ESTRO 36 Abstract Book

S855 ESTRO 36 2017 _______________________________________________________________________________________________

positions where experimental data were not available. A formula was also derived to predict cell death and chromosome damage for a different cell line exposed to a given ion type and energy, basing on the response of a reference cell line to the same radiation quality. For both endpoints, the increase of effectiveness along the plateau was quantified. A non-negligible increase was found also for protons, associated to high levels of damage beyond the distal dose fall-off, due to the lower energy and thus the higher biological effectiveness.

Figure 1: ROC curves of the model validation. Conclusion

The combination of deep learning and radiomics features has similar performance to conventional radiomics modelling strategies. However, feature selection is no longer a required component as all features can be included in the network. This is a major advantage as feature selection is a computationally intractable task for which only heuristic solutions exist.

Conclusion In line with other studies, this work suggests that assuming a constant RBE along a proton SOBP may be sub-optimal. More generally, this work represents an example of therapeutic beam characterization avoiding the use of experimental RBE values, which can be source of uncertainties. Acknowledgements: this work was partially supported by INFN (project ETHICS, P.I. L. Manti, local P.I. F. Ballarini; MC-INFN/FLUKA, P.I. P. Sala, local P.I. A. Fontana) EP-1607 Secondary cancer risk after particle therapy for organs distal or lateral to the target volume L. Toussaint 1 , L. Muren 1 , G. Engeseth 2 , C. Stokkevåg 2 1 Aarhus University Hospital, Medical Physics, Aarhus C, Denmark 2 Haukeland University Hospital, Department of Oncology and Medical Physics, Bergen, Norway Purpose or Objective Proton therapy is the most used particle therapy modality, but carbon ions are also increasingly being applied for specific tumour entities. Particle therapy in general has a known potential of reducing the irradiated volumes of normal tissues, although protons and carbon ions have distinctively different dose distribution characteristics. Protons have a steeper dose fall-off distally while carbon ions have a sharper lateral dose penumbra. In addition, carbon ions have a higher biological effect due to increased cell inactivation, but also for the end-point cell mutation associated with carcinogenic potential. The aim of this study was therefore to compare the risk of secondary cancer (SC) from dose distributions in the thyroid and lungs, particularly radiosensitive organs located distally and laterally to the target volume during craniospinal irradiation (CSI). Since pre-clinical data indicates that the carbon ions RBE for cell mutation may be higher than for cell inactivation, we included this in the models. Material and Methods CSI treatment plans with a prescribed dose of 23.4Gy(RBE) were generated on CT-scans from six pediatric patients (Syngo, Siemens) using pencil beam scanning protons (IMPT) and carbon ions (C-ions). Relative risks (RRs) of radiation induced cancer (IMPT/C-ions) for the thyroid and the lungs were analysed by applying a bell-shaped dose- response model (J Radiol Prot 2009; 29(2A): A143-157). The model accounts for RBE, fractionation as well as for

References 1 Aerts, H. et al, Nat. Commun. 2014 , 5 , 4006.

EP-1606 Calculating ion-induced cell death and chromosome damage by the BIANCA biophysical model M.P. Carante 1,2 , F. Ballarini 1,2 1 Istituto Nazionale di Fisica Nucleare INFN, Section of Pavia, Pavia, Italy 2 University of Pavia, Physics Department, Pavia, Italy Purpose or Objective To calculate probabilities of cell death and chromosome aberrations following cell irradiation with ion beams of different energy. Material and Methods A biophysical model called BIANCA (BIophysical ANalysis of Cell death and chromosome Aberrations) [Carante M.P. and Ballarini F. Front. Oncol. 6:76 2016] was refined and applied to simulate cell death and chromosome aberrations by therapeutic protons and heavier ions. The model, which assumes a pivotal role for DNA cluster damage, is based on the following assumptions: i) a DNA “Cluster Lesion” (CL) produces two independent chromosome fragments; ii) chromosome fragment un- rejoining, or distance-dependent mis-rejoining , gives rise to chromosome aberrations; iii) certain aberrations (dicentrics, rings and large deletions) lead to cell death. The CL yield is an adjustable parameter, as well as the probability that a chromosome fragment remains un- rejoined even if possible partners for rejoining are present. The model, implemented as a MC code providing simulated dose-response curves comparable with experimental data, was applied to different beams, including beams available at the CNAO hadrontherapy centre in Pavia, Italy, and at the CATANA facility in Catania, Italy. Results The model allowed reproduction of experimental survival curves for cell lines characterized by different radiosensitivity, supporting the model assumptions. Furthermore, cell death and chromosome aberrations along SOBP dose profiles were predicted also for depth