If someone develops cancer from radiation exposure, such as medical imaging radiation... would the cancer be treatable or is it always fatal?

There is: a very small theoretical increased cancer risk from medical imaging, however it is cumulative, so the more imaging, the higher the theoretical risk(see xrayrisk.com). The link would be difficult to prove due to the relatively long latency, and the fact that the hypothetical cancers are no different than cancers that arise in that particular body part for other reasons.