ESTRO 36 Abstract Book

S190 ESTRO 36 2017 _______________________________________________________________________________________________

The ACE doses for the single seed in water agree with MC simulations on average within 4.4 ± 2.1% in a 60x60x60 mm 3 cube centered on the seed, with the largest differences near the end-welds of the seed. Percent differences between ACE and MC doses along the plaque central axes (CAX) for all eye plaque plans are shown in Figure 1. The agreement improves beyond ~3 mm from the outer scleral surface, and is generally better for the fully loaded plaques than the single seed plaques, due to more overlapping dose from each seed washing out ray effects caused by the ACE calculation. Compared to using the previous minimum calculation grid size of 1 mm 3 , the smaller 0.5 mm 3 grid size results in less voxel averaging, and therefore more accurate doses immediately adjacent to the plaques, though both agree well with MC in the eye region (Figure 2).

differences in the MC simulation codes used to generate the data, and scaling of the ACE dose distribution in water to match TG-43 data in OcB. Updated seed models will be used to investigate this discrepancy. The good level of agreement indicates that further investigation of ACE in applications involving a virtual, voxelized eye phantom, and patient CT datasets, is warranted. OC-0359 Microdosimetric evaluation of intermediate- energy brachytherapy sources using Geant4-DNA G. Famulari 1 , P. Pater 1 , S.A. Enger 1,2 1 McGill University, Medical Physics Unit, Montreal, Canada 2 McGill University Health Centre, Department of Radiation Oncology, Montreal, Canada Purpose or Objective Recent interest in alternative radionuclides for use in high dose rate brachytherapy (Se-75, Yb-169, Gd-153) with average energies lower than Ir-192 has triggered the investigation of the microdosimetric properties of these radionuclides. A combination of Monte Carlo Track Structure (MCTS) simulations and track sampling algorithms was used to predict the clinical relative biological effectiveness (RBE) for fractionated radiotherapy at relevant doses and dose rates. Previous studies have concluded that the dose mean lineal energy in nanometre-sized volumes is approximately proportional to the α-ratio derived from the linear-quadratic (LQ) relation in fractionated radiotherapy in both low-LET and high-LET radiation. Material and Methods Photon sources were modelled as point sources located in the centre of a spherical water phantom with a radius of 40 cm using the Geant4 toolkit. The kinetic energy of all primary, scattered and fluorescence photons interacting in a scoring volume were tallied at various depths from the point source. Electron tracks were generated by sampling the photon interaction spectrum, and tracking all the interactions following the initial Compton or photoelectric interaction using the event-by-event capabilities of Geant4-DNA. The lineal energy spectra were obtained through random sampling of interaction points and overlaying scoring volumes within the associated volume of the tracks. Results For low-LET radiation, the dose mean lineal energy ratio was approximately equal to the α-ratio in the LQ relation for a volume of about 30 nm (Fig 1). The weighting factors (often denoted clinical RBE) predicted were 1.05, 1.10, 1.14, 1.19 and 1.18 for Ir-192, Se-75, Yb-169, Gd-153, and I-125, respectively (Fig 2). The radionuclides Se-75, Yb- 169, and Gd-153 are 5-14 % more biologically effective than current Ir-192 sources. There is little variation in the radiation quality with depth from the source.

Conclusion Overall, good agreement is found between ACE and MC dose calculations in front of the eye plaques in water. The consistent difference of ~3-4% observed for all comparisons with MC simulations is potentially due to