Volume 30, Issue 3, March 2003
Index of content:
- PH. D. THESES ABSTRACTS
Optimized treatment planning using intensity and energy modulated proton and very high energy electron beams30(2003); http://dx.doi.org/10.1118/1.1541253View Description Hide Description
Intensity and energy modulated radiotherapydose 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 photonradiation modalities in therapy has been developed. The new ITP program uses a very fast protondose 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).
30(2003); http://dx.doi.org/10.1118/1.1541254View Description Hide Description
Inaccurate dose calculations and limitations of optimization algorithms in inverse planning introduce systematic and convergence errors to treatment plans. This work was to implement a Monte Carlo based inverse planning model for clinical IMRT aiming to minimize the aforementioned errors. The strategy was to precalculate the dose matrices of beamlets in a Monte Carlo based method followed by the optimization of beamlet intensities. The MCNP 4B (Monte Carlo N-Particle version 4B) code was modified to implement selective particle transport and dose tallying in voxels and efficient estimation of statistical uncertainties. The resulting performance gain was over eleven thousand times. Due to concurrent calculation of multiple beamlets of individual ports, hundreds of beamlets in an IMRT plan could be calculated within a practical length of time. A finite-sized point source model provided a simple and accurate modeling of treatment beams. The dose matrix calculations were validated through measurements in phantoms. Agreements were better than 1.5% or 0.2 cm. The beamlet intensities were optimized using a parallel platform based optimization algorithm that was capable of escape from local minima and preventing premature convergence. The Monte Carlo based inverse planning model was applied to clinical cases. The feasibility and capability of Monte Carlo based inverse planning for clinical IMRT was demonstrated. Systematic errors in treatment plans of a commercial inverse planning system were assessed in comparison with the Monte Carlo based calculations. Discrepancies in tumor doses and critical structure doses were up to 12% and 17%, respectively. The clinical importance of Monte Carlo based inverse planning for IMRT was demonstrated.