ESTRO 38 Abstract book

S497 ESTRO 38

was to minimize the dose in the organs at risk with specific focus on the hippocampi, while ensuring adequate target coverage. Material and Methods Twenty skull-base meningioma patients with a target volume diameter of > 3 cm were included in this study. A dose of 28x1.8 Gy RBE was prescribed, in ten cases to the 100% isodose (conventional), in ten other cases to the 80% isodose (stereotactic). A system for fully automated plan generation was used to calculate a) a coplanar, dual arc VMAT plan, b) an IMRT plan with nine non-coplanar beams with optimized gantry and couch angle, c) an IMPT plan with three patient-specific selected non-coplanar beams. For all plans, the same set of constraints and prioritized objectives was used. The photon plans were generated with 2mm CTV-PTV margin; for IMPT a mini-max robust optimization was used with ±2mm setup and ±3.5% range robustness. For fair comparison, all plans were rescaled to the same target coverage, i.e., 98% of the PTV (IMRT/VMAT) and 98% of the CTV for the worst case robustness scenario (IMPT) should receive 47.88 Gy (conventional), or 50.4 Gy (stereotactic). For statistical comparison, a two-sided p-value ≤ 0.05 was considered statistically significant. Results Compared to IMRT and VMAT, IMPT allowed for a much better dose conformity to the target volume (Figure 1). Consequently, large dose reductions in organs at risk were observed (Figure 2). In particular, with respect to IMRT, the mean dose and D40% in the bilateral hippocampus were on average reduced by 41% and 71% (conventional), and by 64% and 79% (stereotactic), respectively (ANOVA p≤0.02). Emphasis on conformity during optimization led to a high benefit for the normal brain dose as well. With IMPT, the mean dose in the normal brain and the volumes receiving 10-30 Gy were 28-55% reduced (ANOVA p≤0.03) compared to IMRT. Also the mean doses in the optic nerves, retina, cochlea, brainstem and cerebellum were significantly lower. When comparing IMPT and VMAT, even larger dose differences were observed, mainly due to the coplanar beam setup in VMAT.

while a statistically significant improvement was observed in gradient index (p<0.01) and Paddick conformity index (p=0.01). Data are presented in Table 1.

The improvement in gradient is due to the MCO effectively reducing the constraint on the ring around the PTV. An example of dose distribution is shown in Figure 1.

A general improvement in dose to OARs was observed (Table 1), that resulted to be statistically significant for chest wall V 30Gy and total lungs V 20Gy (p<0.01) and for cord D 0.1cc (p=0.02). Even a small reduction in doses to OARs, although already below constraints, can be clinically significant in the light of patient retreatment. Concerning dosimetric verifications, no statistically significant differences were observed in γ passing rates (3%,3mm, TH10%, local), that were on average (95.4±0.1)% and (96.6±1.4)% for manual and MCO plans, respectively. No manual plan changes were required: all the 20 plans automatically generated were considered clinically acceptable. Average planning time was 10 minutes. Conclusion This study describes a novel approach for automating the planning process and demonstrates the feasibility of developing a class solution for lung VMAT SBRT that produces clinically acceptable plans with a high conformity in a time-efficient manner. PO-0927 Lower dose to hippocampi and other OARs with IMPT than with VMAT and IMRT for skull-base meningiomas M. Florijn 1 , A.W.M. Sharfo 2 , R.G.J. Wiggenraad 3 , J.P.C. Van Santvoort 1 , A.L. Petoukhova 1 , M.S. Hoogeman 4 , M.E. Mast 3 , M.L.P. Dirkx 2 1 Haaglanden Medical Centre, Medical Physics, Leidschendam, The Netherlands ; 2 Erasmus MC Cancer Institute, Radiation Oncology, Rotterdam, The Netherlands ; 3 Haaglanden Medical Centre, Radiation Oncology, Leidschendam, The Netherlands ; 4 Holland Proton Therapy Centre, Medical Physics & Informatics, Delft, The Netherlands Purpose or Objective Meningiomas are considered benign in 90% of the cases. When surgery is not feasible radiotherapy (RT) is used, however potential late RT effects include neurocognitive impairment. Proton therapy might be beneficial, especially in case of larger target volumes, because of the steep dose fall-off beyond the target volume. In this study we applied fully automated treatment planning to systematically evaluate the dosimetric differences between intensity modulated proton therapy (IMPT), volumetric modulated arc therapy (VMAT) and intensity modulated radiotherapy (IMRT). The purpose of this study