I have a patient with a large lesion and a flow of pus that never stops. How can I proceed?

In large lesions or true cystic formations, pus drainage does not stop completely in some cases despite multiple root canal treatment visits. In such cases apical surgery may be unavoidable. In this situation I do not fill the canals before surgery. During surgery, after curetting the lesion, I place gauze in the curetted cavity and fill the apical part of the canal with MTA from the coronal aspect of the tooth. I then remove the gauze and clean away the remaining MTA. When the patient returns for suture removal, I fill the remaining portion above the MTA with a conventional root canal filling.