Brachytherapy treatment apparatus are provided that include an elongate body including a core member and an outer member surrounding and movable relative to the core member. One or more helical catheters are provided on a distal portion of the elongate body, e.g., including distal ends coupled to the core member and proximal ends coupled to the outer member. The outer member is movable relative to the core member, e.g., axially and/or rotationally, to direct the helical catheters between a collapsed configuration for introduction through a tissue tract to a target location, e.g., a lumpectomy cavity, and an expanded configuration. Each helical catheter includes a lumen, and, after expansion to the expanded configuration, a source of radiation may be introduced along the helical catheters and/or a lumen of the core member for delivering radiation to the target location. Optionally, the apparatus may include a balloon surrounding or within the helical members.