OC-0514: radiation necrosis after proton beam therapy - when and where does it happen?

S271 ESTRO 36 _______________________________________________________________________________________________ 70.2% infratentorial, the tumor grade was anaplastic in 62.0%, the extent of resection was complete in 85.1% and 64.5% of patients received a dose >54 Gy. The median PRTV was 43.8 cc (1.1-287.9), and the median CEPTV was 13.3 cc (0-71.4). The median PRTVF was 49.4 cc (0-336.7) and the median POTVF was 4.6 cc (0-118.7). A statistically significant benefit in survival was seen with a POTVF equal to 0 cc in univariate analysis for the DFS and the OS (71.9% versus 40.3% p=0.006) and (93.7% versus 72.37% p=0.023) respectively. In multivariate analysis, POTVF was also statistically significant for OS (p=0.05) and almost significant for DFS (p=0.06). Conclusion In this retrospective study, POTVF was found to be significant predictor of overall survival after ependymoma radiotherapy. POTVF was the more significant predictor of survival compared with PRTV, suggesting that this volume and residual contrast-enhancing tumor may be a more accurate and meaningful reflection of the pathobiology of ependymoma. OC-0513 Radiation necrosis following stereotactic RT and immunotherapy for melanoma brain metastases O. Kaidar-Person1, T. Zagar1, A. Deal2, S. Moschos3, M. Ewend4, D. Sasaki-Adams4, C. Lee3, F. Collichio3, D. Fried1, L. Marks1, B. Chera1 1 University of North Carolina- Chapel Hill- North Carolina- USA-, Department of Radiation Oncology, Chapel Hill, USA 2 UNC Lineberger Comprehensive Cancer Center- Chapel Hill- North Carolina- USA, Statistics, Chapel Hill, USA 3 University of North Carolina- Chapel Hill, Medicine, Chapel Hill, USA 4 University of North Carolina- Chapel Hill, Neurosurgery, Chapel Hill, USA Purpose or Objective Stereotactic radiotherapy (SRT) is the standard treatment for patients with limited number of brain metastases. In the past few years, newer immunotherapies (immune checkpoint inhibitors) have been proven to prolong survival in patients with metastatic melanoma. The safety of the combination of SRT and immunotherapy for brain metastases is unknown. Material and Methods We retrospectively identified patients with melanoma brain metastases treated with SRT between 2007 and 2015. Patients who did not have at least 3 months of follow-up with imaging after SRT were excluded from the analysis. Outcomes were compared between patients who were treated with or without immunotherapy. Results A total of 58 patients were included, of these 29 were treated with SRT and immunotherapy. MAPK inhibitors (BRAF, MEK inhibitors) were used more often in the immunotherapy group (9 vs. 2 patients). There was a higher incidence of intracranial complications in patients treated with immunotherapy and SRT. Eight patients had radiation necrosis, all occurred in patients who were treated with immunotherapy. Nine patients had hemorrhage, of which 7 occurred in patients who were treated with immunotherapy (p=0.08). However, patients treated with immunotherapy and SRT had a significant overall survival advantage compared to SRT without immunotherapy (15 vs. 6 months, p = 0.0013). Conclusion Patients treated with SRT and immunotherapy, have a higher incidence/risk of intracranial complications but a longer overall survival. OC-0514 radiation necrosis after proton beam therapy - when and where does it happen? S. Harrabi1,2,3, C. Gudden1, S. Adeberg1,2,3, N. Bougatf2,3, T. Haberer3, S. Rieken1,2, J. Debus1,2,3, K. Herfarth1,2,3 1 University Hospital Heidelberg, Radiation Oncology,

Heidelberg, Germany 2 Heidelberg Institute of Radiation Oncology, HIRO, Heidelberg, Germany 3 Heidelberg Ion-Beam Therapy Center, HIT, Heidelberg, Germany Purpose or Objective Radiation necrosis after irradiation of central nervous system tumors is a rare but severe side effect. The differentiation on magnetic resonance imaging between postoperative changes, gliosis and therapy associated changes remains a challenge and is not always possible with absolute certainty. Available data almost exclusively refer to conventional radiotherapy with photons (XRT). Since the use of proton beam therapy (PRT) is constantly increasing – especially for the treatment of neurooncologic diseases – we set out to determine the safety of proton irradiation by evaluating the incidence of radiation necrosis. Material and Methods We reviewed 430 patients with a median age of 37 years (4 – 85 years) who received radiotherapy between 2009 and 2015 for meningioma or low grade glioma with either protons (n=276) or photons (n=154). Median applied dose was 54 Gy (50 – 60 Gy). Clinical and radiological information of regular follow-up examinations were analyzed resulting in nearly 3.000 available magnetic resonance imaging (MRI) examinations with a minimum follow up of 12 months (median 30 months, range 12 – 82 months). Findings on MRI were delineated in the treatment plan system and correlated with parameters of the treatment plan. Complementary calculations for dose distribution, linear energy transfer (LET) and relative biological effectiveness (RBE) for the original treatment plan using different models (Monte Carlo, Wedenberg, Carabe) were made. Results The cumulative incidence of radiation necrosis after PRT in our cohort was 3.3 % with a median time to occurrence of 12 months (6 – 32 months). No risk factor could be identified with regard to treatment specific parameters such as optimization algorithm (single beam optimization vs intensity modulated proton therapy), number of used beams (one vs multiple), concomitant chemotherapy or applied dose (≤54 GyE vs >54 GyE). However, the observed radiation necrosis affected significantly often the periventricular border and were almost exclusively at the distal edge of the spread-out Bragg peak (SOBP). Conclusion Radiation necrosis after PRT can be a severe side effect but is as rare as after XRT. The accumulation of incidence at the distal edge of the SOBP and at the periventricular border warrants further radiobiological investigation. OC-0515 Radiation necrosis in children with brain tumours treated with pencil beam scanning proton therapy B. Bojaxhiu1, F. Ahlhelm2, M. Walser1, L. Placidi1, U. Kliebsch1, L. Mikroutsikos1, P. Morach1, A. Bolsi1, T. Lomax1, R. Schneider1, D.C. Weber1 1 Paul Scherrer Institute, Center for Proton Therapy, Villigen, Switzerland 2 Cantonal Hospital Baden, Department of Radiology, Baden, Switzerland Purpose or Objective To assess the rate of radiation-induced brain necrosis (RN) and related neurologic symptoms in paediatric patients with primary brain tumours treated with Pencil Beam Scanning (PBS) proton therapy (PT) with or without concomitant chemotherapy at the Paul Scherrer Institute, Switzerland. Material and Methods One hundred and seventy-one children and adolescent young adults (AYA) (<18 years) with brain tumours were