ESTRO 36 Abstract Book

S413 ESTRO 36 2017 _______________________________________________________________________________________________

3 Heidelberg Ion Beam Therapy Center HIT, Clinical Research Group Radiotherapy with Heavy Ions, Heidelberg, Germany Purpose or Objective Due to the lack of a reliable model, current analytical treatment planning for proton and heavier ions cannot account for the degradation of the sharp distal fall-off of the Bragg peak caused by microscopic density heterogeneities, which cannot be resolved by clinical CT. Here, we present a systematic study of Bragg peak degradation in stationary lung parenchyma to provide a comprehensive analytical parametrization for implementation in treatment planning systems (TPS) – aiming at the reduction of dose uncertainties in radiotherapy of the lung. Material and Methods We developed a compact model describing the lung parenchyma microscopic geometry based on few geometrical and physical variables allowing for flexible Monte Carlo (MC) simulations of lung specific features (alveolar dimension, lung density) and breathing state parameters (air filling state, water equivalent thickness traversed, WET). To benchmark the accuracy of the simulated model, we performed a MC study to assess the specific contributions of the cumulative physical sources of degradation and a series of transmission experiments on lung-like phantoms with clinical proton and carbon beams at the Heidelberg ion-therapy center (HIT). We adopted the benchmarked model to provide a parametrization of the Bragg peak degradation on the beam and on the previously mentioned lung parameters. Throughout this work, we tested and used a Gaussian convolution of the undegraded Bragg peak (U. Titt et al, 2015) to parametrize the degradation. Furthermore, the model was used to investigate the effects on clinical spread out Bragg peak (SOBP) and on the relative biological effectiveness (RBE). Results Fluctuations in the WET were found the major degradation factor, contributing more than 75% (40%) to the cumulative distal falloff widening for a carbon (proton) Bragg peak. The simulated lung parenchyma model (Figure 1) was capable to reproduce the experimental data with a slight underestimation of the degradation parameters, yet guaranteeing the correct reproduction of all the relevant characteristics in the degraded dose distribution. The Gaussian filtration unified the description for different beam particles and provided a compact and complete characterization with specific dependencies with respect to each lung parameter. Moreover, the description was found independent from the initial beam energy resulting in deviations mainly about the SOBP distal falloff while the plateau remains unaffected. Finally, the impact on the biological dose was mainly driven by changes to the physical dose due to the limited deviations in the RBE.

All measurements were performed in an PTW MP3 watertank except for TLDs and Gafchromic EBT3 film which were performed in solid water. Results Figure 1 displays the results of the energy dependence investigation for each detector in the study. The response of each detector was normalised to 1 at 6MV.

Figure 2 displays a comparison between MC calculated versus detector measured out-of-field dose.

Conclusion In general the results of the energy dependence investigation predicted the response of the detectors to out-of-field radiation except for the case of the Pinpoint, TLD and microDiamond detectors. Energy dependence was thought to be the leading source of variation in detector response to out-of-field radiation due to the relative increase in low-energy photons. However, dose-rate and angular dependencies can exist in detector responses but were not investigated as part of this study. Other factors such a charge multiplication and cable effects can contribute to a change in response as observed with the Pinpoint detector. This study highlights the need for careful selection of appropriate detectors when accurate out-of-field dosimetry is required and offers a guide and improved understanding of detector response to out-of- field radiation. The waterproof Farmer chamber showed best agreement with MC calculated out-of-field dose and is recommended for out-of-field dose measurements. PO-0787 A compact and complete model for Bra gg peak degradation in lung tissue R. Dal Bello 1 , C. Möhler 1,2 , S. Greilich 1,2 , O. Jäkel 1,2,3 1 German Cancer Research Center DKFZ, Division of Medical Physics in Radiation Oncology, Heidelberg, Germany 2 National Center for Radiation Research in Oncology NCRO, Heidelberg Institute for Radiation Oncology HIRO, Heidelberg, Germany

Conclusion We provide a comprehensive characterization of Bragg peak degradation that can readily be implemented in a TPS. Such implementation is crucial for a more complete description of lung treatments, adding to the effect of macroscopic structures (e.g. bronchi, CT resolvable) the