I thought of this today after hearing the news,
The US has drones capable of giving people cancer from the air, this is the first high profile target. This drone has a device capable of delivering radiation in high doses with laser accuracy. It would simply sweep across the body and lay waste to the DNA in cells, mutated DNA being the catalyst for cancer.
All the technology already exists. There was directional radiation weapons developed years ago, drones are capable of circling a target for days from high altitude etc..
I don't believe this to be true, I just think it would be possible.