In recent years, doctors and researchers at the University of Pennsylvania had come up with a very affective leukemia treatment called Kymriah or CAR-T therapy. Kymriah works by removing a patient's white blood cells and finding their T-cells. The lab then introduces a virus to the T-cell that will carry new genetic material to them, making them form the ability to recognize and kill cancer cells as they come across them.
For the fact that this is still a relatively new treatment for cancer patients, it is still considered experimental. The young man in this case had an extremely persistent and strong form of leukemia, and unfortunately was killed because of a rare occurrence inside the treatment of his cancer. Normally, the doctors will inject the white blood cells and new T-cells back into the body. Some leukemia cells would come out with the white blood cells, but ultimately be killed due to the new genetics of the T-cells, or so it was thought.
In this rare case, one of the leukemia cells which was picked up during the initial process of collecting the cells, was genetically modified by the virus that was supposed to be only affecting the T-cell. In a result of this, the leukemia cell which was produced was immune to the treatment by the T-cells, resulting in the old leukemia being killed off, and the new leukemia reproducing soon after that.
Now that scientists and researchers know that this is a possible outcome for this new treatment, they are working on making sure that all of the leukemia cells which are collected with the white blood cells at the beginning of patient treatment are gone before undergoing viral exposure. Although very sad, I believe this extremely rare death to be beneficial to the science community in that we will be able to learn from the mistake of what could possibly happen and be able to create a new drug or treatment in which will better serve these sick individuals.