ESTRO 36 Abstract Book

S768 ESTRO 36 _______________________________________________________________________________________________

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 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 The study shows that for this specific value of kB studied and all the limitations of the model, the quenching effect will not significantly affect the scintillator output factor measurements in small 6 MV photon fields, and the quenching correction factor will be therefore close to the unity (u=1%). The impact of the stem signal (i.e. Cerenkov and fluorescence light produced in the optical fiber cable during irradiations) therefore remains the main influencing factor on such measurements. EP-1441 Repurposing of a small clinical x-ray source for radiobiology irradiations M. Barry 1 , R. Jones 1 , M. Fay 1 , D. Butler 2 , J. Lehmann 1 1 Calvary Mater Newcastle, Department of Radiation Oncology, Newcastle- NSW, Australia 2 Australian Radiation Protection and Nuclear Safety Agency, Radiation Therapy Section, Yallambie, Australia Purpose or Objective Around the clock availability of irradiation capability is desirable for creative design of radiobiology experiments. Clinical radiation systems are often only available after hours. Dedicated commercial cell irradiation systems are bulky and expensive. They might not be feasible due to financial or administrative constraints. This work reports on the repurposing of a retired clinical intraoperative kV irradiation system (Intrabeam™) for cell irradiations. Material and Methods The Intrabeam system is designed to deliver spherical dose distributions to surgical cavities. The in the adaption for cell irradiations, one of the supplied applicators has been modified to be fitted with custom collimators aiming to deliver a homogenous field across the cell dish. Several collimator designs have been tested and measured used using radiochromic EBT3 film. Additionally, measurements without a collimator were done in comparison and to support Monte Carlo simulations. Film calibrations were performed with national standard beams covering the energy range of the device. The BEAMnrc code and the 'NRC swept BEAM” source model have been used to characterize the dose distributions and to aid collimator development. Results Using the film measurements, the parameters of the Monte Carlo source model (swept angle and beam radius) were tuned to produce the final model. Very good agreement between measured and simulated dose profiles for the open source at 5, 7.5 and 10 cm distance from the tip was observed. (Figure 1)