Optimized treatment planning using intensity and energy modulated proton and very-high energy electron beam

Intensity and energy modulated radiotherapy dose planning with protons and very-high energy (50--250 MeV) electron beams has been investigated. A general-purpose inverse treatment planning (ITP) system that can be applied to any combination of proton, electron and photon radiation modalities in therapy has been developed. The new ITP program uses a very fast proton dose calculation engine and employs one of the most efficient optimization algorithms currently available. First, the ITP program was employed to investigate intensity-modulated proton therapy (IMPT) dose optimization for prostate cancer. The second application was to evaluate the potential of intensity-modulated very-high energy electron therapy (VHEET) for dose conformation. For an active proton beam delivery system the required energy resolution to reasonably implement energy modulation was found to be a function of the incident beams' energy spread and became coarser with increasing energy spread. For passive proton beam delivery systems the selection of the required depth resolution for inverse planning may not be critical as long as the depth resolution chosen is at least equal to FWHM/2 of the primary beam Bragg peak. In the study of the number of beam ports selected for IMPT treatment of the prostate, it was found that a maximum of three to four beams is required. Using proton beams for inverse planning of the prostate instead of photon beams gave the same or better target coverage while reducing the sensitive structure dose and normal tissue integral dose by up to 30% and 28% of the prescribed target dose, respectively. In evaluating the potential of VHEET beams for dose conformation, it was found that electron energies greater than 100 MeV are preferable for VHEET treatment of the prostate and that implementation of energy modulation in addition to intensity modulation has only a modest effect on the final dose distribution. VHEET treatment employing approximately nine beams was sufficient to give good dose conformation. Compared to intensity-modulated x-ray therapy (IMXT) of the prostate, VHEET provided comparable target coverage but significantly greater sparing of the sensitive structures (10%) and normal tissues (12% of the prescribed target dose).