Dosimetric characterization of an ion chamber matrix for radiotherapy QA

A commercial ion chamber matrix was examined of its characteristics and performance for radiotherapy quality assurance. The device was the I'mRT MatriXX (Scanditronix-Wellhofer, Schwarzenbruck, Germany). The MatriXX device consists of a 1020 vented ion chamber array, arranged in 24x24 cm2 matrix. Each ion chamber has a volume of 0.08 cm3, spacing of 0.762 cm and minimum sampling time of 20 min. For the investigation of the characteristics, dose linearity, output factor, short term reproducibility and dose rate dependency were tested. In the testing of dose linearity, it has shown a good signal linearity within 1% in the range of 1-800 cGy. Dose rate dependency was found to be lower than 0.4% (Range: 100-600 MU/min) relative to a dose rate of 300MU/min as a reference.Output factors matched very well within 0.5% compared with commissioned beam data using an ionization chamber (CC01, Scanditronix-Wellhofer, Schwarzenbruck, Germany) in the range of field sizes 3x3-24x24 cm2. Short term reproducibility (6 times with an interval of 15 min) has also shown a good agreement within 0.5%, when the temperature and the pressure were corrected by each time of measurement. In addition, we compared enhanced dynamic wedge (EDW, Varian, Palo Alto, USA) profiles from calculated values in the radiation planning system with those from measurements of the MatriXX. Furthermore, a non-uniform IMRT dose fluence was tested. All the comparison studies have shown good agreements. In this study, the MatriXX was evaluated as a reliable dosimeter, and it could be used as a simplistic and convenient tool for radiotherapy quality assurance