Actually your case is of external root resorption in which a part of root is dissolved may be becoz of infection or any old trauma. So the root canal filling which may be normal at that Time and now its overextended. So dont worry its not a big deal. Either u can treated by re rct (repeat root canal) or by the small root resection surgery.

Please share what is your age and what symptoms you have to be able to guide you..as it is pertinent here. May be share the actual image too so that severity of overextended filling can be commented upon. Why did you undergo RCT, trauma or caries?

Hello..external root resorption may be due recurrence of infection .kindly post x ray to determine actual cause.visit endodontist for 2nd opinion.

Report is an opinion of the radiologist who has viewed that x-ray..... it would be better if you post the image of the X-ray for us to understand the problem in a better way..... slight over extension of the root canal filling is not a major concern if it doesn't produce any clinical symptoms...... what I am actually surprised is that why a CBCT was advised for evaluating the root canals of a single tooth??????? nonetheless, please put the image of CBCT or x-ray so that we can guide you better..... also over extension might be seen at present because the otherwise normal filling would appear over extended if external root resorption takes place, as is mentioned in the report.....

As per your history and reports ,better option to go for surgery (Apicoectomy) rather than delay. Your reports show some discontinuity in labial cortical plate so you might be develop sinus formation and pus discharge through gums in relation to that tooth in latter stages.