ESTRO 36 Abstract Book

S178 ESTRO 36 2017 _______________________________________________________________________________________________

biological systems and endpoints studied, but also to the actual linear energy transfer (LET) in the biological systems. To provide accurate estimates of the relative biological effects of protons, high precision cell experiments are needed together with detailed knowledge of the LET at a given measurement depth. The objective of this study was to estimate the LET distribution along the depth dose profiles from a low energy proton beam, using Monte Carlo (MC) simulations adjusted to match Dose measurements were performed at the experimental proton beam line at the Oslo Cyclotron Laboratory (OCL) employing 17 MeV protons. A Markus ionization chamber and GafChromic films were used to measure the dose distribution at 28, 88 and 110 cm from the beam exit window. At each position, measurements were performed along the depth dose profile (using increasing thickness of paraffin- and Nylon6 sheets). A transmission chamber was used for monitoring beam intensity. The geometry of the experimental setup was reproduced in the FLUKA MC code. The dose profiles were calculated using FLUKA, and MC parameters relating to beam energy and beam line components were optimized based on comparisons with measured doses. LET-spectra and dose-averaged LET (LET d ) were also scored using FLUKA. Results The measured pristine Bragg peak from the OCL cyclotron covered about 200 µm (Figure 1a). The MC simulations of the beam line were validated by comparing simulated dose profiles with measured data (Figure 1a). The simulated LET d increased with depth, also beyond the Bragg peak (Figure 1a and Table 1). Also, LET d at target entrance increased with distance from the beam exit window due to the presence of air (Table 1). The LET spectrum was narrow at the target entrance, and considerably broadened at BP depth (Figure 1b). measured dose profiles. Material and Methods

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

Conclusion