ESTRO 36 Abstract Book

S767 ESTRO 36 _______________________________________________________________________________________________

used for alignment and positioning. The xyz-stage also allows for scanning and accurate field-size measurements at the reference position. The system has been characterized using an ensemble of 11 thimble ionization chambers of the types PTW 30013, IBA FC65G, NE 2571, and NPL2611. Results Automated procedures were implemented for measurement of absorbed dose to water calibration coefficients. Source irradiations and positioning was found to be highly reproducible. The relative standard deviation of dose-rate measurements with the 11 ionization chambers was less than 0.03% within each specific measurement session carried out over a period of 120 days. The collimator and the shutter systems were characterized using randomized tests run continuously over 24 h periods. Setting the field size to different values in a random order resulted in a relative standard deviation for dose rates within each filed size of less than 0.05%. Conclusion The ability to computer control irradiations has enabled development of automatic calibration and measurement procedures. This in turn has resulted in an improved quality of measurements and implementation of more comprehensive measurement sequences relative to what would have been feasible using an irradiator system with only manual source control. The special rig and the optical alignment system allowed for precise (better than 0.1 mm) positioning of ionization chambers. The system was therefore found to be highly suitable for research and calibrations involving ionization chambers and other dosimeters used in radiotherapy . EP-1438 Experimental determination of correction factors for reference dosimetry in Gamma Knife Perfexion E. Zoros 1 , E.P. Pappas 1 , K. Zourari 2 , E. Pantelis 1 , A. Moutsatsos 1 , G. Kollias 3 , C.I. Hourdakis 2 , P. Karaiskos 1 1 National and Kapodistrian University of Athens, Medical School - Medical Physics Laboratory, Athens, Greece 2 Greek Atomic Energy Commission, Division of Licensing and Inspections, Athens, Greece 3 Hygeia Hospital, Gamma Knife Center, Athens, Greece Purpose or Objective To experimentally determine machine-specific reference (msr) field correction factors (CFs) for a set of commercially available ion chambers and two dosimetry phantoms which are commonly used for the calibration of the Gamma Knife Perfexion (GK PFX) radiosurgery unit. Material and Methods Measurements were performed for both plastic spherical phantoms, referred to as acrylonitrile butadiene styrene (ABS) and Solid Water (SW), which are used in GK PFX reference dosimetry. CFs were obtained for IBA CC01, IBA CC13, PTW 31010 and Exradin A1SL ion chambers using the formalism proposed by Alfonso et al. (2008) for the dosimetry of small and non-standard photon fields. The determination of absorbed dose to water in phantom material for the msr field (16mm collimator size) was performed using EBT3 radiochromic films and alanine pellets as reference passive dosimeters whose calibration is traceable to a primary standard and do not exhibit substantial beam quality dependence. However, in order to determine absorbed dose to water in water, film and alanine measurements were corrected using phantom- dose conversion factors obtained by Monte Carlo simulations using a recently introduced EGSnrc simulation

model. Special custom made inserts to accommodate ion chambers and alanine pellets were fitted into the inserts of the ABS and SW phantoms. Detectors’ central axis was aligned with the z axis of GK PFX stereotactic space for SW measurements, while placed on x-y plane for the ABS phantom. A scanning technique was implemented for the accurate alignment of detectors’ reference point of measurement with GK PFX radiation focus. In order to estimate statistical uncertainties of the CFs five measurements were performed for each detector. Regarding ion chambers, measurements were averaged for positive and negative polarity and the obtained readings were corrected for ion recombination, temperature and pressure effects. Results For the majority of ion chambers in SW phantom CFs were up to 1.01, except of the IBA CC01 were a correction of 4% is needed mainly due to perturbation of the high density central electrode. Regarding ABS phantom larger corrections are needed up to 1.05 for IBA CC01 and CC13 and up to 1.02 for PTW 31010 and Exradin A1SL, attributed to the different orientation of the detectors in GK PFX stereotactic space. Conclusion An experimental procedure is proposed for the determination of CFs for the GK PFX radiosurgery unit and CFs were determined for a set of ion chambers allowing for accurate dosimetric measurements. Acknowledgement: This work was financially supported by the State Scholarships Foundation of Greece through the program “Research Projects for Excellence IKY/SIEMENS”. EP-1439 Small field dosimetry: preliminary characterization of a nano-chamber with a focus on stem effect A. Stravato 1 , G. Reggiori 1 , P. Mancosu 1 , F. Lobefalo 1 , L. Paganini 1 , F. Zucconi 1 , V. Palumbo 1 , S. Tomatis 1 , M. Scorsetti 1 1 Istituto Clinico Humanitas, Department of radiotherapy and radiosurgery, Rozzano Milan, Italy Purpose or Objective Micro and nano-chambers cannot be as small as solid state detectors but present some advantages in terms of energy independence and absolute dose measurement that make them fundamental for small field dosimetry in the SBRT scenario. A preliminary characterization of a nano- chamber prototype (Razor Nanochamber, IBA) was carried out with a particular focus on stem effect. Response under 10 MV FFF beams was observed too. Material and Methods The study included characterization of leakage, dose rate and dose per pulse dependence, measurement of small beam profiles, and depth dose curves. Profiles were acquired both in orthogonal (i.e chamber axis orthogonal to beam axis) and parallel (i.e chamber axis parallel to beam axis) configuration. Ten repeated inline profile measurements were performed in head-foot and foot- head direction to better quantify the stem effect. Ion collection efficiency and polarity effects were measured. The values of P ion were verified with 1/V versus 1/Q curves (Jaffé plots). The 6 MV and 10 MV FFF photon beams of a Varian EDGE were used. Output factors for small fields were compared with Razor Diode (IBA) and FOD scintillator The 2mm diameter guarantees a very high spatial resolution comparable to some commercially available diodes, with penumbra values 0.5-0.8mm larger than those measured with a Razor Diode for the same fields (Figure 1). When used with the chamber axis perpendicular to the beam axis a strong stem (and cable) effect was observed leading to asymmetric inline profiles values. Results