(CNN) In January of 2017, Jennifer Swenson and her daughter, Marit, were training for a half-marathon. By August, Marit was dead.

Swenson, of North Oaks, Minnesota, will run in Sunday's New York City Marathon in honor of her daughter, whom she described repeatedly as her "best friend," someone "smart, athletic, kind — genuinely the perfect girl."

The 16-year-old's life was cut short by a pediatric high-grade glioma with an H3.3K27M mutation. In layman's terms, the teen died of cancer, but Swenson's explanation to CNN was littered with the sorts of medical terms — epigenetic information, diffuse intrinsic pontine gliomas or DIPG tumors — that a person only learns when they absolutely have to.

In Swenson's case, her learning took place in a crash course. Marit's tumor was found on Valentine's Day, just half a year before her death.

Marit Swenson with her parents, Jennifer and Peter Swenson, about a year before her diagnosis.

Now, Swenson's understanding of the disease is almost encyclopedic. Beyond being able to define, let alone pronounce, all of those multisyllabic words no parent should ever have to learn, she rattles off facts about the form of cancer with ease.

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