A system and method for localized delivery of a therapeutic or diagnostic agent within a vessel is provided. The system and method provide for adjustability of the length of the treatment area and for reducing of pressure within the treatment area. A catheter system includes an inner elongated element, an outer elongated element positioned coaxially with respect to the inner elongated element, and a blood-release element at a distal end of the inner elongated element. A proximal occlusion element is positioned at the distal end of the outer elongated element, proximal to an outlet port. A distal occlusion element is positioned at a distal end of the inner elongated element. The distal end of the inner elongated element is distal to and movable with respect to the outer elongated element distal end.