ESTRO 2020 Abstract book

S423 ESTRO 2020

each brain extracted T1w MRI. Sigmoid and transverse sinuses mimicking cerebellar tissue were removed using the CE T1w MRI. The cerebellum was segmented into grey matter (GM), white matter (WM) and cerebrospinal fluid (CSF) 6 . Its volume was calculated as the sum of all GM and WM probabilities and normalized to the baseline value for each patient. The mean relative cerebellar volume change per year was estimated for each patient with a linear regression. The resulting rate of volume change per year was plotted against the mean dose delivered to the cerebellum across all patients, and the rate of volume change per year per dose was subsequently estimated using a linear regression. Results Figure 1 illustrates segmentation of the cerebellum. Mean cerebellar dose for patients treated with protons and photons was 1.7Gy ± 2.1Gy and 5.7Gy ± 4.6Gy, respectively. The linear model estimated a cerebellar volume loss of approx. 1.8% per 10Gy per year (p<0.001; Fig. 2). This is similar to that estimated in the cerebrum of the same patient cohort 2 . Conclusion Cerebellar volume loss after radio(chemo)therapy is related to the mean cerebellar dose. The atrophy rate is similar to that previously found in the cerebrum. Additional work is needed to further validate those findings and relate them to cognitive and motor performance. [1] Prust et al. Neurology 2015;85:683-691 [2] Petr et al. Radiother Oncol 2018;128:121-127 [3] Zhou et al. Sci Rep-UK 2017;7:46181 [4] Eekers et al. Clin Transl Radiat Oncol 2018;8:22-26 [5] Avants et al. NeuroImage 2011; 54:2033–2044 [6] Avants et al. Neuroinformatics 2011;9:381-400

Proffered Papers: Proffered papers 35 : Late-breaking abstracts

OC-0692 Phase 1 study of olaparib as radiosensitizer for locally advanced non-small cell lung cancer R. De Haan 1 , J. Van Diessen 1 , H.M.U. Peulen 1 , E. Van Werkhoven 2 , A.J. De Langen 3 , F. Lalezari 4 , C. Vens 5 , J. Schellens 6 , N. Steeghs 7 , M.M. Van Den Heuvel 3 , M. Verheij 1 , B. Van Triest 1 1 netherlands Cancer Institute, Radiotherapy, Amsterdam, The Netherlands ; 2 netherlands Cancer Institute, Biometrics, Amsterdam, The Netherlands ; 3 netherlands Cancer Institute, Thoracic Oncology, Amsterdam, The Netherlands ; 4 netherlands Cancer Institute, Radiology, Amsterdam, The Netherlands ; 5 netherlands Cancer Institute, Cell Biology, Amsterdam, The Netherlands ; 6 netherlands Cancer Institute, Pharmacology, Amsterdam, The Netherlands ; 7 netherlands Cancer Institute, Medical Oncology, Amsterdam, The Netherlands Purpose or Objective The addition of a tumor specific radiosensitizer to (chemo- )radiotherapy has the potential to improve outcome for patients with locally advanced non-small cell lung cancer (LA-NSCLC). PARP inhibitors are such promising radiosensitizers. To date, clinical data on safety and tolerability of PARP inhibitors in combination with high dose thoracic (chemo-)radiotherapy is still lacking. The purpose of this study was to identify the maximum tolerated dose (MTD) of the PARP inhibitor olaparib in combination with high dose radiotherapy with or without concurrent cisplatin for the treatment of LA-NSCLC. Material and Methods Olaparib dose was escalated in two parallel study arms: radiation (66Gy/24fractions) with and without concurrent daily low dose cisplatin (6mg/m 2 ). Patients with stage II-III inoperable disease were included. After a protocol amendment, patients with stage IV oligometastatic disease and a good response after induction chemotherapy This abstract will be published n th day of its presentation