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can’t be learned from textbooks and it is difficult to show how to handle it in a manual.

selected metastatic site. Disease stage was classified according to the International Neuroblastoma Staging System (INSS). We evaluated the recurrence pattern (local, systemic), progression free survival and overall survival. Results A Total of 33 patients with high risk neuroblastoma were included in this study. The median age of patients was 4 years (range, 1-11 years). Twenty four patients (73%) were INSS stage 4 and eight patients were INSS stage 3. At the median follow up of 63 months, there were 6 cases of local recurrence and 10 cases of systemic recurrence. Among six local failure cases, four relapsed adjacent to the radiation field. Other two relapsed in the radiation field (para-aortic, retroperitoneal). Among these, one underwent R2 resection and tumor recurred at residual lesion. The main sites of distant metastasis were bone marrow, bone, lymph nodes. Median overall survival was 63 months and median progression free survival was 58 months. Conclusion Radiation therapy directed at the primary tumor site shows good local control. It appears to be adequate for local control for patients with high-risk neuroblastoma after chemotherapy and surgical resection. Because only 33 patients were analyzed in this study, additional study is needed. EP-1642 Evaluation of inter-observer variations in target delineation – A dose based approach I. Kristensen 1 , K. Nilsson 2 , T. Knöös 1 , P. Nilsson 1 Substantial inter-observer variations in target delineation have been presented previously. Target delineation for paediatric cases is particular difficult due to the small number of patients, the variation in paediatric targets, the number of study protocols, and the individual patient’s specific needs and demands. Uncertainties in target delineation might lead to under- or over-dosage. The aim of this work is to apply the concept of a consensus volume combined with good quality treatment plans to visualise and quantify inter-observer target delineation variations in dosimetric terms in addition to conventional geometrically based volume concordance indices. Material/methods: Two paediatric cases were used to demonstrate the potential of adding dose metrics when evaluating target delineation diversity; a Hodgkin’s disease (case 1) and a rhabdo–myosarcoma of the parotid gland (case 2). The variability in target delineation (PTV delineations) between six centres was first quantified using the generalised conformity index, CI gen , generated for volume overlap. Secondly, the STAPLE algorithm, as implemented in CERR, was used for both cases to derive a consensus volume. STAPLE is a probabilistic estimate of the true volume generated from all observers. Finally, dose distributions created by each centre for the original 1 Skane University Hosplital Lund- Lund University, Radiation Physics, Lund, Sweden 2 Clinical Oncology- Uppsala University Hospital, Department of Immunology- Genetics and Pathology- Experimental and Clinical Oncology- Clinical Oncology, Uppsala, Sweden II. Purpose or Objective Background/urpose:

EP-1640 Impact of radiation therapy on outcome in high-risk neuroblastoma N. Bouzid 1 , D. Valteau Couanet 2 , S. Bolle 1 1 Gustave Roussy, Radiation Oncology Department, Villejuif, France 2 Gustave Roussy, Children and Adolescent Oncology Department, Villejuif, France Purpose or Objective To evaluate the impact of radiotherapy (RT) on local control (LC) and overall survival (OS) in patients with high-risk neuroblastoma (NBL) after multimodality treatment including high-dose chemotherapy (HD-CT). Material and Methods We reviewed the records of 78 patients with high-risk NBL treated in our institution according the HR- NBL1/SIOPEN protocol between 2003 and 2014. Results After HD-CT, 65 patients were eligible for RT. Thirty- nine received RT and 26 did not. Median dose to tumor bed was 21 Gy (range, 19,2-30,6 Gy). More patients in the RT group had gross residual disease than in the non-RT group (25,6% vs 7,7%). At a median follow-up of 38,5 months (range, 7-150 months), 28 patients died of disease (12 in the RT group and 16 in the non-RT group). Distant failure was observed in 20 patients of whom 13 after RT. Six patients who failed locally have presented synchronous metastasis, only one was initially treated with RT. In patients with macroscopic residue, one patient had local progression after RT while all patients did in absence of RT. 5-year LC rate were 90% in the RT group compared to 70% in the non-RT group (p=0,03). 5-year OS rate was 58% in the RT group compared to 32% in the non-RT group (p=0,014). Radiation-induced side effects were mainly hematologic and gastro-intestinal. No grade 3 or greater toxicity was noted. Conclusion Our data suggest that RT influence LC but also OS in high-risk NBL treated with multimodality therapy. A dose of 21Gy could be sufficient for local control in case of gross residual disease after HD-CT. EP-1641 Patterns of Recurrence after Radiation Therapy for High-risk Neuroblastoma J.H. Jo 1 , S.D. Ahn 1 , E.K. Choi 1 , J.H. Kim 1 , S.W. Lee 1 , S.Y. Song 1 , S.M. Yoon 1 , Y.S. Kim 1 , J.H. Park 1 , S.S. Kim 1 , J.H. Jung 1 1 Asan Medical Center- Univ. Ulsan, Radiation oncology, Seoul, Korea Republic of Purpose or Objective To investigate the patterns of recurrence in neuroblastoma patients who received radiation therapy We retrospectively analyzed patients with high risk neuroblastoma who received definitive treatment (chemotherapy, surgical resection, stem cell transplantation) with radiation therapy to primary tumor site, Between January 2003 and December 2015. Patients treated with three dimensional conformal radiation therapy or intensity modulated radiation therapy. A total 14 Gy – 24 Gy in 7 - 14fx was delivered to the planning target volumes which include the primary tumor bed and to primary tumor site. Material and Methods