The cause behind the infection is proximal caries in the tooth that is under going root canal therapy , if the antibiotic therapy and local disinfection dont respond , rct of the teeth next to the current tooth under treatment may be required..... Conservative treatment using antibiotics is possible but may not be successful....rct is adviced in such situation....... The x ray you posted is quite old , are you keeping up with your appointments as per schedule .... If you are not visiting the dentist on the required date specified ...the root canal therapy might not be successful either... Visit you treating dentist at he earliest .....
Next Steps
visit your treating dentist