Intensity modulation methods for proton radiotherapy

The characteristic Bragg peak of protons or heavy ions provides a good loca lization of dose in three dimensions. Through their ability to deliver late rally and distally shaped homogenous fields, protons have been shown to be a precise and practical method for delivering highly conformal radiotherapy . However, in an analogous manner to intensity modulation for photons, prot ons can be used to construct dose distributions through the application of many individually inhomogeneous fields, but with the localization of dose i n the Bragg peak providing the possibility of modulating intensity within e ach held in two or three dimensions. We describe four different methods of intensity modulation for protons and describe how these have been implement ed in an existing proton planning system. As a preliminary evaluation of th e efficacy of these methods, each has been applied to an example case using a variety of field combinations. Dose-volume histogram analysis of the res ulting dose distributions shows that when large numbers of fields are used, all techniques exhibit both good target homogeneity and sparing of neighbo uring critical structures, with little difference between the four techniqu es being discerned. As the number of fields is decreased, however, only a f ull 3D modulation of individual Bragg peaks can preserve both target covera ge and sparing of normal tissues. We conclude that the 3D method provides t he greatest flexibility far constructing conformal doses in challenging sit uations, but that when large numbers of beam ports are available, little ad vantage may be gained from the additional modulation of intensity in depth.