And since doctors see so few young women with breast cancer, there is a gap in research about fertility, early-onset menopause and other effects of diagnosis, treatment and outcomes in young women.
Chemotherapy may affect a young woman in many ways, including her ability to have children in the future. But for teenagers, concerns such as body image, sexuality, beauty and peers loom larger.
"At that time, as a teen, you think you're invincible," Bryndza said. "I sort of saw the whole thing as a big inconvenience."
But as Bryndza began chemotherapy following her surgery, her cancer became more than an inconvenience. And while she had the unfailing support of family and friends, Bryndza could not forget how singular her situation was.
"Sometimes I felt so lonely," Bryndza said. "My friends were there for me but they couldn't fully understand what I was going through. Nor could a woman who is double my age who went through breast cancer."
Younger women have two striking disadvantages when it comes to breast cancer diagnosis because of their age.
"If they haven't generally been as tested in life, there is a theory that [younger patients] could be less resilient, if this is their first major challenge," Partridge said. "And they do have more difficulty adjusting to the diagnosis at the time of diagnosis and during follow up."
Younger women are more likely to be affected to the point of depression if they feel overwhelmed by the disease. In addition, unlike older breast cancer patients, they generally lack a strong peer support system
"I think when you're older you expect it more... it's not something that's atypical for your peer group," said Bryndza's doctor, Dr. Dawn Hershman, co-director of the breast cancer program at the Herbert Irving Comprehensive Cancer Center, Columbia University Medical Center. "When you're young you feel like you're the only one. Everybody wants to help but no one knows what it's like."
But younger women may not want empathy, craving normality instead. Often, the greatest source of anxiety for a young woman with breast cancer is not the disease -- it's whether their peers will treat them differently. Both Thompson and Bryndza said they felt the most anxious about heading back to school.
"Because she was so young, she did not know exactly what [her condition] was, and that helped her deal with it," Anderson said. "But she was worried about her peers -- if they were going to talk about her as if she had a [contagious] disease... She didn't want a lot of young people to know. I guess because she didn't understand herself what was going on, they might not understand either."
And the intensity of the cancer experience can be too much for some. Bryndza had a boyfriend when she was diagnosed with breast cancer and said he was supportive, but eventually, the stress took a toll on their relationship.
"It was hard because I was such a wreck, emotionally, sometimes," Bryndza said. "I needed to focus on myself and my health and it was hard to be in a relationship when I had to worry about myself."
Anderson said that the only time Thompson became overwhelmed with stress was because her surgery prevented her from participating in summer activities with her friends.
"I wanted to have fun," Thompson said. "But now I feel good."