Carter was being treated with a cancer drug called Keytruda that uses the immune system to fight off cancerous cells.

Keytruda, made by pharmaceutical company Merck, was originally approved by the FDA in September 2014 to treat melanoma, a deadly form of skin cancer that can also show up in other organs of the body, as it did in Carter's case.

In someone with melanoma, certain proteins called PD-1 stop the immune system from doing its job and fighting the cancerous cells. Keytruda works by getting in the way of those proteins, allowing the immune system to access the cancer cells. Then, with the help of radiation therapy, which works to shrink tumors by killing cancer cells, it can knock the cancer out.

The drug is delivered intravenously every three weeks, costing about $12,500 a month.