Balloon catheter, applicator, particularly for invasive radiotherapy, and method of controlling the balloon catheter with respect to its shape and position in a tissue to be treated prior to an irradiation. The balloon catheter provides a catheter shaft for introducing a probe, an outer balloon and an inner balloon, disposed inside the outer balloon, the inner space of the inner balloon connecting to a channel for connection to a media inlet and intermediate space between the outer balloon and the inner balloon connecting to a channel for connection to a media storage unit. In order that the balloon catheter can be securely controlled with respect to its shape and position in a patient in a simple but clear manner prior to irradiation, the outer balloon is formed of a flexible, non-extendible material and the inner balloon is formed of a flexible, extendible material.
