A system for positioning a patient on a treatment couchtop for SBRT is provided. The system includes a patient support panel and at least one positioning/fixation component for releasable mounting on the patient support panel at a desired position to immobilize a portion of the patient's body. The patient support panel includes a pair of longitudinally extending side rails having a series of longitudinally spaced indexing apertures for receipt of a locking bar for mounting a positioning/fixation component thereon at a discrete index position. The at least one positioning component includes a pair of clamping mechanisms for releasably securing that component to the side rails at any longitudinal position along the side rails.