A balloon applicator for irradiation of a cavity within living tissue is provided, comprising a double-lumen shaft and with a balloon connected to the shaft at a proximal and a distal position, in which a movement of a first portion of the shaft relative to a second portion of the shaft can be effected. In addition, a tool is provided which can be inserted into a lumen of the shaft and which is designed to effect the relative movement. Furthermore, a system is provided composed of balloon applicator and tool.