ESTRO 38 Abstract book

S506 ESTRO 38

The recalculated dose distributions agreed well with the dose distributions calculated in the clinically used TPS. The variable RBE-models predicted an increase of 7-12 Gy (RBE) in the near-maximum D RBE (D RBE, 2% ) for the structures with the observed toxicities resulting in not fulfilling the clinical goals. The NTCP for the structures with the observed toxicities increased from 0.8, 0.0 and 0.1% (RBE=1.1) to 15.5, 3.6 and 1.8% (Wedenberg RBE-model) for the three patients, respectively. All alternative plans produced physical dose distributions which were similar or better than the clinical plans, while also generally allowing for substantial D RBE , LET d , and NTCP reductions in several critical structures assuming the variable RBE- models. In all cases, alternative plan 4 resulted in the lowest values, with a LET d reduction of 50%, or more, compared to the clinical plan.

PO-0940 The effects of Bragg curve degradation in proton therapy of bronchial carcinomas V. FLatten 1,2 , K. Baumann 1,2 , U. Weber 3 , S. Lautenschläger 1 , F. Eberle 1 , R. Engenhart-Cabillic 1 , K. Zink 2 1 University Medical Center Giessen-Marburg, Department of Radiotherapy and Radiooncology, Marburg, Germany ; 2 University of Applied Sciences Giessen THM, Institute of Medical Physics and Radiation Protection, Giessen, Germany ; 3 GSI Helmholtzzentrum für Schwerionenforschung, Biophysics Division, Darmstadt, Germany lung parenchyma cause a degradation of the Bragg curve. State-of-the-art treatment planning systems are unable to consider the degradation in the treatment planning process ignoring the influence on the dose distribution. Potentially, this can result in an underdosage of the PTV and an overdosage of distal normal tissue. In a previous study we presented a strategy to consider this Bragg peak degradation by applying a density modulation to the voxels associated with the lung [1]. In this study we use this tool to analyse the effects of this Bragg curve degradation on CT based phantoms, enabling a thorough investigation of the dependencies of parameters effecting the degradation and present data for five treatment plans on patient CTs. Material and Methods Stereotactical proton treatment plans were optimised using the treatment planning system Eclipse (VARIAN) on phantom and patient CTs. Each plan was then recalculated using the Monte Carlo toolkit TOPAS [2]. In a first scenario, the treatment plans were recalculated using the original density values from the treatment planning CT which correlates to the dose distribution predicted by the treatment planning system. In a second scenario, the density values of each voxel within the lung were modulated using a mathematical model [1], thus giving the actual dose distribution in the patient. The phantoms that were used covered a range of different distances of the treatment volume in lung as well as various tumour volumes. Results Purpose or Objective Sub-millimetre-sized heterogeneities like

Conclusion Although a study of this size and design could not establish direct causality between RBE and toxicity, the analysis indicates that the enhanced RBE, due to high LET d , could be a potential cause of the observed toxicities. Combining IMPT with alternative beam arrangements and/or objectives beyond physical dose allow for D RBE , LET d , and NTCP reductions in several critical structures for intracranial lesion, without compromising the target dose. Such planning strategies might hence be a future tool in order to maximize the benefit of proton therapy.

In Figure 1 an exemplary depth-dose curve through one of the phantom dose distribution shows the effect of the Bragg peak degradation in comparison to the curve predicted by the treatment planning system.