A balloon brachytherapy applicator has either a single or double chambers, with the capability of “blistering” outwardly at a specified area on one side of the balloon. This is useful to move sensitive tissue farther away from a radiation source within a source guide inside the balloon. In one case a secondary compartment is formed on the primary balloon, and is separately inflatable so as to allow blistering as desired when needed. In another form the balloon comprises a single chamber but with a “window” or area in which the balloon wall is elastic, or substantially more elastic than the main balloon wall, so that inflation of the balloon up to a certain point forms a substantially regular balloon shape, and the addition of further pressure will blister out the elastic area to increase the distance between the shaft and the sensitive tissue in a desired orientation of the balloon.