The OD1500 array's dosimetric properties showed the applicability of the array for clinical dosimetry with the possibility to increase the spatial sampling frequency and the coverage of a dose distribution with the sensitive areas of ionization chambers by merging two measurements.
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.
Purpose: To characterize the dose response function K(x) and to analyze the dependence on changing photon spectra of a novel synthetic single crystal diamond prototype detector (microDiamond, T60019, PTW‐Freiburg). Methods: The K(x) of the microDiamond was examined by scanning a narrow photon field, with the detector symmetry axis arranged towards the photon source, at 6 and 15 MV. The same dose profiles were scanned with a Si diode, for which the K(x) is already known, to obtain D(x). In a search process, the D(x) were numerically convolved with normalized one‐dimensional Gaussian kernels K(x) of varying o. The best fit between the convolution product D(x)*, K(x) and the measured profile M(x) of the microDiamond was used to determine σ. Furthermore, profiles were compared with ion‐chamber (PTW Semiflex 31010) measurements at different field sizes and depths to study its spatial resolution, output factor and out‐of‐field measurement characteristics. The EPOM was determined by comparing the PDDs against those obtained with a Roos chamber. Results: The optimal σ of K(x) of the microDiamond was found to be 1.14 mm, which is comparable to the detector dimensions (radius = 1.1 mm). The microDiamond profiles agree well with the ion‐chamber measurements within regions where the volume effect of the ion‐chamber can be neglected. At 10 cm depth and for field sizes between 4×4 cm2 and 20×20 cm2 the output factors measured with the microDiamond and ion‐chamber agree better than 1% thus setting a limit for a possible energy dependence of the detector. This is underpinned by the good agreement of the out‐of field doses between ion‐chamber and microDiamond. The vendor specified EPOM was also verified (1.3 mm below the surface). Conclusion: Our study indicates that the characteristics of the microDiamond detector are well suited for accurate dosimetry within the investigated field sizes and depth limits.
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