an outer catheter having an opening, at least one open aperture at a distal end, and a lumen in communication with respect to the opening and the aperture, the aperture being coaxial with the lumen and the outer catheter being adapted for insertion into the brain of the patient; and

at least first and second inner catheters chosen from a plurality of different inner catheters for delivering a treatment agent to a select tissue region of the patient's brain, each inner catheter having a substantially identical outer diameter so that each inner catheter is adapted to be individually received within the lumen, the lumen snugly receiving each inner catheter and guiding each inner catheter to the tissue region, each inner catheter being adapted to extend through the aperture when inserted into the lumen and having a passageway and at least one port in communication with the passageway, the first inner catheter having a first length for delivering the treatment agent to a first tissue region and the second inner catheter having a second length for delivering the treatment agent to a second tissue region.