Room: Exhibit Hall | Forum 4
Purpose: The correction factors necessary for reference and non-reference measurement conditions, as well as the effective point of measurement (EPOM) and the lateral dose response functions K(x), have been determined for a novel PinPoint ionization chamber (type 31023, PTW Freiburg, Germany) replacing the 31014 chamber.
Methods: Beam quality correction factors have been determined by Monte-Carlo simulations. The saturation correction factors have been measured for different dose-per-pulse (DPP) values. The polarity effect correction factors were obtained for field sizes ranging from 5x5 cmÂ² to 40x40 cmÂ². For 6 MV photon beam, non-reference condition correction factors have been computed by varying the field size and measurement depth. The EPOM of the chamber has been determined by comparing the measured PDD curve with a reference curve obtained with the Roos chamber. The functions K(x) have been approximated by Gaussian functions.
Results: Due to a thicker central electrode and new guard ring design, the beam quality correction factors of the novel chamber are closer to unity than those of the 31014. The saturation correction factors are between 1.0017 and 1.0024 for the DPP used. Under reference conditions, the polarity effect correction factors (TG-51) are 0.9989 and 0.9996 for 6 MV and 10 MV respectively. Non-reference condition correction factors deviate by not more than 1% from unity. For 6 MV photon beam, the EPOM was shifted by 0.55 mm (radial chamber orientation) and 0.97 mm (axial chamber orientation) from the reference point towards the source. The standard deviation of the K(x) is 0.75 mm (radial chamber orientation) and 0.80 mm (axial chamber orientation) for 6 MV photon beam.
Conclusion: Correction factors as well as the EPOM and K(x) have been determined for the new chamber. The new chamber also shows improved dosimetric characteristics compared its predecessor in terms of saturation behaviour and the polarity effect.