ESTRO 38 Abstract book

S502 ESTRO 38

J. Hofmaier 1 , R. Bodensohn 1 , S. Garny 1 , M. Reiner 1 , M. Eder 1 , Y. Dinc 1 , S. Corradini 1 , C. Belka 1,2 , M. Niyazi 1 1 University Hospital LMU Munich, Department of Radiation Oncology, Munich, Germany ; 2 German Cancer Consortium DKTK, partner site Munich, Munich, Germany Purpose or Objective For a long time whole brain radiotherapy (WBRT) has been the only treatment option for multiple cerebral metastases whereas a few lesions (usually up to four) were treated with SRS. In the recent decade SRS has become an option also for patients with more lesions. The Elements TM Multiple Brain Mets SRS (MBMSRS) tool (Brainlab, Munich, Germany) enables the simultaneous treatment of up to 15 metastases on LINACs with a single isocenter using non- coplanar dynamic conformal arcs (DCAT). This study compares the dosimetric properties of this technique with non-coplanar volumetric modulated arc therapy (VMAT) (Elekta Monaco®). Material and Methods Datasets of 20 patients with a total of 66 lesions who were treated with MBMSRS version 1.5 at an Elekta Versa HD TM were included in this comparison. A single fraction dose of 15-20 Gy was prescribed to the 80% isolevel encompassing 98% of the PTV (GTV plus a margin of 1mm). All patients were re-planned using VMAT in Elekta Monaco® 5.11. For all plans, the total number of monitor units and the volume of the brain receiving more than 4, 5 and 8 Gy where compared using the Wilcoxon signed rank test (WSRT). Using a density based clustering algorithm, dose clouds of 10 Gy, 12 Gy and half of the prescription dose were identified and assigned to the corresponding lesion. The V10Gy and V12Gy around each lesion were compared using WSRT. Targets with overlapping 10 Gy clusters were joined and treated as one lesion (2 lesions joined: 5 times, 3 lesions joined: once). Paddick conformity index and gradient index (GIPTV) were calculated and compared only for lesions which could be separated in the clusters corresponding to their half prescription dose. To identify a possible predictor for VMAT being superior, the correlation of the differences between V10Gy and V12Gy for both techniques with the sphericity (a measure of similarity to a perfect sphere) of the corresponding PTV was assessed using Spearman’s rank correlation coefficient. Results The MBMSRS plans showed superior results in all the investigated metrics (see Table 1). All differences were significant (p<0.05). A moderate correlation of the difference of the V10Gy and V12Gy between the two techniques with the PTV sphericity (Spearman’s rho = 0.27 and 0.30, for V10Gy and V12Gy, respectively and p<0.05 for both parameters) was found. Lesions with higher sphericity tended to have a better healthy brain sparing with MBMSRS.

Table 1 - Investigated metrics for the 20 treatment plans, the 59 lesions and the 53 lesions for which the target indices were computed.

Fig.1 - Difference in V10Gy and V12Gy scattered against the sphericity of the respective PTV. A sphericity value of 1 corresponds to a perfect sphere, values <1 to shapes deviating from a sphere. Conclusion In most cases, MBMSRS can generate plans with steeper dose gradients, superior healthy brain sparing and less MU as compared to VMAT. In particular cases where the shape of the treated lesions deviates substantially from a sphere, VMAT can be superior. PO-0934 Physical and biological doses with increasing number of proton beams for pediatric brain irradiation L. Toussaint 1 , D.J. Indelicato 2 , K.S. Holgersen 1 , J.B.B Petersen 1 , C.H. Stokkevåg 3 , Y. Lassen-Ramshad 4 , O. Casares-Magaz 1 , C. Pedro 5 , R. Mikkelsen 6 , A. Vestergaard 4 , H. Hasle 7 , L.P. Muren 1 1 Aarhus University Hospital, Department of Medical Physics, Aarhus, Denmark ; 2 University of Florida Health Proton Therapy Institute, Department of Radiation Oncology, Jacksonville, USA ; 3 Haukeland University Hospital, Department of Oncology and Medical Physics, Bergen, Norway ; 4 Danish Center for Particle Therapy, Department of Oncology, Aarhus, Denmark ; 5 Instituto Português de Oncologia Francisco Gentil, Department of Radiotherapy, Lisbon, Portugal ; 6 Aarhus University, Department of Biomedicine, Aarhus, Denmark ; 7 Aarhus University Hospital, Department of Pediatrics, Aarhus, Denmark Purpose or Objective Dose conformity and normal tissue sparing of photon - based radiotherapy has improved considerably with the introduction of arc delivery techniques. The concept of proton arc therapy is just becoming commercially/clinically available. Besides the potential for improved physical dose conformity, proton arc therapy might also lessen the concern of elevated linear energy transfer (LET) and relative biological effectiveness (RBE) at the end of the proton range. The aim of this project was therefore to simulate the effects of proton arc therapy for pediatric brain irradiation with respect to dose coverage and conformity, including LET-weighted biological doses. Material and Methods Two cylindrical phantoms were used to create a series of treatment plans with varying number of equiangular coplanar beams (2, 3, 4, 6, 8, 10 and 12), where a dose of 54 Gy was prescribed to a cylindrical target. CTs and target volume delineations of three pediatric brain tumor patients were used to construct plans with 3, 6 and 12 equiangular coplanar beams. Doses delivered to a large