ESTRO 36 Abstract Book

S75 ESTRO 36 2017 _______________________________________________________________________________________________

One beam (1 GyE) was applied to a CIRS head phantom and monitored with the PGI slit camera. To investigate the influence of the spot dose, the same beam with 5 GyE was also delivered and measured. Global and local (5 cm in diameter) range shifts were introduced and the PGI profiles (prompt-γ counts over depth) with and without shifts were compared. Sum profiles containing prompt-γ counts over the entire fraction were used for the comparison of DS and PBS. Moreover, PGI profiles measured in PBS were analyzed spot-wise and will also be compared with simulated profiles for absolute range determination. Results A good agreement between introduced and measured global shifts was found in the sum profile evaluation for both modalities, PBS and DS (Table 1). Relative differences were below 2, 7 and 12 % for the 10, 7 and 4 mm shifts, respectively. Local shifts are not detectable using sum profiles. For the applied local shifts, a spot-wise comparison of PGI profiles in PBS allows the detection and localization of global and local shifts (Figure 1). For interpretation, neighboured spots should be clustered, as shifts detected for single spots are less reliable due to low statistics. Higher doses (5 vs. 1 GyE) allow the detection of smaller shifts as shown in Figure 1 for the 4 mm local shift. Table 1: Measured global shifts between sum profiles in DS and PBS with 1 and 5 GyE.

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.

Figure 1: Spot-wise analysis of the determined range shifts: The points represent PBS spots of one energy layer, the size corresponds to the dose per spot, the color to the detected shift between two PGI profiles. Spots influenced by the local shifts (black ring) are highlighted with a black edge. Global and local shifts with 1 and 5 GyE were measured.