Percutaneous conformal radiotherapy with keV-photons using a large number of incident beam directions and artificial dose enhancement

TitlePercutaneous conformal radiotherapy with keV-photons using a large number of incident beam directions and artificial dose enhancement
Publication TypeConference Paper
Year of Publication2009
AuthorsPetersheim, M, Hesser, Jürgen, Wenz, F
Conference NameDEGRO2009
PublisherDeutsche Gesellschaft für Radioonkologie
Abstract

Percutaneous conformal radiotherapy with keV-photons using a large number of incident beam directions and artificial dose enhancement Purpose: This study investigates a dedicated strategy for percutaneous conformal radiotherapy with keV-photons. Despite the striking advantages of less equipment cost and less shielding overhead, low energetic X-rays suffer from the lack of the build-up effect demanding sophisticated methods to ensure suitable tumor conformity using dose enhancement and many focused beam directions. Method and Materials: A 50 kVp X-ray source (Axxent S700, Xoft Inc.) was modeled based on its AAPM task group 42 emission properties. The source was equipped with a cylindrical collimator to enable directional irradiation distal to the tube only. A phase-space of the emitted particles was calculated and validated using the Geant4 Monte Carlo toolkit. In the next step, irradiation of a water phantom with a defined tumor volume (depth of tumor center = (50mm), tumor volume = (50mm x 50 mm x 50mm)) was simulated and single beamlet dose distributions were acquired for an optimal arrangement of 13 different incident polar angles and 5 distances. Cylindrical symmetry was used to expand these dose distributions over 45 different incident azimuthal angles by rotating the voxel grid around the propagation axis, thus gaining a total number of 2950 dose kernels. A linear programming based optimization algorithm was implemented to achieve the optimal weightings of each beamlet. These weighting factors correspond to irradiation time in a real treatment. In the simulation, a superposition of the weighted dose kernels led to the resulting dose distribution and hence to the depth dose curve in the water phantom. For further amplification of tumor dose, the simulation was repeated modeling different dosages of uniformly distributed gold particles in the tumor volume. Comparison of the resulting depth dose characteristics should give information about the suitability of dose enhancement by secondary electrons from artificially induced high-Z materials. Results: It was found, that even with optimized combination of a large number of different beams from different angles, keV-photon irradiation alone is not suitable for external radiotherapy of structures below the skin. In combination with dose enhancement, however, a two times higher dose to the tumor, compared to skin could be achieved. With very narrow collimation, even a dose ratio of five could be generated, though only to a very small focal volume (8mm x 8mm x 16mm). Conclusion: Since primarily the analysis of the local dose enhancement was promising, further exploration of this method is suggested, especially transferring the simulation to real patient data and expanding the analysis to micro-dosimetric measures. In combination with intra-cavitary electronic brachytherapy, the artificial dose enhancement could also be very supportive for ensuring local dose conformality.

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