A patient consulted a doctor about recurrent pain in the frontal part of his upper jaw. He has history of previous treatment for pulpitis. Objectively: the crown of the 12 tooth is restored with a filling. X-ray picture of the upper jaw shows an area of bone tissue destruction (up to 1 cm in diameter) at the root apex of the 12 tooth. The root channel of the 12 tooth is filled up to the top. The patient was diagnosed with cystogranuloma of the 12 tooth. Choose the best method of treatment: