Would it be feasible for them to overdose someone when giving radiation treatments for cancer?

Of course: This is why radiation oncology is its own unique medical specialty and persons undergoing XRT have such close tracking of their treatments as well as subsequent imaging (xray & ct) studies.

Possible: But highly unlikely. Numerous safeguards and culture of safety, but miscalculations, equipment programming mistake, not running phantom checks of multi-leaf collimators before imrt plan made headlines, pick a center with a trustworthy doctor more than a glitzy ad or emphasis on technology. People keep you safe.