ABSTRACT
A spread-out Bragg peak (SOBP) is the foundation of forward treatment planning for Three Dimensional (3D)-conformal proton therapy with uniform scattered fields. It is used to achieve a longitudinal conformality of the required dose to the target. In the early days of proton therapy, the wide variety of clinically required combinations of range and SOBP modulation required a large number of premanufactured wheels, with separate wheels required for shallow and deep tumors, one wheel for a close set of modulation width. The task of the planner is then to iteratively adjust the relative contributions, or “weights,” of multiple beams and to combine their doses so that the resulting distribution suits a particular set of requirements. Pencil beam scanning (PBS) delivery allows one to achieve highest conformality of proton dose distributions to the target volume and best sparing of healthy tissue. Dosimetric or other planning objectives may be defined for volumes or individual voxels.
