Abstract Book

S884

ESTRO 37

target volumes were then applied to this consensus volume. Results A considerable variation in target segmentation was seen in both cases. For case 1 the variation was 374-960 cm 3 (average 669 cm 3 ) and for case 2; 65-126 cm 3 (average 109 cm 3 ).

could be used in paediatric brains and to evaluate the feasibility of hippocampal delineation in childhood. Material and Methods Brain MR images of 20 children (12 M, 6 F, age ranging from 8 months to 8 years) were retrospectively evaluated. All studies were performed on a 3 Tesla magnet. Subjects underwent a standardized protocol which included T1 weighted sequences. Inclusion criteria were the absence of pathological findings and more specifically the presence of a normal myelination pattern related to subject age. As image quality was paramount for our purpose, sedation was required in a number of subjects. Hippocampal region contouring was performed on axial slices by a panel of radiation oncologists and radiologists according to the atlas provided by the Radiation Therapy Oncology Group (RTOG) trial 0933. Qualitative analysis of contours was performed by comparing the pediatric brains with RTOG images obtained from adult subjects in terms of morphologic appearance and anatomic landmarks. Results In our cohort adult brain appearance was reached at about 12-15 months of age on T1-weighted images analysis. These results are consistent with findings in literature. In children older than 12-15 months hippocampus appearance was similar to adults for contornation purposes. In children younger than 12-15 months the poor contrast between white and grey matter did not allow to differentiate hippocampal structures on T1 weighted images. Specifically, one of the main contouring rules proposed in RTOG atlas, i.e. to contour only the hypointense grey matter and not the white matter could not be used. The cerebrospinal fluid containing structures indicated as landmarks in RTOG atlas were instead visible in this subgroup. Specifically, the temporal horns of the lateral ventricles and the quadrigeminal cistern could be used to delineate the temporo-mesial structures from the lateral portions of the temporal lobe. Conclusion Hippocampal contouring in childhood is feasible as soon as the myelination process allows to discriminate the hippocampal structures on T1-weighted imaging (12-15 months). In this group RTOG atlas recommendations can be used. In younger subjects accurate hippocampal contouring is not feasible because the myelination process is still incomplete. One possible solution could be sparing all temporo-mesial structures. Further studies are required to develop and validate ad hoc atlases. EP-1644 Potential gain of MRI-guided IMRT planning versus current clinical CBCT-guided VMAT for Wilms’ tumor F. Guerreiro 1 , E. Seravalli 1 , G.O. Janssens 1 , J.J.W. Lagendijk 1 , B.W. Raaymakers 1 1 UMC Utrecht, Department of Radiotherapy and Imaging Division, Utrecht, The Netherlands Purpose or Objective At University Medical Center Utrecht, radiotherapy treatment (RT) for Wilms' tumor is currently done using a cone beam computed tomography-guided volumetric modulated arc therapy (VMAT CBCT ) workflow. By adding real-time imaging guidance to the RT with the introduction of the magnetic resonance imaging (MRI)- linac, the use of small safety margins due to the better visualization of the target and the organs at risk (OAR) can be achieved. The purpose of this study is to quantify the potential reduction of planning target volume (PTV)

Figure 1. Case 1 (left) and case 2 (right) with all centres’ delineations and the consensus volume in yellow.

CI gen were 0.53 and 0.70, respectively. Each dose distribution were applied to both its own target as well as to the consensus volume. The DVHs in absolute volume displayed for the delineated target volume as well as for the consensus volume adds information on both “compliant” target volumes as well as outliers which are hidden with just the use of concordance indices.

Figure 2. Examples of individual DVHs for target volumes (dashed lines, centres 3 to 4) for case 1 compared to the consensus volume (solid yellow line). To the left a substantial under-dosage of the consensus volume is present, and to the right an over-dosage of the consensus volume is present. Conclusion DVHs in absolute volume adds information that is more understandable and interpretable compared to various indices for evaluating uniformity in target delineation. The DVHs displayed for the consensus volume adds information on both “compliant” target volumes as well as outliers which are hidden with only the use of concordance indices. This approach should be reported together with descriptive statistics, concordance indices and statistical measures of agreement to get a complete evaluation of delineation studies. EP-1643 Applicability of reference atlases to hippocampal contouring in paediatric radiotherapy S. Di Biase 1 , M. Trignani 1 , C. Di Carlo 1 , I.P. Voicu 2 , L. Gentile 2 , A. Augurio 1 , M. Caulo 2 , A. Vinciguerra 1 , D. Genovesi 1 1 "SS. Annunziata" Hospital- "G. D'annunzio" University, Radiotherapy, Chieti, Italy 2 "SS. Annunziata" Hospital- "G. D'annunzio" University, Radiology, Chieti, Italy Purpose or Objective Brain maturation and myelination is a dynamic process starting in fetal life and evolving during the first years of life. Magnetic resonance based atlases for hippocampus contouring in radiotherapy of brain tumors in adults are available, while similar tools are not currently available for pediatric population. The aim of our study was to verify if reference atlases for hippocampal contouring