GLAaS: An absolute dose calibration algorithm for an amorphous silicon portal imager. Applications to IMRT verifications
To define for the calculation a cut to the “maximum significant dose” in the field was applied. The method is described with an example: (a) a dose map for an IMRT field and (b) the dose-area histogram relative to the same field with three cut thresholds at 2%, 3%, and 5% shown on the graph. The maximum significant dose is defined as the maximum dose in the histogram after having removed the high 2 (or 3 or 5)% tail. (c) The effect of the cut in conservatively reducing the acceptance area in the space: one-dimensional representation of ellipsoidal DTA and D test for no cut (black line) and after application of the 5% cut.
Dosimetric characterization of PV-aS500. In the first row: linear response to primary or transmitted radiation for a representative case and for the different experimental conditions (measurement depths). In the second and third rows: the linear response to primary radiation for the four setups and for different field sizes from . Note the maximum (minimum) spread of data at depth.
Dosimetric characterization of PV-aS500. OF measured with PV-aS500 plotted as a function of EwwF (points) and the fit to Eq. (2) (solid line). To better assess the specificity of the a-Si detector, for comparison the dashed line refers to ion chamber measurements.
Dosimetric characterization of PV-aS500. (a) OF data as a function of EwwF for all configurations together. (b) Experimental data and fit results of vs. OF according to Eq. (3). Note the larger spread of data for depth.
Qualitative assessment of the GLAaS algorithm for three fields. From left to right: dose maps derived from PV-aS500 measurements, map comparing measured maps against Eclipse dose calculations, 1D absolute dose profiles (along the dashed lines shown in the 2D maps). Solid line is Eclipse calculations, dashed is for PV-aS500 data, the vertical scale is in Gy. Data are shown for measurements performed at depth.
Quantitative assessment of GLAaS performance for the 34 IMRT fields for which GLAaS and film dosimetry were performed at all the different depths. Data are shown as scatter plots of the percentage of field area %FA with for GLAaS (in ) versus the same quantity calculated for film dosimetry (in ). Note the good correlation for 3.8 and 1.5 depths (no data beyond the acceptance threshold of 10%) and the poorer agreement for (showing, however, an improvement in results with GLAaS compared to films).
Quantitative assessment of GLAaS performance. First row: histograms of the distribution of %FA with for the first 240 IMRT fields verified with GLAaS versus a similar group of fields verified with film. Second row: histograms of %FA with for the 110 split and 130 not-split fields verified with GLAaS. As reported in the text, significant statistical differences were observed between GLAaS and film dosimetry while no difference was observed between split and nonsplit fields.
Quantitative assessment of GLAaS performance for variable gantry angles. Scatter splot of %FA with for fields verified at 90° (diamonds) or 270° (triangles) versus the corresponding measurements at 0°. A high correlation among data sets was observed as proved by the absence of any statistical difference as reported in the text.
Summary of index evaluation on the 34 fields in the main validation experiment.
Article metrics loading...
Full text loading...