ESTRO 37 Abstract book

S1032

ESTRO 37

Material and Methods Eleven left sided breast cancer patients with lymph node involvement were retrospectively analysed with split and non-split arc designs using RapidArc optimiser (Varian Medical Systems, Palo Alto, CA). The patient selection criteria were high lung and heart dose in conventional planning. The traditional non-split arc techniques consisted of two 190° arcs. In the split arc technique, the 190° arcs were split into half at the gantry angle 65° and refocusing the fields to the PTV shielding the lungs and heart by the medial jaw (see Figure 1). Same optimisation criteria were used in both plans.

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 Purpose or Objective Sub-millimetre-sized heterogeneities like lung paren- chyma 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.

Results Results are shown in Table 1. No significant difference could be seen in PTV coverage and contralateral lung mean dose between the different VMAT designs. Ipsilateral lung dose and heart V5Gy were significantly lower than those in non-split VMAT design. The contralateral breast dose V5Gy was significantly larger for split VMAT plans than in non-split VMAT designs.

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).

Conclusion The proposed split VMAT technique was shown to be superior to previously published non-split arc technique significantly reducing dose to the ipsilateral lung and