Abstract: A variable stop collimator for use in a radiation therapy machine having a radiation source for producing a radiation beam directed toward a patient. The variable stop collimator includes a plurality of radiation attenuating leaves, with each leaf having an opposing leaf to collimate a ray of the radiation beam. For each leaf, the collimator includes a pneumatic cylinder configured to independently move each leaf between the open and closed state and at points between the open and closed states. Each pneumatic cylinder is connected at one end to the leaf and, at the other end, to a variable stop. The stop may include a raised spiral projection that permits a smooth stop at any point deemed appropriate for the motor. The collimator includes a support configured to guide the plurality of radiation attenuating leaves transverse to the beam plane.