A device for circumscribing a target site with a beam. The target site is located within a target body. The path of the beam is varied rotationally so as to form a cone with an isocenter at the cone's apex. The isocenter is fixed on the approximate center of the target site. The target body is rotated about a vertical axis passing approximately through the center of the target site, and the rates of rotation of the beam path and body, respectively correspond so that the beam intersects an axis passing through the target site at an approximately constant angle.