ESTRO 36 Abstract Book

S76 ESTRO 36 _______________________________________________________________________________________________

Purpose or Objective Particle therapy has many advantages over conventional photon therapy, particularly for treating deep-seated solid tumors due to its greater conformal energy deposition achieved in the form of the Bragg peak (BP). Successful treatment with protons and heavy ions depends largely on knowledge of the relative biological effectiveness (RBE) of the radiation produced by primary and secondary charged particles. Different methods and approaches are used for calculation of the RBE-weighted absorbed dose in treatment planning system (TPS) for protons and heavy ion therapy. The RBE derived based on microdosimetric approach using the tissue equivalent proportional counter (TEPC) measurements in 12 C therapy has been reported, however large size of commercial TEPC is averaging RBE which dramatically changes close to and in a distal part of the BP that may have clinical impact. Moreover, the TEPC cannot be used in current particle therapy technique using pencil beam scanning (PBS) delivery due to pile up problems associated with high dose rate in PBS. Material and Methods The Centre for Medical Radiation Physics (CMRP), University of Wollongong, has developed new silicon-on- insulator (SOI) microdosimeter with 3D sensitive volumes (SVs) similar to biological cells, known as the “Bridge” and “Mushroom” microdosimeters, to address the shortcomings of the TEPC. The silicon microdosimeter provides extremely high spatial resolution and can be used for in-field and out-of-field measurements in both passive scattering and PBS deliveries. The response of the microdosimeter was studied in passive and scanning proton and carbon therapy beam at Massachusetts General Hospital (MGH), USA, Heavy Ion Medical Accelerator in Chiba (HIMAC) and Gunma University Heavy Ion Medical Center (GHMC), Japan, respectively. Results Fig 1a shows the dose mean lineal energy, and frequency mean lineal energy, measured using the SOI microdosimeter irradiated by the 131.08 MeV pencil proton beam as a function of depth in water. The value was around 2 keV/µm in the plateau region, then approximately 3 to 5 keV/µm in the proximal part of the BP, and increasing dramatically to 9 to 10 keV/µm at the end of the BP. Fig 1b shows derived RBE along the BP for 2Gy dose delivered in a peak. Fig 2 shows the distribution with depth for the 290 MeV/u 12 C ion pencil beam at GHMC. The inset graph in the left corner of Fig. 2 shows a detailed view of the distribution at the BP measured with submillimetre spatial resolution. It can be seen that the distribution at the peak illustrates the effect of ripple filter used in this facility which is impossible to observe with any TEPC based microdosimeters. RBE values and dose equivalent obtained near the target volume are also derived using the SOI microdosimeters and the results will be presented in a full paper.

Conclusion This work presented an application of SOI micodosimeters for RBE determination in passive and scanning proton and 12 C ion therapy and silicon microdosimetry has demonstrated a simple and fast method for routine Quality Assurance in charged particle therapy. OC-0153 Sensitivity evaluation of prompt γ-ray based range verification with a slit camera L. Nenoff 1 , M. Priegnitz 2 , A. Trezza 1 , J. Smeets 3 , G. Janssens 3 , F. Vander Stappen 3 , L. Hotoiu 3 , D. Prieels 3 , W. Enghardt 1,4,5,6 , G. Pausch 1,5 , C. Richter 1,4,5,6 1 OncoRay - National Center for Radiation Research in Oncology, Faculty of Medicine and University Hospital Carl Gustav Carus- Technische Universität Dresden- Helmholtz-Zentrum Dresden - Rossendorf, Dresden, Germany 2 Helmholtz-Zentrum Dresden-Rossendorf, Institute of Radiation Physics, Dresden, Germany 3 Ion Beam Applications SA, Louvain-la-Neuve, Belgium 4 Faculty of Medicine and University Hospital Carl Gustav Carus- Technische Universität Dresden, Department of Radiation Oncology, Dresden, Germany 5 Helmholtz-Zentrum Dresden-Rossendorf, Institute of Radiooncology, Dresden, Germany 6 German Cancer Consortium DKTK and German Cancer Research Center DKFZ, Dresden, Germany Purpose or Objective The dose distribution and range of proton beams are exceedingly prone to uncertainties and anatomical changes, demanding for an in-vivo range verification. A promising approach is prompt γ-ray imaging (PGI), which was recently implemented clinically in Dresden using a so- called PGI slit camera [1,2] in double scattering (DS). However, the detectability of local range shifts, affecting only part of the lateral field in DS, is limited. The spot- wise dose deposition in pencil beam scanning (PBS) promises a finer spatial resolution of range shifts. The purpose of this study is to comprehensively investigate the sensitivity to detect range shifts in DS and PBS using a head phantom in a clinical setup. Material and Methods For a realistic brain tumor treatment, treatment plans in DS and PBS (2 beams, 60 GyE, 2 GyE/fx), were created.