ESTRO 36 Abstract Book

S867 ESTRO 36 _______________________________________________________________________________________________

along SOBP dose profiles were predicted also for depth 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.

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 the competing events of cell mutation and inactivation. The RBE variation for the different end-points was included by introducing and varying the ratio (k) between cell mutation and inactivation for C-ions. The median and range of the patient-specific RRs were calculated from the physical dose distributions and the published input model parameters. Results The dose distributions (Fig 1) illustrated the sharper lateral penumbra of C-ions, which resulted in lower lung doses compared to protons, while the C-ion fragmentation tail contributed to higher doses to the thyroid than from protons. The SC risk estimates strongly depended on the ratio k, and the RR decreased for increasing k for both organs (Fig 2). For the thyroid, the RR was higher from the C-ion plans for the entire scanned range of k. Despite a better sparing of the lungs with C-ions, the carcinogenic potential of C-ions was not consistently lower than for protons: Not including a difference in end-point resulted in RRs in favour of C-ions, while increasing the ratio k gave higher risks for C-ions compared to protons. For the lungs, the median RR turned in favour of IMPT at a threshold value k=1.1.

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