 Agglutination of the fistula tract is a central part of this study. Agglutinative properties of doxycycline are frequently used by pneumologists for pleurodesis artificial adhesion of the pleurae to occlude the pleural space for the treatment of pneumothorax or other conditions. Doxycycline or tetracycline is thereby instilled into the pleural space, leading to local inflammation and finally pleurodesis. Pleurodesis is an efficient procedure with closure rates 80. The investigators are expecting to achieve high closure rates also for fistulae. 