(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose: Monte Carlo simulations of inverse Compton x-ray sources (ICS), which offer low divergence, monochromatic x-rays in the 50-200 keV range were performed. These sources have sharp penumbra and strong high-Z contrast enhancement, while providing depth-dose fall-off equivalent to Bremsstrahlung sources at 5-6 times higher kVp energies.
Methods: Simulations were performed using the egs brachy user code of EGSnrc to utilize low-energy photon variance reduction techniques. The ICS source divergence and spectra were obtained from the literature. Nominal photon energies of 80keV and 150keV were investigated. An energy of 80keV represents optimal gold dose-enhancement, and a bone dose-enhancement factor of 2.5-3 versus water. The 150keV energy provides less gold enhancement, but greater penetration (bone dose-enhancement 1.15). Simulations were performed in a 30x30x30cc water phantom including cortical-bone slabs placed at various depths. Further calculations include three test CT cases including brain and spine treatments performed for both the ICS and clinical 6MV photon beams.
Results: The ICS profiles have an extremely sharp fall-off (<0.1 mm wide) with a broad low-dose tail that starts at 5-30% of the dose on the central axis depending on depth and energy. Without adding gold-nanoparticles (GNP), the brain case gave comparable plan quality for the ICS calculations and 6MV. Adding 10mg/g of GNP to the target volume reduced the V12Gy of the normal brain volume by a factor 40 for 80keV versus 6MV. The Paddick conformity index also significantly improved for ICS calculations with GNP’s compared to 6MV. Better plan quality was achieved in the spine cases where the dose-to-bone enhancement of the ICS beams enables better conformity and better spinal cord sparing compared to 6MV.
Conclusions: The sharp penumbra and preferential absorption of the ICS by high-Z materials indicates that inverse Compton x-ray sources could be useful for stereotactic radiosurgery that require steep dose gradients.
Keywords
Monte Carlo, Stereotactic Radiosurgery
Taxonomy
TH- External Beam- Photons: Computational dosimetry engines- Monte Carlo
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