Sony has shown that it can (accidentally/negligently) short circuit its hardware via firmware updates. I’m sure cyberwarfare agencies have the capabilities to remotely detonate your phone’s battery. The CIA have already allegedly hijacked a vehicle and killed an American journalist.

But just from a pragmatic perspective, it doesn’t make sense to give someone cancer especially through phones. You want to kill or permanently incapacitate your target as quickly as possible. Or in the case of Russia, you want to slowly poison your target with radiation because they’re traitors who need to be made examples of - but those cases are odd because the Brits don’t allow any evidence or witnesses to be disclosed even in court. But cancer relies too much on the person’s genetics. If they’re strong, then they have a chance of beating it, not to mention cancer is universally disliked and garner a lot of sympathy which you want to avoid.