Computational aspects of treatment planning for neutron capture therapy
Author(s)Albritton, James Raymond, 1977-
Massachusetts Institute of Technology. Dept. of Nuclear Science and Engineering.
Dr. William S. Kiger, III.
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Boron Neutron Capture Therapy (BNCT) is a biochemically targeted form of binary radiation therapy that has the potential to deliver radiation to cancers with cellular dose selectivity. Accurate and efficient treatment planning calculations are essential to maximizing the efficacy of BNCT and ensuring patient safety. This thesis investigates computational aspects of BNCT treatment planning with the aim of improving both the accuracy and efficiency of the planning process as well as developing a better understanding of differences in computational dosimetry that exist between the different BNCT clinical sites around the world. A suite of computational dosimetry reference problems were developed as a basis for comprehensively testing, comparing, and analyzing current and future BNCT treatment planning systems (TPSs) under conditions relevant to both patient planning and planning system calibration. Using these reference problems, four of the TPSs that have been used in clinical BNCT (MacNCTPlan, NCTPlan, BNCTRtpe, and SERA) were compared to reference calculations performed with the well-benchmarked Monte Carlo radiation transport code MCNP5. The comparison of multidimensional dose data in the form of dose profiles, isodose contours, dose difference distributions and dose-volume histograms yielded many clinically significant differences. Additional calculations were performed to further investigate and explain significant deviations from the reference calculations.(cont.) A combined 81 brain tumor patients have been treated in dose escalation trials of Neutron Capture Therapy (NCT) in the USA at Harvard-Massachusetts Institute of Technology (MIT) and Brookhaven National Laboratory (BNL). Pooling the clinical data from these and other trials will allow the evaluation of the safety and efficacy of NCT with more statistical rigor. However, differences in physical and computational dosimetry between the institutions that make a direct comparison of the clinical dosimetry difficult must first be addressed before clinical data can be compared. This study involves normalizing the BNL clinical dosimetry to that of Harvard-MIT for combined NCT dose response analysis using analysis of MIT measurements and calculations with the BNL treatment planning system (TPS), BNCTRtpe, for two different phantoms. The BNL TPS was calibrated to dose measurements made by MIT at the Brookhaven Medical Research Reactor (BMRR) in the BNL calibration phantom, a Lucite cube, and then validated by MIT dose measurements at the BMRR in an ellipsoidal water phantom. Using the newly determined TPS calibration, treatment plans for all BNL patients were recomputed, yielding reductions in reported mean brain doses of 10% on average in the initial 15 patients treated with the 8 cm collimator and 27% in the latter 38 patients treated with a 12 cm collimator. These reductions in reported doses have clinically significant implications for those relying on reported BNL doses as a basis for initial dose selection in clinical studies and reaffirm the importance of collaborative dosimetric comparisons within the NCT community.(cont.) The dosimetric adjustments allowed the BNL clinical data to be legitimately combined with the Harvard-MIT clinical data for a combined dose response analysis of the incidence of radiation-induced somnolence syndrome. Probit analysis of the composite data set for the incidence of somnolence yielded ED5o values of 5.76 Gyw and 14.4 Gy, for mean and maximum brain dose. The applicability and optimization of variance reduction techniques for BNCT Monte Carlo treatment planning calculations were investigated using MCNP5. The preexisting variance reduction scheme in the Monte Carlo model of the fission converter beam (FCB) at MIT was optimized, resulting in improved energy-dependent neutron and photon weight windows. Using these weight windows, a more precise surface source representation of the FCB was produced downstream at the patient position with improved statistical properties that increased the mean efficiency of in-phantom dose calculations by a factor of 9. The variance reduction techniques available in MCNP were also explored as a means of increasing the efficiency of dose calculations in the patient model. By disabling implicit neutron capture and using fast neutron source biasing and photon production biasing techniques, the mean efficiency of dose calculations was improved by a factor of 2.2. Constructing an accurate description of a neutron beam is critical to achieving accurate calculations of dose in NCT treatment planning.(cont.) This study compares two methods of neutron beam source definition commonly used in BNCT treatment planning calculations, the phase space file (MCNP surface source file) and source variable probability distributions (MCNP SDef). To facilitate the comparison, a novel software tool was developed to analyze MCNP surface source files and construct MCNP SDef representations. This tool was applied to the MIT FCB, which has a well-validated Monte Carlo model. Each source type (surface source file and SDef) was used to simulate transport of the beam through voxel models of the modified Snyder head phantom, where doses were calculated. Compared to the surface source file, the initial dose calculations with the SDef produced significant errors of ~15%. Using a patched version of MCNP that allowed the observed radial dependence of the relative azimuthal angle to be modeled in the SDef, errors in all dose components in the head phantom at Dmax were reduced to acceptably small levels with none being statistically significant except for the induced photon error of 0.5%. Errors in the calculated doses introduced by sampling the azimuthal component of particle direction uniformly in the SDef vary spatially, are phantom-dependent, and thus cannot be accurately corrected by a simple scaling of doses.
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Nuclear Science and Engineering, February 2010.Cataloged from PDF version of thesis.Includes bibliographical references.
DepartmentMassachusetts Institute of Technology. Dept. of Nuclear Science and Engineering.
Massachusetts Institute of Technology
Nuclear Science and Engineering.