The goal of this clinical trial is to learn if adjuvant antimicrobial photodynamic therapy (aPDT) can help in the treatment in patients with pieces with apical periodontitis and the presence of fistula, in terms of reducing symptoms, resolution of fistula and the clinical signs of inflammation at the oral mucose, as well as, evaluation radiographic parameters. Participants will undergo conventional endodontic treatment at Group I (n=15) and will undergo conventional endodontic treatment combined with antimicrobial photodynamic therapy at Group II(n=15).