Synchrotron stereotactic radiotherapy (SSRT) is a treatment that involves the targeting of high- elements into tumors followed by stereotactic irradiation with monochromatic x-rays from a synchrotron source, tuned at an optimal energy. The irradiation geometry, as well as the secondary particles generated at a higher yield by the medium energy x-rays on the high- atoms (characteristic x-rays, photoelectrons, and Auger electrons), produces a localized dose enhancement in the tumor. Iodine-enhanced SSRT with systemic injections of iodinated contrast agents has been successfully developed in the past six years in the team, and is currently being transferred to clinical trials. The purpose of this work is to study the impact on the SSRT treatment of the contrast agent type, the beam quality, the irradiation geometry, and the beam weighting for defining an optimized SSRT treatment plan.Methods:
Theoretical dosimetry was performed using theMCNPX particle transport code. The simulated geometry was an idealized phantom representing a human head. A virtual target was positioned in the central part of the phantom or off-centered by 4 cm. The authors investigated the dosimetric characteristics of SSRT for various contrast agents: Iodine, gadolinium, and gold; and for different beam qualities: Monochromatic x-ray beams from a synchrotron source (30–120 keV), polychromatic x-ray beams from an x-ray tube (80, 120, and 180 kVp), and a 6 MV x-ray beam from a linear accelerator. Three irradiation geometries were studied: One arc or three noncoplanar arcs dynamic arc therapy, and an irradiation with a finite number of beams. The resulting dose enhancements, beam profiles, and histograms dose volumes were compared for iodine-enhanced SSRT. An attempt to optimize the irradiation scheme by weighing the finite x-ray beams was performed. Finally, the optimization was studied on patient specific 3D CT data after contrast agent infusion.Results:
It was demonstrated in this study that an 80 keV beam energy was a good compromise for treating human braintumors with iodine-enhanced SSRT, resulting in a still high dose enhancement factor (about 2) and a superior bone sparing in comparison with lower energy x-rays. This beam could easily be produced at the European Synchrotron Radiation Facility medical beamline. Moreover, there was a significant diminution of dose delivered to the bone when using monochromatic x-rays rather than polychromatic x-rays from a conventional tube. The data showed that iodine SSRT exhibits a superior sparing of brain healthy tissue in comparison to high energy treatment. The beam weighting optimization significantly improved the treatment plans for off-centered tumors, when compared to nonweighted irradiations.Conclusions:
This study demonstrated the feasibility of realistic clinical plans for low energy monochromatic x-rays contrast-enhanced radiotherapy, suitable for the first clinical trials on brainmetastasis with a homogeneous iodine uptake.
The authors would like to thank Professor Franck Verhaegen for providing the spectra data, ESRF SCISOFT team, especially Rainer Wilcke and Dr. Claudio Ferrero for MCNPX and other software issues, and Mathias Vautrin and Pierre Deman for DICOM segmentation.
II. MATERIAL AND METHODS
II.A. Monte Carlo simulations: Source code
II.B. Monte Carlo simulations: Geometries and irradiation parameters
II.B.1. Human head analytical phantom
II.B.2. Braintumor, patient specific data
III.A. Dose enhancement in the tumor
III.B. Dose distribution versus beam quality
III.C. Dose distributions according to irradiation geometry
III.C.1. Axial DVHs
III.C.2. Volumetric DVHs
III.D. Use of margins for increasing dose distribution homogeneity
III.D.1. Dose homogeneity versus tumor position
III.E. Optimization of dose distribution by weighting the beams
III.E.1. Analytical human head phantom
III.E.2. Patient specific data
IV.A. Contrast agent enhanced radiotherapy using synchrotron radiation
IV.A.1. Contrast agent type and concentration
IV.A.2. Optimal irradiation energy
IV.B. Dose distribution optimization
V. CONCLUSION: TOWARD A TREATMENT PLANNING SYSTEM THAT CAN BE USED IN CONTRAST-ENHANCED CLINICAL TRIALS
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