34A Small field dosimetry for electron beams using four types of detectors

Abstract

Introduction Accurate calculation of dose by Treatment Planning System (TPS) depends on the quality of the measured data used to configure the beam models. It is advisable to choose the most suitable detector for the experimental conditions and the need of precision required for the knowledge of the absorbed dose and its spatial distribution. Methods The purpose of this work is to study the dosimetric characterization of the small electron fields ranging from 2 to 6 cm for energies of 6, 9, 12, 1 6 and 20 MeV generated by the linear accelerator ClinaciX (Varian). Percentage depth dose (PDD), OAR dose profile, and output factor (OF) were performed by the four different types of detectors manufactured by PTW-Freiburg® (Cylindrical chamber Micro-ion chamber PTW Pinpoint 3D 0.016 cm3, plane parallel ion chamber Markus PTW 23343 and two solid detectors: PTW60017 unshielded diode, and PTW 60019 Micro-Diamond). Results The PDDs showed an agreement of ±1% between values measured by the diode and the microdiamond detector. The two solid detectors are characterized by their small sensitive volume suitable for dosimetry in small fields. These results are similar when compared to those found by Laub et al. [1] . A discrepancy of 2.5% was found between the Markus chamber and the Micro-Diamond. This disagreement was most important in the region of high gradient dose and in the area near the surface where the detectors are partially in the air. The diamond detector and the diode provided the curve directly in dose while the depth-dependent corrections were necessary for the ionization chamber Markus [2] . The diode has the narrowest penumbra with a mean difference of 0.6 mm compared to the one measured by the diamond detector. PinPoint ionization chamber shows the lowest spatial resolution, with a widening of the penumbra compared to the one measured by the diode and the diamond. The difference found between the output factors OF between the Markus chamber and the diamond detector was less than 2% except for the field size of 2 × 2 cm2 and this for all energies and that because of the relatively large volume of the Markus ionization chamber. Finally, a good agreement between the two solid detectors was reported except for 6 MeV for the field size of 2 × 2 cm2. Conclusion Dosimetric properties observed indicate that microdiamond and diode detectors are the most efficient for dosimetry of small electron field used in radiotherapy.