ESTRO 36 Abstract Book

S74 ESTRO 36 2017 _______________________________________________________________________________________________

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.

which introduces errors in the SECT derived RSPs. The DECT method determines the effective atomic number and relative electron density and on basis of these physical parameters enables a more accurate estimate of the RSP.

Conclusion The developed DECT method is more accurate in prediction of relative proton stopping powers than the SECT calibration method for a wide range of materials and tissues and can be of benefit to proton therapy treatment planning. OC-0152 Innovative solid state microdosimeters for Radiobiological effect evaluation in particle therapy T.L. Tran 1 , L. Chartier 1 , D. Bolst 1 , D. Prokopovich 2 , A. Pogossov 1 , M. Lerch 1 , S. Guatelli 1 , A. Kok 3 , M. Povoli 3 , A. Summanwar 3 , M. Reinhard 2 , M. Petesecca 1 , V. Perevertaylo 4 , A. Rozenfeld 1 1 University of Wollongong, Centre for Medical Radiation Physics, Wollongong, Australia 2 Australian Nuclear Science and Technology Organisation, Engineering Material Institute, Lucas Heights, Australia 3 SINTEF, Microsystems and Nanotechnology, Oslo, Norway 4 SPA-BIT, SPA-BIT, Kiev, Ukraine