A treatment couch having a tabletop that has a radiolucent region, and a lower-torso assembly that has a first slide member coupled to a slot of the tabletop, a second slide member coupled to a slot of the first slide member, and a link member coupled to the first and second slide members. The lower-torso assembly may be configured to adjust a patient along a longitudinal direction relative to a shoulder line of the tabletop while the lower-torso assembly remains outside the radio lucent region of the tabletop.