SU‐E‐T‐135: Dosimetric Properties of the OCTAVIUS Detector 1500

Purpose: In this study the dosimetric properties of the Octavius Detector 1500 array (PTW‐Freiburg‐Germany) are investigated. Methods: The chambers of the array, each with an entrance window of 4.4 × 4.4 cm2, are arranged in a checkerboard pattern in a measurement area of 27 × 27 cm2 with a sampling frequency of 0.1 mm−1 along each row which can be doubled by merging two measurements shifted by 5 mm. Linearity, stability and output factors were measured with either a Semiflex 31013 or 31010 as a reference detector. Output factors were additionally measured with a Diode 60012. The effective point of measurement was determined by comparing TPR curves of the array with Roos chamber 34001 measurements. The lateral dose response function of a single chamber was determined by comparison with a high resolution diode. An IMRT field verification was carried out with a merged OD1500 measurement. Results: The OD1500 was stable within ±0.15 %. Deviations in linearity did not exceed 1% from 5 to 1000 MU. The effective point of measurement was 8.2 mm below the surface. Deviations in output factors were below 0.77 % from 5 × 5 to 27 × 27 cm2. As expected for the smallest field of 1 × 1 cm2, the deviation from the diode was significant. The widths of the lateral dose response functions were σ6 = (2.07 ± 0.03) mm and σ1 5 = (2.09 ± 0.03) mm. Gamma Index passing rates for typical IMRT and VMAT plans were above 90 % compared to film and TPS calculations for a local 3 mm / 3 % criterion. Conclusion: The first measurements with the OD1500 array show the excellent applicability of the array for clinical dosimetry. The response of the array to the mean photon energy and dose per pulse are under investigation.