Abstract Book

S111

ESTRO 37

Material and Methods 11 advanced NSCLC patients are included in the study. The three strategies for layer selection are implemented in the treatment planning system Hyperion; a) constant energy steps, b) exponentially increasing energy steps, and c) an adaptive approach, where the spot weights are exposed to a group sparsity penalty in combination with layer exclusion during optimization. Within each strategy, four levels of layer reduction are used; a) increasing constant energy increment, b) increasing energy multiplication factor, and c) an increasing removed percentage of layers. The plans are created with three treatment fields on the mid ventilation 4DCT phase to simulate gating. Optimum target and OAR constraints are found by a patient specific reference treatment plan, where the distance between the layers is 2 MeV. Identical target constraints are used for all patients and strategies. The plan quality is assessed by the homogeneity index (HI), the maximum target dose and the outcome of a robustness evaluation, where the patient is shifted 2 mm or 4 mm in the 6 major directions. Results Robustness and target coverage as a function of reduction level are similar for all strategies. Figure 1 shows the D98%, D02% and the HI for the CTV. All MFO plans are clinically acceptable, while the SFO plans starts to degrade after what corresponds to 50% layer reduction as compared to the reference plan. The robustness, however, is independent of the number of energy layers for both SFO and MFO plans. Figure 2 shows the robustness evaluation results as a D98% dose difference between the planned dose and the mean shifted dose at either 2 mm or 4 mm. The SFO plans are significantly more robust than the MFO plans with all p-values below 0.001 (Wilcoxon signed-rank). The overall mean D98% dose difference is at 2 mm: 0.7 Gy (SFO) and 1.9 Gy (MFO), and at 4 mm: 3.2 Gy (SFO) and 5.4 Gy(MFO).

Conclusion The number of layers in MFO plans can be reduced substantially more than in SFO plans without compromising the plan quality. The robustness is independent of the number of layers, and the SFO plans are significantly more robust than the MFO plans. Combining the two conclusions, however, shows that if the level of robustness is acceptable or enforced via robust optimization, MFO plans could be candidates for treatment time reductions via energy layer reductions as neither the plan quality nor robustness is compromised. PV-0204 Comparison of IMPT and VMAT for diffusion tensor image guided treatment planning of gliomas J.B. Petersen 1 , S. Lukacova 2 , M. Jensen 1 , T. Guldberg 2 , A. Harbøll 1 , J. Kallehauge 1 1 Aarhus University Hospital, Medical Physics- Department of Oncology, Aarhus C, Denmark 2 Aarhus University Hospital, Department of Oncology, Aarhus C, Denmark Purpose or Objective Recently, the use of clinical target volume (CTV) driven by the differential migrations patterns of gliomas in grey (GM) and white matter WM has shown promising results in predicting the location of marginal recurrences. The resulting CTVs however have also been shown to be more irregular in shape and will thus require a more highly modulated radiation treatment technique. The aim of this pilot study was to compare the dose distributions from Intensity Modulated Proton Therapy (IMPT) and Volume Modulated Arc Therapy (VMAT) for both anisotropic and isotropic migration driven CTVs. Both techniques are supposed to deliver highly conformal dose distributions to complex targets. Material and Methods Two high grade glioma patients were scanned using Diffusion Tensor Imaging (DTI) on a 1.5T Philips Ingenia MR scanner. Their brains were segmented into GM, WM and Cerebral Spinal Fluid (CSF). The extent of the microscopic spread (CTV) was estimated using the gross tumor volume (GTV) as starting point and the subsequent migration was different in the three segmented regions. In the isotropic case the migration of tumor cells was 10 times greater in WM compared to GM. No migration was allowed in CSF and across other known anatomical barriers: brainstem wall, falx, and tentorium. The anisotropic case extends this solely by using the DTI information about the WM fiber tract directions and adds this as the preferential direction of the migration. A 3mm Planning Target Volume (PTV) was added isotopically to