ESTRO 38 Abstract book

S473 ESTRO 38

Results Particle type dependent WET-offset values < 1 mm were measured, while chamber length was found in agreement to the value provided by the vendor. Measurements showed a high repeatability: mean relative standard deviation was within 0.5% for all channels and both particle types. Moreover, the detector response was linear with dose (R 2 > 0.99) and independent on the dose rate. The mean deviation over the channel-by-channel readout respect to the reference beam flux was equal to 0.2% and 0.6% for protons and carbon ions respectively. The long- term stability of the gain calibration was very satisfying for both particle types, with values of channel mean relative standard deviation less than 1% for all the acquisitions performed at different times. Merging of measured data with and without 1-mm plate provided the right trade-off between accuracy and measurement time for QA purposes. Against reference curves in water, deviations in BP position were < 1 mm for both particle types in the whole investigated energy range. Similar results were found for modulated SOBPs against expected values.

Conclusion This study presents a novel method combining 3D PG and TL dose measurements for the purpose of absolute 3D dose measurements that can also be applied in complex anthropomorphic phantoms using only a single measurement. The method was validated for two different irradiation geometries including a homogeneous large field as well as a small field irradiation with sharp dose gradients. PO-0894 Characterization of a multilayer ionization chamber for relative depth-dose curves in particle beams A. Vai 1 , D. Maestri 1 , G. Magro 1 , A. Mairani 1,2 , E. Mastella 1 , A. Mirandola 1 , S. Molinelli 1 , S. Russo 1 , M. Togno 3 , S. La Civita 3 , M. Ciocca 1 1 National Center for Oncological Hadrontherapy CNAO, Medical Physics Department, Pavia, Italy ; 2 Heidelberg Ion Beam Therapy Center HIT, Physics Department, Heidelberg, Germany ; 3 IBA Dosimetry, Research and Development, Schwarzenbruck, Germany Purpose or Objective To characterize a commercial multilayer ionization chamber (MLIC) detector primarily designed for measurements of integrated depth-dose (IDD) curves of both proton and carbon ion scanning beams, for QA purposes. Material and Methods The commercial MLIC Giraffe (IBA Dosimetry, Schwarzenbruck, Germany) is a device developed for integrated depth-dose (IDD) curves measurements of a central axis pencil-beam, with FWHM ranging between 0.5 to 3 cm. Preliminary, water-equivalent calibration factory values were verified against the reference curves acquired with the Peakfinder water column (PTW GmbH, Freiburg, Germany). For both protons and carbon ion beams, the device was tested in terms of short- and long term repeatability, linearity and dose-rate independence. Accuracy of the detector was tested by evaluating range (distal R 90 ) of IDD curves for a set of representative energies for both particle species against reference acquisitions in water. Two modulated proton spread-out Bragg peak (SOBP) were measured and range compared to theoretical values. To increase the native spatial resolution (approx. 2 mm), consecutive acquisitions were acquired respectively with and without build-up PMMA plates. The possibility of a double- (none + 1-mm WET thick) or triple-merge (none + 1-mm + 0.44-mm WET thick) was investigated in terms of accuracy.

Conclusion To our best knowledge, this is the first time the detector has been used with carbon ion beams. The Giraffe was proved to be accurate, linearly responding with dose, precise and easy to handle for QA beam energy checks of both protons and carbon ion beams. PO-0895 Anthropomorphic breathing phantom with lung and liver components for testing MR-guided radiotherapy E. Colvill 1 , M. Krieger 1 , Y. Zhang 1 , S. Safai 1 , D.C. Weber 1 , A.J. Lomax 1 , G. Fattori 1 1 Paul Scherrer Institute, Center for Proton Therapy, Villigen, Switzerland Purpose or Objective Magnetic resonance (MR) imaging is widely recognised as a key element in many new frontiers of radiotherapy such as daily treatment adaptation and motion compensated dose delivery. Currently available phantoms for treatment quality assurance are however limited in realism and provide poor MR imaging texture. Therefore, renewed effort went to develop an anatomical model with good imaging contrast for testing new sequences and image- guided radiotherapy techniques. Material and Methods We expand on an existing anthropomorphic breathing thorax phantom by making use of 3D printing to develop a new, anatomically correct, lung model with improved mechanical properties and tissue characteristics. The respiratory tract including trachea and main bronchi has been segmented from clinical data and used as inlet for ventilation. The lung infill modulated during printing to