No one can say with any real authority why recurrences happen. Radiation induced possible but unlikely. If we all had to look forward to getting a second cancer down the road from our cancer radiation treatments no one would get them.

Radiolucent areas can be lots of things, not necessarily cancer. You don’t describe this well enough but it sounds like it’s in a root. Cancer doesn’t happen in teeth. If it’s around the root I’d be more concerned that osteoradionecrosis is an issue. That is a big deal. Not cancer but really a huge problem. This needs to be pursued with someone outside of a dentist that is familiar with looking at ORN. The dark area will increase in size over time if this is it. If it’s down by the root tip I’d be thinking more of an apical abscess. If it extends down the side of a root from the crown root junction, it’s more likely common periodontal disease. Either way further exploration is in order to determine what this is. Good luck, hoping this is one of the simple choices.