Sci—Wed PM: Delivery—07: Dosimetric Characterization of a New Pixel Ionization Chamber Array

A new ionization chamber array developed by IBA Dosimetry (Germany) is evaluated at our centre. The dosimetric characteristics of this device such as dose and dose rate dependence, stability, energy dependence, profiles and error detection ability were studied. COMPASS is an array with 1600 parallel plate ion chambers. Each chamber is 3.8 mm in diameter and 2mm in height spaced 6.5 mm apart. The array is mounted onto the wedge slot of the Varian Clinac 2100iX (Varian Medical Systems, Palo Alto, CA) linear accelerator. Linearity of the detector with dose and response to dose rate were done using Clinac 2100iX 6 MV beam. The detector showed excellent linearity response with dose and a variation of <0.2% with dose rates. Short term and long term reproducibility results showed a standard deviation of 0.05%. Energy dependence of COMPASS was evaluated by response of the device to beam energies ranging from 60 Co to 18 MV X‐rays. K Q , determined by COMPASS increases with energy as opposed to a decreased response with energy of a thimble ion chamber. Profiles measured with COMPASS were similar to Markus (plane parallel) ion chamber for field sizes ≤ 20×20 cm 2 . The chamber array showed increased dose response off‐axis for larger fields. The penumbra measured by COMPASS was 3.14± 0.03 mm vs. 1.25± 0.02 mm for film measurements for a range of field sizes. A leaf gap error of 1 mm was detected by the device using an IMRT test field. Research partially sponsored by IBA Dosimetry.