ESTRO 37 Abstract book

ESTRO 37

S589

Dollinger 2 , T.E. Schmid 1 1 Klinikum rechts der Isar Technische Universität München, Department of Radiation Oncology, Munich, Germany 2 Universität der Bundeswehr München, Institute for Applied Physics and Metrology, Neubiberg, Germany Purpose or Objective The biological optimization of radiotherapy requires an accurate knowledge of how the relative biological effectiveness (RBE) depends on the spatial dose distribution and thus linear energy transfer (LET). The RBE of high LET radiation is attributed to the highly inhomogeneous spatial dose deposition due to formation of complex DNA damage which is more difficult to repair, compared to the quasi-homogeneous distribution of damage in the case of low LET particles. The aim of the study was to assess the influence of different spatial distributions of DNA damage with respect to the endpoints of the biological responses cell survival, apoptosis induction and DNA double strand break (DSB) repair. Material and Methods Chinese hamster ovary cells (CHO-K1) were irradiated using the ion microbeam SNAKE at the Munich tandem accelerator with low LET protons. The cells were exposed either to a grid like irradiation pattern where a certain number of protons were focused to sub-micrometer spots or to a homogeneous proton irradiation. However, the mean dose was the same in all irradiation patterns, only the spatial dose distribution was varied. Biological endpoints of cell survival, apoptosis induction and DNA DSB repair were assessed by clonogenic assay, caspase 3/7 activity and γ-H2AX flow cytometry after irradiation. Results Our results showed that focusing of low LET protons to sub-micrometer spots resulted in a significant increase in RBE for the cell survival (RBE= 2.27±0.29) compared to quasi-homogeneous proton application (RBE= 0.90±0.32). The DNA damage repair after grid like focused irradiation is meant to result in more lethal chromosome aberrations [1] and thus less cell survival due to the increased local density of DSBs compared to a homogeneous proton application. In line with these findings, the fraction of the residual unrepaired DSBs was 22% higher as measured by the relative mean γ-H2AX fluorescence intensity after focused proton irradiation. Moreover, the results suggested that changing the spatial dose by focusing low LET protons might efficiently trigger an early apoptotic response and induce about 50% higher apoptotic rate compared to a homogeneous proton irradiation. These results indicated that the increased local density of DSBs and thus more complex DSBs after focused dose deposition might have an influence on the apoptotic response in cells. Conclusion Our results provided novel evidence that the increased RBE of heavy ions can be approached by spot application of protons. This approach opens a new test field to understand the creation of higher RBE values through high LET radiation by modeling these effects by focused low LET radiation. Acknowledgement: Supported by the BMBF projects 02NUK031A and 02NUK031B and by the DFG-Cluster of Excellence ‘Munich-Centre for Advanced Photonics’. 1. Low LET protons focused to submicrometer shows enhanced radiobiological

effectiveness. Phys Med Biol, 2012. 57 (19): p. 5889-907.

2.

PO-1048 Radiobiological characterization of clinical proton, helium-, carbon- and oxygen ion beams I. Dokic 1 , A. Mairani 2,3 , M. Niklas 1 , F. Zimmermann 1 , D. Krunic 4 , J. Debus 1 , T. Haberer 2 , A. Abdollahi 1 1 German Cancer Consortium- Heidelberg University Hospital- National Center for Tumor Diseases- German Cancer Research Center- Heidelberg Institute of Radiation Oncology- National Center for Radiation Research in Oncology- Heidelberg Ion-Beam Therapy Cen, 2 Heidelberg Ion-Beam Therapy Center, Physics, Heidelberg, Germany 3 National Center for Oncological Hadrontherapy, Medical Physics, Pavia, Italy 4 German Cancer Research Center, Light Microscopy Facility, Heidelberg, Germany Purpose or Objective To analyze and compare biological efficacy of therapeutic proton, helium, carbon and oxygen ion beams at Heidelberg Ion-Beam Therapy Center (HIT), using a panel of radiobiological assays. Material and Methods Particle irradiations were performed at HIT with the raster-scanning technique. For irradiation with proton, helium-, carbon- and oxygen ion beams, cells were positioned in the middle of a 1 cm wide spread out Bragg peak (SOBP) centered at about 3.5 cm water-equivalent depth. Plans have been optimized applying a research treatment planning system (TPS) available at HIT. Photon irradiation was performed using a linear accelerator (LINAC, 6 MV, Artist, Siemens). To assess the biological efficacy of the beams, series of different radiobiological readouts have been performed, using human non-small lung cancer cells A549. This cell line was selected due to its relatively low background foci levels. To investigate cytotoxic potential of different beams, and define relative biological effectiveness (RBE) of different particle beams, clonogenic survival assay, caspase 3/7 dependent apoptosis and reactive oxygen species (ROS) accumulation were monitored. For studying DNA damage response and repair kinetics, surrogate of DNA damage, γ-H2AX foci were analyzed. The cell fluorescence ion track hybrid detector ( Cell-Fit-HD ) technology was employed to detect particle traverse per cell nucleus. Monte Carlo (MC) predictive models were run against experimentally obtained cell survival and particle traverse per nucleus data. Results Clonogenic survival and RBE mainly correlated with the radiation modality-specific spatio-temporal pattern of DNA double strand break (DSB) formation and repair kinetics. The size and the number of residual nuclear γ- H2AX foci increased as a function of linear energy transfer (LET) and RBE ( 1 H< 4 He< 12 C< 16 O), reminiscent of enhanced DNA-damage complexity and accumulation of non-repairable DSB. Conclusion These data confirm the high relevance of complex DSB formation as a central determinant of cell fate and reliable biological surrogates for cell survival/ RBE. The correlation of foci formation/area and clonogenic survival as function of LET opens the possibility of extending the MC predictions incorporating a calculation scheme for foci as a function of particle energy and dose level.