ESTRO 36 Abstract Book

S179 ESTRO 36 _______________________________________________________________________________________________

OC-0341 Monte Carlo dose calculations using different dual energy CT scanners for proton range verification I.P. Almeida 1 1 Maastricht Radiation Oncology MAASTRO clinic, Physics Research, Maastricht, The Netherlands Purpose or Objective To simulate the dose profile for proton range verification by means of Monte Carlo calculations and to quantify the difference in dose using extracted values of relative electron densities ( ρ e ) and effective atomic numbers ( Z eff ) for three commercial dual-energy computed tomography (DECT) scanners from the same vendor: a novel single- source split-filter (i.e. twin-beam), a novel single-source dual-spiral and a dual source device. This study aims also to provide a comparison between the use of different DECT modalities and the conventional single-energy CT (SECT) technique in terms of dose distributions and proton range. Material and Methods Measurements were made with three third generation DECT scanners: a novel dual spiral at 80/140 kVp, a novel twin-beam at 120 kVp with gold and tin filters, and a dual- source scanner at 90/150kVp with tin filtration in the high energy tube. Images were acquired with equivalent CTDI vol of approximately 20 mGy and reconstructed with equivalent iterative reconstruction algorithms. Two phantoms with tissue mimicking inserts were used for calibration and validation. Monte Carlo proton dose calculations were performed with GEANT4, in which the materials and densities were assigned using the DECT extracted values of ρ e and Z eff for both phantoms. Simulations were done with monoenergetic proton beams impinging under directions to the cylindrical phantoms, covering different tissue-equivalent inserts. Dose calculations were also performed on images from a third generation SECT scanner at 120 kVp. Simulations based on DECT and SECT images were compared to a reference phantom. Results Range shifts on the 80% distal dose fall-off (R80) were quantified and compared for the different beam directions and media involved to a reference phantom. Maximum R80 range shifts from the reference values for the calibration phantoms based on DECT images were 3.5 mm for the twin-beam, 2.1 mm for the dual-spiral and for the dual- source. For the same phantom, simulations based on SECT images had a maximum range shift of 4.9 mm. 2D stopping power maps were computed and compared for the different techniques.

Figure 1. Illustration of the beam 1 direction in the calibration phantom with different tissue-equivalent inserts.

Figure 2: Dose to water profile for one beam direction in the Gammex RMI 467 phantom. The dose is laterally integrated and the R80 is measured. Conclusion A comparison study between the use of SECT and DECT images for proton dose distribution is performed to understand the differences and potential benefit of DECT for proton therapy treatment planning, using different CT scanners. The final aim is to decrease uncertainty in dose delivery, possibly allowing narrower treatment margin than currently used. In most scenarios, the different modalities of DECT produced results closer to the reference, when compared with the SECT based simulations. Small differences were found for the different DECT scanners. OC-0342 Monte Carlo simulations of a low energy proton beam and estimation of LET distributions T.J. Dahle 1 , A.M. Rykkelid 2 , C.H. Stokkevåg 3 , A. Görgen 2 , N.J. Edin 2 , E. Malinen 2,4 , K.S. Ytre-Hauge 1 1 University of Bergen, Department of Physics and Technology, Bergen, Norway 2 University of Oslo, Department of Physics, Oslo, Norway 3 Haukeland University Hospital, Department of Oncology and Medical Physics, Bergen, Norway 4 Oslo University Hospital, Department of Medical Physics, Oslo, Norway Purpose or Objective The physical advantage of protons in radiotherapy is mainly due to the ‘Bragg peak’ of the proton depth dose distribution. However, there is still a controversy on the biological effects of protons, in particular around the Bragg peak. This relates both to the variability of