ABSTRACT
This chapter describes precisions and uncertainties associated with proton therapy planning and delivery and currently understood mitigation strategies. Proton beams are used in radiation therapy because of their physical characteristics of energy loss as they penetrate into matter. A broad proton beam can be formed by passive scattering or by dynamic scanning of a pencil beam, both laterally and in depth. In passive scattering, the placement of scattering material in the beam provides a near-uniform dose within the field; a variable thickness propeller that rotates in the beam gives uniform dose in depth. Passively scattered proton beams utilize physical range compensators to achieve distal conformance of the dose distribution to the target volume. The thickness profile of the compensator is calculated based on the difference between the distal ranges of the given ray to that of the global maximum for the beam. Computer tomography artifacts, contrast agents, and metal implants can introduce significant range uncertainties.
