An insertion device for use during surgical procedure to enlarge an opening in a patient's tissue to facilitate access to an internal treatment site with a surgical instrument. The insertion device includes an anoscope including a flange, and an elongate body having proximal and distal ends extending distally from the flange along a longitudinal axis. The anoscope may include a configuration that is asymmetrical about a plane extending along the longitudinal axis that bisects the flange.
