Experimental and Monte-Carlo characterization of the novel compact ionization chamber PTW 31023 for reference and relative dosimetry in high energy photon beams

Abstract

IntroductionThe aim of the present work is to perform dosimetric characterization of a novel vented PinPoint ionization chamber (PTW 31023, PTW-Freiburg, Germany). This chamber replaces the previous model (PTW 31014), where the diameter of the central electrode has been increased from 0.3 to 0.6mm and the guard ring has been redesigned. Correction factors for reference and non-reference measurement conditions were examined. Materials and methodsMeasurements and calculations of the correction factors were performed according to the DIN 6800-2. The shifts of the effective point of measurement (EPOM) from the chamber's reference point were determined by comparison of the measured PDD with the reference curve obtained with a Roos chamber. Its lateral dose response functions, which act according to a mathematical convolution model as the convolution kernel transforming the dose profile D(x) to the measured signal M(x), have been approximated by Gaussian functions with standard deviation σ. Additionally, the saturation correction factors kS have been determined using different dose-per-pulse (DPP) values. The polarity effect correction factors kP were measured for field sizes from 5cm×5cm to 40cm×40cm. The influence of the diameter of the central electrode and the new guard ring on the beam quality correction factors kQ was studied by Monte-Carlo simulations. The non-reference condition correction factors kNR have been computed for 6MV photo beam by varying the field size and measurement depth. Comparisons on these aspects have been made to the previous model. ResultsThe shifts of the EPOM from the reference point, Δz, are found to be −0.55 (6MV) and −0.56 (10MV) in the radial orientation and −0.97mm (6MV) and −0.91mm (10MV) in the axial orientation. All values of Δz have an uncertainty of 0.1mm. The σ values are 0.80mm (axial), 0.75mm (radial lateral) and 1.76mm (radial longitudinal) for 6MV photon beam and are 0.85mm (axial), 0.75mm (radial lateral) and 1.82mm (radial longitudinal) for 15MV photon beam. All σ values have an uncertainty of 0.05mm. The correction factor kS was found to be 1.0034±0.0009 for the PTW 31014 chamber and 1.0024±0.0007 for the PTW 31023 chamber at the highest DPP (0.827mGy) investigated in this study. Under reference conditions, the polarity effect correction factor kP of the PTW 31014 chamber is 1.0094 and 1.0116 for 6 and 10MV respectively, while the kP of the PTW 31023 chamber is 1.0005 and 1.0013 for 6 and 10MV respectively, all values have an uncertainty of 0.002. The kP of the new chamber also exhibits a weaker field size dependence. The kQ values of the PTW 31023 chamber are closer to unity than those of the PTW 31014 chamber due to the thicker central electrode and the new guard ring design. The kNR values of the PTW 31023 chamber for 6MV photon beam deviate by not more than 1% from unity for the conditions investigated. DiscussionsCorrection factors associated with the new chamber required to perform reference and relative dose measurements have been determined according to the DIN-protocol. The correction factor kS of the new chamber is 0.1% smaller than that of the PTW 31014 at the highest DPP investigated. Under reference conditions, the correction factor kP of the PTW 31023 chamber is approximately 1% smaller than that of the PTW 31014 chamber for both energies used. The dosimetric characteristics of the new chamber investigated in this work have been demonstrated to fulfil the requirements of the TG-51 addendum for reference-class dosimeters at reference conditions.