1. A system comprising: a beam generator configured to generate at least one beam to treat a patient; at least one computing device coupled to the beam generator, wherein said at least one computing device is configured to: obtain at least one three-dimensional image of a target structure and surrounding tissue of the patient, wherein the target structure is configured to move and be repositioned with respect to the surrounding tissue; create a treatment plan that includes at least one first three-dimensional probability distribution of a position of the patient and at least one second three-dimensional probability distribution of an internal position of the repositioned target structure; combine the at least one first three-dimensional probability distribution with the at least one second three-dimensional probability distribution to generate a joint distribution of the position of the patient and the internal position of the repositioned target structure; select a probability level from the joint distribution such that the probability level defines an enclosed surface, wherein a distance defined between a portion of the enclosed surface and a point of origin in at least one direction is at least equal to a predefined threshold value of a parameter of the repositioned target structure in the at least one direction; and project the enclosed surface to a plane that is positioned relative to a direction of the at least one beam to facilitate the generation of at least one beam-specific planning target volume design output.