daughter her age. It was definitely a moment that will make my life highlight reel: seeing her walking, seeing her smiling, seeing her dancing. That was just incredible. I'll never forget that."
This weekend, Delaney and her family are planning to travel to Washington for a childhood cancer rally called CureFest. There, she will be advocating for childhood cancer research funding and performing a dance routine on stage.
As for the future, "she has her whole life ahead of her," Melissa said. "We wanted to give her the best chance of being able to do as many things as she would want to do and not be limited. We definitely feel like we made a good decision."
Although the doctors say they were able to remove all the cancer, about a third of osteosarcoma patients are expected to relapse, so Delaney will need monitoring for the rest of her life.
What would Delaney say to other children who might be in her situation?
"What I would say to another teenager who has cancer is to keep your personality," she said. "When I heard I had cancer, I said 'I want to be an inspiration and I just kept smiling and doing what I always did.'
"Don't say 'I can't.' Just try it, and if you can't do it then, that's fine, but if you never actually tried it, you should. It's like just a little stop in the road, but then you got to just keep going."