Abstract Book

S1031

ESTRO 37

significantly reducing dose to the ipsilateral lung and heart. However, this came with the expense of an increase in the dose to the contralateral breast. Split VMAT technique is also found convenient when DIBH is used, as the splitting of arcs also aids in keeping the beam on time feasible. EP-1902 Analysing the effects of Bragg curve degradation due to lung parenchyma in treatment planning K. Baumann 1 , V. Flatten 1 , U. Weber 2 , R. Engenhart- Cabillic 1 , K. Zink 3 1 University Medical Center Giessen-Marburg, Department of Radiotherapy and Radiooncology, Marburg, Germany 2 GSI Helmholtzzentrum für Schwerionenforschung, Biophysics Division, Darmstadt, Germany 3 University of Applied Sciences, Institute of Medical Physics and Radiation Protection, Giessen, Germany lung parenchyma cause Bragg peak degradation. If not considered in treatment planning for particle therapy, this can significantly influence the dose distribution in lung cancer patients potentially resulting in an underdose of the PTV and an overdose of distal normal tissue. However, this consideration is not possible since the fine structure of human lung parenchyma is not resolved in typically used treatment- planning CTs. 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 the treatment plans of real patients. Material and Methods Stereotactical treatment plans of lung cancer patients were optimised for protons using the treatment-planning system ECLIPSE (VARIAN). The dose of the treatment- planning volume was set to 30 Gy. 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 – this gives the actual dose distribution in the patient [1]. Results In Figure 1 an exemplary patient (tumor size: 67,3 cm 3 ) and the corresponding dose distribution optimised with ECLIPSE is shown. The PTV is marked in red, the trachea in yellow and the right lung in green. Purpose or Objective Sub-millimetre-sized heterogeneities like

It can be seen clearly that when calculating with the modulated density – which correlates to the act ual dose distribution in the patient – the PTV gets less dose compared to the case with the original density from the treatment-planning CT which correlates to the dose distribution predicted by the treatment-planning system. The dose deposited in the trachea is underestimated by the treatment-planning System. Conclusion We are able to analyse the effects of Bragg curve degradation due to human lung parenchyma in the treatment planning of lung cancer patients. Doing so we could show that the Bragg peak degradation must be considered in the treatment planning of lung cancer patients to avoid an underdose of the PTV and an overdose of distal normal tissue. [1] Baumann K-S, Witt M, Weber U, Engenhart-Cabillic R and Zink K 2017 An efficient method to predict and include Bragg curve degradation due to lung-equivalent materials in Monte Carlo codes by applying a density modulation Phys. Med. Biol. 62 3997-4016 [2] Perl J, Shin J, Schuemann J, Faddegon B and Paganetti H 2012 TOPAS: an innovative proton Monte Carlo platform for research and clinical applications Med. Phys. 39 6818–37 EP-1903 Is it always possible to boost breast tumour bed with conformal photon beams? M. Colomer 1 , D. Amat 1 , A. López 1 , G. Frontera 1 , D. Navarro 1 , R. Gómez 1 , E. Ambroa 1 , J. García-Miguel 1 , M. Parceisa 1 , R. Pujol 1 , T. Ramírez 1 1 Consorci Sanitari de Terrassa, Medical Physics Unit - Radiation Oncology Department, Terrassa, Spain Purpose or Objective In a very busy department with three linacs, one in a satellite location, commissioning and measuring all available electron beams is very time consuming. More than 70% of electron beams are used for breast boost radiotherapy and we would like to compare, in terms of PTV coverage and low doses to heart and ipsilateral lung, if we can give priority to photon plans in order to not measure all electron beams and gradually reduce them. Material and Methods We retrospectively selected 293 patients who underwent radiotherapy to the whole breast at a dose of 50 Gy in 25 sessions plus a tumour bed boost of 16 Gy in 8 sessions with either photons or electrons from 2013 to 2017. Patients treated with other fractionations were not considered. Only dosimetric boost data was taken into account. 192 patients were treated with electrons (110 left-sided breast and 82 right) and 101 with photons (56 left and 45 right).

In Figure 2 the dose-volume histograms are shown for the PTV (red), the trachea (blue) and the right lung (green) for both scenarios (modulated density and original density from the treatment-planning CT).