The present invention relates to a balloon catheter (10) as well as an applicator (50), particularly for invasive radiotherapy. The balloon catheter (10) provides a catheter shaft (11) for introducing a probe (40), a first, outer balloon (20) and a second, inner balloon (30), disposed inside the outer balloon (20), the inner space (31) of the second, inner balloon (30) connecting to a channel (32) for connection to a media inlet (41) and intermediate space (21) between the first, outer balloon (20) and the second, inner balloon (30) connecting to a channel (22) for connection to a media storage unit (42). 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, it is provided according to the invention that outer balloon (20) is formed of a flexible, non-extendible material and that inner balloon (30) is formed of a flexible, extendible material.
