ESTRO 38 Abstract book

S911 ESTRO 38

polyethylene with a cavity adapted to the size of the ion chamber used was employed (fig. 1). This cavity was constructed so as to make the center of the phantom coincide with the reference point of the chamber. The phantom is comprised of a half sphere joined to a cylindrical region. The center of the phantom was located at 10 cm depth from the spherical region surface. The phantom center was positioned at the unit isocenter. The phantom spherical region pointed upwards, being the chamber perpendicular to the couch plane. In this way, it was possible to rotate the gantry between 90º and 270º without moving the couch. In each measurement 200 MU were delivered using a 600 MU/min dose rate and a 4x4 cm 2 field size. Gantry angles between 90º and 270º in 10º steps were used, performing three measurements per angle and calculating the average and the standard deviation for each position. Results were normalized to the value obtained at 0º (ion chamber parallel to the radiation beam).

Figure 2: CC04 angular dependence in percentage deviation with respect to 0º value as a function of gantry angle. Results agree with the IEC 60731 document, which states that the angular dependence of an ion chamber should not exceed a 1% deviation in the angular interval comprised between ±40º (maximum deviation was 0.45% at -40º). However, for gantry angles greater than 40º an increasing angular dependence was found. In figure 2 the percent deviations of the normalized values as a function of the gantry angle is depicted. Some symmetry with respect to 0º is observed mainly due to the cylindrical geometry of the chamber as well as due to the irradiation geometry. Deviations up to 3.6% with respect to the 0º value were observed (at -90º). For each angle the standard deviation did not exceed 0.1%, so it was decided not to perform a higher number of measurements per gantry position.Figure 2: CC04 angular dependence in percentage deviation with respect to 0º value as a function of gantry angle. Conclusion The angular dependence of a CC04 ion chamber was obtained for an angular interval between 90º and 270º. This dependence is in accordance with the IEC 60731 document, but it increases as the angle deviates from 0º up to 3.6% at -90º. Considering these results it is recommended that the angular dependence of the detectors used be known by the user, being this data provided by the manufacturer or measured by the user itself. A proper characterization of this dependence could be employed to calculate correction factors to be applied when using an ion chamber. The angular dependence of a CC04 ion chamber was obtained for an angular interval between 90º and 270º. This dependence is in accordance with the IEC 60731 document, but it increases as the angle deviates from 0º up to 3.6% at -90º. Considering these results it is recommended that the angular dependence of the detectors used be known by the user, being this data provided by the manufacturer or measured by the user itself. A proper characterization of this dependence could be employed to calculate correction factors to be applied when using an ion chamber. EP-1694 Evaluation of a new portal dosimetry solution for dose quality control of Elekta and Varian linacs S. Couespel 1 , N. Delaby 1 , S. Sorel 1 , C. Boutry 2,3 , C. Lafond 1,4 1 Centre Eugène Marquis, Ille-et-Vilaine, Rennes, France ; 2 Groupe Oncorad Garonne, Tarn-et-Garonne, Montauban, France ; 3 Dream SAS, Haute-Garonne, Toulouse, France ; 4 Univ Rennes- CLCC Eugène Marquis- Inserm- LTSI-UMR 1099, Ille-et-Vilaine, Rennes, France Purpose or Objective The aim of this study is to evaluate a new portal dosimetry solution (ARTISCAN Beam QA, AQUILAB - France) for independent QA of photon and electron beams on Elekta and Varian linacs.

Figure 1: Home-made phantom used with a cavity adapted to the CC04 ion chamber size. Measurements were performed on a Varian Clinac iX using a 6MV beam energy. A CC04 ion chamber and a Bahnhofstrasse 5 (IBA Dosimetry) electrometer were utilized. A home-made phantom made of high-density polyethylene with a cavity adapted to the size of the ion chamber used was employed (fig. 1). This cavity was constructed so as to make the center of the phantom coincide with the reference point of the chamber. The phantom is comprised of a half sphere joined to a cylindrical region. The center of the phantom was located at 10 cm depth from the spherical region surface. The phantom center was positioned at the unit isocenter. The phantom spherical region pointed upwards, being the chamber perpendicular to the couch plane. In this way, it was possible to rotate the gantry between 90º and 270º without moving the couch. In each measurement 200 MU were delivered using a 600 MU/min dose rate and a 4x4 cm2 field size. Gantry angles between 90º and 270º in 10º steps were used, performing three measurements per angle and calculating the average and the standard deviation for each position. Results were normalized to the value obtained at 0º (ion chamber parallel to the radiation beam).Figure 1: Home-made phantom used with a cavity adapted to the CC04 ion chamber size. Results Results agree with the IEC 60731 document, which states that the angular dependence of an ion chamber should not exceed a 1% deviation in the angular interval comprised between ±40º (maximum deviation was 0.45% at -40º). However, for gantry angles greater than 40º an increasing angular dependence was found. In figure 2 the percent deviations of the normalized values as a function of the gantry angle is depicted. Some symmetry with respect to 0º is observed mainly due to the cylindrical geometry of the chamber as well as due to the irradiation geometry. Deviations up to 3.6% with respect to the 0º value were observed (at -90º). For each angle the standard deviation did not exceed 0.1%, so it was decided not to perform a higher number of measurements per gantry position.