The 2D-ARRAY seven29 A new way of dosimetric verification of - PTW

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a b Figure 9: DICOM export of the correct dose plane with y=0. Note, that the center of the exported dose matrix is automatically positioned at the isocenter. 5.5 Phantom setup and calibration measurement The Phantom setup for the measurements is shown in Fig. 10. The gantry and collimator angles are set to 0°. The 2D-ARRAY is located on top of a package of 3 cm RW3 material and adjusted in a way that the isocenter is positioned at the effective measuring point of the central ion chamber, i.e., 5 mm below the front plate of the array (Fig. 10a). Then another stack of 5 cm RW3 material is added above the array (Fig. 10b). Finally, the SSD of this arrangement should be 94.5 cm. a Figure 10: Phantom setup for the verification measurements. The isocenter is adjusted to the effective measuring point of the central ion chamber of the 2D-ARRAY. Although the 2D-ARRAY is already calibrated in absorbed dose to water, normally each measurement must be corrected for different air pressure and temperature, for the used photon quality and for possible non-water equivalent properties of the phantom. To avoid these corrections, which are time-consuming and susceptible to errors, we use a simple calibration field with a known dose, which is delivered before each verification measurement: • Within the treatment planning system, a simple “calibration plan” with only one homogeneous field with a field size of 10x10 cm 2 and a gantry angle of 0° is created. Beam quality and isocenter position are identical to those of the IMRT fields. • The monitor units of that field are adjusted to an isocenter dose Dcal of e.g., 2.00 Gy. • After positioning of the phantom (Fig. 10), this calibration field is always treated as the first field of a verification session and the dose is measured with the 2D-ARRAY using MatrixScan. 8 / 16 b
• The readout M of the central chamber is noted. This chamber can be selected in the “options” menu of MatrixScan (chamber coordinates [14,14]). The dose is then visible at the status bar of MatrixScan. • A calibration factor f = Dcal/M is calculated, which is used for the correction of all later verification measurements (see Section 5.7). This calibration method has the additional advantage that it also corrects for possible deviations in the output calibration of the linear accelerator. 5.6 Verification measurements In Berlin, all verification measurements are performed in the clinical mode under the control of VARiS Vision. The verification fields are prepared within VARiS Vision in its own treatment course, which is linked to the patient for whom the verification is performed. Unlike the real treatment fields, however, the verification fields do not add up any dose to the patient within VARiS Vision. Immediately after the delivery of the calibration field (see section 5.5), the IMRT fields are irradiated one after the other and measured with the acquisition software MatrixScan. In Fig. 11, a screenshot of MatrixScan during the measurement of an IMRT field is depicted. To block as little machine time as possible, only the raw data are saved and no further corrections are performed during the measurements. If the calculated DICOM dose plane data were already saved in a special patient folder as proposed in section 5.4, it makes sense to store the measurements within the same folder. 5.7 Analysis of the measurements using VeriSoft and documentation of the results To guarantee a smooth and fast data analysis process, we suggest using the following procedure for each calculated and measured field pair: • The calculated data set is read into matrix A. For small fields, it is worth defining an appropriate region of interest to avoid too many data points outside the interesting area. • The corresponding measured data set is read into matrix B. • The measured data set is calibrated by pushing the “Calibrate” button. Within the calibration window (Fig. 12), the measured data set is now multiplied by the determined correction factor f = Dcal/M with Dcal = 2.00 Gy (see section 5.5). In the example shown in Fig. 12, the measured central chamber dose M of the calibration field was M = 2.02 Gy. 9 / 16 Figure 11: Screen shot of MatrixScan during a measurement of an IMRT field. Figure 12: Calibration procedure. In this example, the measured data set is corrected by a factor 2.00/2.02 = 0.99.
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The 2D-ARRAY seven29 A new way of dosimetric verification of - PTW

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