ESTRO 36 Abstract Book

S122 ESTRO 36 2017 _______________________________________________________________________________________________

maximum). All irradiation modes deposit a mean dose of 1.7 Gy. For RBE determination dose response curves of reference radiation were used. Results The RBE values, as determined by measuring dicentrics in human-hamster hybrid (AL) cells, are significantly higher when 117 protons were focused to a 0.78 µm spot within a 5.4 × 5.4 µm 2 matrix compared to homogenous applied protons (RBE = 1.96 ± 0.16 vs. RBE = 1.30 ± 0.16). By doubling the spot size to 1.6 µm the RBE decreased to 1.52 ± 0.16. By further increasing the spot size to 2.7 µm the RBE was not longer different (RBE = 1.36 ± 0.14) to the homogenous radiation. Conclusion Our experiments demonstrate evidence that low LET radiation focused to sub-micrometer diameters results in an increase in RBE for the induction of dicentrics depending on the spot size. The local density of DSB is increased at the irradiated spots enhancing also the probability for the interaction of the DSB and thus raising the probability of connecting the wrong ends. We hypothesize that a tighter beam spot of protons might further enhance the RBE value. Supported by the DFG-Cluster of Excellence ‘Munich- Centre for Advanced Photonics’, by the BMBF-project 02NUK031A and 02NUK031B “LET-Verbund”. OC-0244 Does the RBE depend on ion type? A. Lühr 1,2,3 , C. Von Neubeck 2,3 , M. Baumann 1,2,3,4 , M. Krause 1,2,3,4 , W. Enghardt 1,2,3,4 1 Helmholtz-Zentrum Dresden - Rossendorf, Institute of Radiooncology, Dresden, Germany 2 OncoRay–National Center for Radiation Research in Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus - Technische Universität Dresden - Helmholtz-Zentrum Dresden - Rossendorf, Dresden, Germany 3 German Cancer Consortium DKTK, Partner Site Dresde, Dresden, Germany 4 University Hospital Carl Gustav Carus at the Technische Universität Dresden, Department of Radiation Oncology, Dresden, Germany Purpose or Objective Currently, modeling of RBE as a simple function of linear energy transfer (LET) receives much attention in the proton therapy community. However, such LET-RBE parametrizations are purely empirical and ion type specific. Additionally, their applicability is restricted by large uncertainties associated with the biological input parameters from proton experiments. In contrast, long term clinical experience on RBE modeling as well as treatment outcome data exist for carbon ion therapy. The aim is to establish a clinically relevant RBE modeling for proton therapy that is directly based on available clinical and pre-clinical experience from carbon ion therapy. Material and Methods The RBE dependence on the radiation field – i.e., on physics – was mathematically derived in a convenient way using assumptions also applied by the local effect model (LEM) and the micro kinetic model (MKM); both used in patient treatment. A large set of in vitro literature data (several hundred data points, including six different ion types) on RBE and the linear-quadratic model parameter α p for particles was used to validate the derived model. Pre-clinical RBE data of the rat spinal cord (one and two fractions) at six different depth positions in a carbon ion treatment field were used to demonstrate the transfer of carbon ion RBE data to proton therapy. Physical properties of the applied carbon treatment field as a function of depth were obtained by Monte Carlo simulation considering the full particle spectrum: dose, LET, beam quality Q = Z 2 /E (Z = ion charge; E = kinetic energy). Results

The derivation revealed for α p and RBE a linear increase with beam quality Q but no dependence on ion type. These findings were well confirmed by the experimental in vitro data for different ions (Fig. 1). Specifically, the independence of ion type holds true for different cell types and irradiation under normoxic and hypoxic conditions. The pre-clinical spinal cord RBE data increased linearly with Q (Fig. 2). The linear slope depends in the same way on fractionation dose as described by the derived model. Due to the apparent independence of RBE on ion type, the experimentally obtained RBE for carbon ions as function of Q could also be used to estimate the RBE in a proton SOBP where Q can be determined at any depth (Fig. 2).

Fig.1 Beam quality dependence: In vitro RBE and α p as function of beam quality Q (Z 2 /E) for HSG and V79 cell lines. H: proton, He: helium, C: carbon, Ne: neon.

Fig.2 Concept of RBE translation : (A) Obtain (pre-) clinical RBE from carbon ion therapy as (B) function of beam quality and (C) use it to optimize dose prescription in proton therapy. Spinal cord RBE; 1 and 2 fractions (Fx). Conclusion The RBE seems to depend on the beam quality Q but not on ion type for clinically relevant treatment situations. This opens up the possibility to directly transfer clinically and pre-clinically obtained parameters from carbon ion to proton therapy. Currently, RBE experiments and Monte Carlo simulations of patient treatments are performed as a next step to translate this approach to proton therapy. OC-0245 Clinical evidence that end-of-range proton RBE exceeds 1.1: lung density changes following chest RT T. Underwood 1,2 , C. Grassberger 1 , R. Bass 1 , R. Jimenez 1 , N. Meyersohn 3 , B. Yeap 1 , S. MacDonald 1 , H. Paganetti 1 1 Massachusetts General Hospital & Harvard Medical School, Department of Radiation Oncology, Boston MA, USA 2 University College London, Department of Medical Physics and Bioengineering, London, United Kingdom 3 Massachusetts General Hospital & Harvard Medical School, Department of Radiology, Boston MA, USA Purpose or Objective Clinical practice assumes a fixed proton relative biological effectiveness (RBE) of 1.1, but it has been postulated that higher RBEs occur at the distal edge of proton spread out Bragg peaks, i.e. within the lung for chest wall patients. We performed retrospective qualitative & quantitative analyses of late-phase lung-density changes (indicative of asymptomatic fibrosis) for chest wall patients treated using protons & X-rays. Our null hypothesis (H0) was that, assuming a fixed proton RBE of 1.1, these changes would