An apparatus and method for interstitial treatment and diagnosis in organs utilizing a flexible cannula (12) having an interstitial locking means (16) thereon. The apparatus includes a shaft for insertion into a body passage adjacent to the organ to be treated and adapted to guide cannula into the organ. The shaft is also adapted to receive an endoscope (28) to visualize insertion of the cannula. The cannula locking means includes a locking member which extends from the distal end of the cannula in response to relative motion between parts of the cannula.
