Characterization of a 2D ionization chamber array for IMRT plan verification

A commercialized array of 2D pixel ionization chambers MatriXX from Scanditronix Wellhöfer was evaluated with the objective to implement for quality assurance in IMRT treatment plan verification. The device consists of 1020 chambers arranged in a 32×32 grid. The distance between the chamber centres is 7.6 mm and the volume of the chamber is 0.08 cm3. The effective point measurement of the MatriXX was verified and it agreed with the MatriXX's manual specifications. The start-up behaviour, and the short- and long-term reproducibilities of the array detector were tested. Dose linearity and energy independence were also analyzed. The results showed that the dose was linear within the range 9–800 cGy and the response of the 2D array was independent of energy for 6 and 10 MV photon beams. The MatriXX was independent of dose rate ranging from 183 to 483 cGy/min. For field sizes 3×3 cm2 and above the output factors of the 2D agreed within 1% with those obtained using the FC65-G ionization chamber. But at field size 2×2 cm2 the percentage difference was 5%. However, there was a poor correlation with differences greater than 1 mm in the penumbra region. The preliminary investigations indicate that the detector is suitable for IMRT plan verifications but corrections have to be applied in regions of high dose gradient.