ESTRO 36 Abstract Book

S269 ESTRO 36 2017 _______________________________________________________________________________________________

treated with PBS PT between 1999 and 2015. Median age at diagnosis was 3.3 years (range, 0.3-17) and the male/female ratio was 1.44. The median delivered dose was 54 Gy(RBE) (range, 40–74.1). Post PT brain alterations (white matter lesions [WML] and RN) were defined as a new area of abnormal signal intensity on T2-weighted images or increased signal intensity on T2-weighted images and contrast enhancement on T1 occurring in the brain parenchyma included in the radiation treatment field, which did not demonstrate any abnormality before PT. RN was graded according to the National Cancer Institute Common Terminology Criteria for Adverse Events v4.03 and EORTC grading systems. The median follow-up period for the surviving patients was 49.8 months (range, 5.9-194.7).

(n=18 patients). The 5-year RN-free survival was 71%. In univariate analysis, neo-adjuvant chemotherapy ( p =0.025) and hydrocephalus before PT ( p =0.035) were significant predictors of RN.

Conclusion Children treated with PT demonstrated a low prevalence (7%) of symptomatic RN. Exposure to chemotherapy before PT and hydrocephalus as an initial symptom were significant risk factors associated with RN in these children/AYAs. OC-0516 Brainstem linear energy transfer in intensity- modulated proton therapy of paediatric brain tumours L.F. Fjaera 1 , Z. Li 2 , K.S. Ytre-Hauge 1 , L.P. Muren 3 , D. Indelicato 2 , Y. Lassen-Ramshad 4 , G.M. Engeseth 5 , M. Brydøy 5 , S. Flampouri 2 , O. Dahl 5 , C.H. Stokkevåg 5 1 University of Bergen, Department of Physics and Technology, Bergen, Norway 2 University of Florida, Department of Radiation Oncology, Jacksonville, USA 3 Aarhus University Hospital, Department of Medical Physics, Aarhus, Denmark 4 Aarhus University Hospital, Department of Oncology, Aarhus, Denmark 5 Haukeland University Hospital, Department of Oncology and Medical Physics, Bergen, Norway Purpose or Objective The enhanced linear energy transfer (LET) and relative biological effectiveness (RBE) at the end of a proton track is usually only accounted for qualitatively during treatment planning. Intensity-modulated proton therapy (IMPT) plans are currently optimised using dose/volume constraints for the brainstem, but there is growing concern about the clinical consequences of the elevated LET surrounding a tumour volume and potentially within nearby organs at risk. For posterior fossa tumours invading or in close proximity to the brainstem, the brainstem may receive an unintended increased biological dose. The aim of this study was to investigate how various posterior fossa tumour locations impact the LET and biological dose distributions in the brainstem. Material and Methods The brainstem was contoured on CT/MRI images on a five year old male patient with a posterior fossa tumour treated with protons. Multiple IMPT treatment plans were generated (in Eclipse, Varian) for different simulated tumour locations relative to the brainstem: full overlap between tumour and brainstem (A), half overlap (B), juxtaposed posteriorly (C) and 1 cm posterior of the brainstem (D). Two lateral and one posterior non-coplanar fields were applied for all plans. The dose prescription was 54 Gy(RBE) and brainstem constraints were applied based on published metrics to keep the risk of brainstem necrosis <5%. All plans were optimized using a fixed RBE value of 1.1. The dose-averaged LET (LET d ) as well as the dose- weighted LET d (LET d x dose) were subsequently calculated using the FLUKA Monte Carlo code. The dose-weighted

Results Twenty-nine (17%) patients developed RN with a median time of 5 months (range, 1-26), the majority of them ( n =17; 59%) being asymptomatic (grade 1). Grade 2, 4 and 5 toxicities were observed in 8, 2 and 2 patients, respectively. The observed toxicity reversed in a majority of cases ( n =18; 62%). The observed WML rate was 11%