ESTRO 36 Abstract Book

S192 ESTRO 36 _______________________________________________________________________________________________

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 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.

Fig 1: Dose mean lineal energy ratios between Co-60 and 100 kVp Fig 2: Dose mean lineal energy ratios as a function of scoring diameter X-rays as a function of scoring diameter. The dotted line corresponds for various brachytherapy sources. to α-ratio of 1.20. Conclusion Currently, the International Commission on Radiation Protection (ICRP) assigns a radiation weighting factor of unity for all photon emitting sources, equating the RBE of high and low energy photon sources. However, the clinical RBE for lower energy brachytherapy sources are considerably above unity and should be taken into account during the treatment planning process, to ensure that the equivalent dose delivered to the tumour is similar for different sources. OC-0360 Dose warping uncertainties for the cumulative rectal wall dose from brachytherapy in cervical cancer L.E. Van Heerden 1 , N. Van Wieringen 1 , C. Koedooder 1 , C.R.N. Rasch 1 , B.R. Pieters 1 , A. Bel 1 1 Academic Medical Center, Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective Brachytherapy (BT) is part of radiotherapy for women with locally advanced cervical cancer; nowadays, BT is commonly given in multiple applications to the tumour area. In clinical practice, the 2 cm 3 receiving the highest dose (D 2cm3 ) in the rectum is calculated by assuming that the high dose volumes overlap for each treatment. To account for rectal deformation due to differences in filling and/or the presence of air, many authors state it is preferable to sum the 3D dose distributions using dose warping after deformable image registration (DIR). However, little is known about the reliability of DIR for dose warping. The purpose of this study is to quantify the dose warping uncertainty in the rectum using a physically realistic model, which describes rectal deformation.