Abstract Book

S926

ESTRO 37

EP-1731 A comparative study of passive detectors in active scanning proton beams A. Thummerer 1 , P. Kuess 1,2 , D. Georg 1,2 , M. Clausen 1,2 1 Medical University of Vienna, Department of Radiotherapy, Vienna, Austria 2 Medical University of Vienna, Christian Doppler Laboratory for Medical Radiation Research for Radiation Oncology, Vienna, Austria Purpose or Objective Radiochromic films and thermo-luminescent detectors (TLDs) have smaller size and higher spatial resolution than ionization chambers. This makes them suitable for use in custom or commercial phantoms and allows obtaining a complete depth-dose curve in a single irradiation for particle beams. Performing depth-dose measurements in this manner saves valuable beam time which is especially beneficial in dose verification with active scanning beam delivery. The purpose of this study is to compare the response of three different types of passive detectors: radiochromic EBT3 and EBT-XD films (GAFchromic, NJ, USA) and TLDs (TLD-100, Thermo Scientific, MA, USA) in proton beam spread-out Bragg peaks (SOBP) delivered by active scanning. Additionally, considerable effort was put into TLD characterization and individual correction factor determination in order to reduce the uncertainties for advanced dosimetric applications, i.e. in anthropo- morphic phantoms. Material and Methods The RayStation treatment planning system (RaySearch, Sweden) was utilized to generate a SOBP of 8 cm in depth. An RW3 slab phantom (PTW, Germany) was used for irradiation. EBT3 and EBT-XD films, both cut to 4x4 cm 2 pieces, were aligned in the phantom in two different configurations. The first consisted of 10 films irradiated perpendicular to the beam axis. The second was a combination of a perpendicular and a parallel orientation, i.e. the first six were oriented perpendicular, the last one parallel. Parallel orientation was performed in order to get a high resolution at the end of SOBP as well as at the distal dose fall-off region. Seven RW3 slabs were customized for housing 5, 6 or 7 of TLD disks (diameter: 4.5 mm, thickness: 0.89 mm), which allows simultaneous irradiation at various depths along the beam. For absolute depth dose measurements a Roos ionization chamber (Type 34001, PTW, Freiburg) was used. Results Figure 1 shows the response of two sets of 41 TLDs, where one set was evaluated with individual correction factors obtained for each TLD. The uncertainties were reduced on average from 3.7 % down to 1.3 %. Figure 2 shows depth dose measurements of EBT3 films and TLDs. The dashed line represents the measurements from the Roos chamber (errors are below 0.6%). The results from EBT-XD film evaluation are currently processed.

Conclusion The performed measurements show that individually calibrated TLD-100 detectors are well suited for proton dosimetry in a SOBP, as demonstrated by the good agreement with Roos chamber measurements. TLD measurements at larger depths need to be conducted to further study quenching phenomena. EBT3 films showed an under response of up to 11% compared to Roos chamber measurements. Orienting the EBT3 film parallel to the beam proved to be feasible in high dose gradient regions. EP-1732 Multimodal range verification for proton irradiation using MR and PET imaging A. Runz 1,2 , M. Runz 1,2 , H. Prokopp 1,2 , R. Dal Bello 1,2 , Y. Berker 1,2 , G. Echner 1,2 , P. Mann 1,2 1 German Cancer Research Center DKFZ, Medical Physics, Heidelberg, Germany 2 Heidelberger Institute for Radiooncology, Medical Physics, Heidelberg, Germany Purpose or Objective In proton therapy, various range verification systems in 1D (ionization chamber, IC), 2D (film) and pseudo-3D (IC- array) are implemented in clinical routine. However, the implementation of a systems allowing for a fast, exact and truly 3D proton range measurement would be a desirable tool which will be presented in this work. Material and Methods Proton range verification was realized by means of a modified polymer gel (PG). The gel used is the so-called PAGAT PG. It consists of 88% H 2 O and two different kind of monomers embedded within a gelatine matrix that start to polymerize upon irradiation. This process causes the relaxation time T 2 to locally vary which can be measured in 3D by magnetic resonance imaging (MRI).