ESTRO 36 Abstract Book

S759 ESTRO 36 2017 _______________________________________________________________________________________________

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 for small fields. Furthermore a difference was observed between profiles performed in head-foot or foot-head direction (Figure 2). Dose rate dependence was found to be <0.3% while dose per pulse dependence showed an increasing trend but still <0.6% for a maximum dpp of 0.2 cGy/pulse. At the nominal operating voltage of 300 V the Razor Nanochamber exhibits a field size dependence of the polarity correction > 2% between the 1x1cm 2 and the 40x40cm 2 field. The OF values were compared with diode and scintillator measurements and show a good agreement for fields >20x20 mm 2 . For smaller fields the volume effect is huge and leads to strongly underestimated values. Conclusion Razor chamber is an interesting option for small field measurements. Its use in orthogonal configuration raises some stem effect issues evident when measuring inline values. Results

profiles. More measurements are required in order to fully characterize this ion-chamber. EP-1440 Monte Carlo determination of scintillator quenching effect for small radiation fields G. Valdes Santurio 1 , C. E. Andersen 1 1 Technical University of Denmark, Nutech, Roskilde, Denmark Purpose or Objective Fiber-coupled organic plastic scintillator detectors are excellent for measurement of the absorbed dose to water in small MV photon fields. This is mostly because their small active volume and their high degree of water equivalence result in an almost negligible perturbation of the radiation field. However, plastic scintillators are less ideal when we consider the signal generation and the signal detection. For the signal generation, it is known that the light yield per absorbed dose for electrons below 100 keV produces less light than electrons with higher energy which is the so-called ionization density quenching. The objective of this work was to investigate the potential implication of this quenching effect for output factor measurements in small 6MV photon beams. Monte Carlo modelling was used to compute changes in light production for different field sizes using Birks formula applied to electrons. Material and Methods The quenching effect can be predicted by the Birk´s formalism which relates the amount of light per distance travelled by a given particle to the ionization density from that particle as expressed by the collision stopping power of the medium (dE/dx). This formalism introduces the quenching parameter ( kB ), which describes that the light produced by low energy electrons is not proportional to its deposited energy. We implemented Birks formula in a modified version of the application egs_chamber which is part of the EGSnrc Monte Carlo system. The modified application scored the light output over the absorbed dose for each field size. This ratio will gives us how much the quenching effect affects for that specific field size and therefore, differences of the quenching effect when changing the field size can be estimated. Moreover, this ratio will give us how much the scintillator output factor changes when the quenching effect is taken into account. We computed light yields for square field sizes down to 0.6x0.6cm 2 with 10x10cm 2 as reference. Results The light output over the absorbed dose was calculated for all field sizes. The uncertainty of all values was less than 0.5%. Figure 1 shows the normalized scintillator light output per dose for all field sizes and their respective uncertainties (1 standard deviation). The straight line represents the mean of all the obtained values and the dashed lines represent 1% of deviation with respect to the reference. As can be deducted from the figure, all the ratios fell inside the range of +/- 1% of the deviation respect the reference.

Conclusion