Abstract: For use with an irradiation system including a radiation source operable to produce a radiation beam towards a target, a beam modulator including a flexible, deformable container at least partially filled with a radiation attenuating fluid, a non-deformable first contacting surface in contact with a first portion of the container, the first contacting surface pivotable about a first axis, and a positioner operable to rotate the first contacting surface about the first axis, wherein as the first contacting surface rotates about the first axis, the first contacting surface deforms the container.