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

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• By default, VeriSoft normalizes each matrix individually to its maximum dose. This is not meaningful for absolute dose measurements with the 2D-ARRAY. Therefore, both data sets have to be normalized to the same meaningful value (e.g., the dose at the isocenter). • The correct positioning of the two data sets is checked using the “Profiles” option. Normally, no alignment should be required. However, positional corrections of ±1 mm in both x and y directions are sometimes necessary due to slight misalignments of the phantom during the measurements. Fig. 13a shows the isodose overlay, which already gives a first impression of the concurrence of the two data sets. This concurrence is quantitatively confirmed by vertical and horizontal line scans through the isocenter (Fig. 13b). Line scans have the disadvantage that an investigation of the complete area is quite timeconsuming. Therefore, VeriSoft provides additional quantitative 2D compare modes. Figure 13a: VeriSoft isodose overlay of a typical IMRT head and neck field. The measured isodoses are depicted as dotted lines. In Fig. 14, a difference plot is shown, where each measured chamber dose Dm(i,j) is compared to the averaged calculated dose according to the following formula: ∆D( i, j) D D = m ( i, j) D − c D c ( i, j) ( i, j) * 100% Note that VeriSoft provides also a percentage difference mode, which is relative to the normalization dose of matrix A. This mode however is not as sensitive to the low dose. 10 / 16 Figure 13b: Horizontal (above) and vertical (below) line scans through the isocenter of the IMRT field of Fig. 13a. Calculations are shown as blue lines and the measured chamber values are depicted as green bars.
a Figure 14: Difference plot of the dose distribution of Fig. 13a. Most differences are below 3% of the local measured dose. Higher deviations are only found at highgradient regions and at very low doses. While dose difference plots work very good within low-gradient areas of the analyzed dose distribution, they may show artificially large deviations in high-gradient parts (Fig. 14). To overcome that problem, which is especially critical for highly modulated IMRT fields, Low et al. developed the gamma analysis method [3]. The gamma method judges the concurrence of 2D dose distributions using two criteria: the local dose difference within low-gradient areas and the distance-toagreement (i.e., the distance of two points, which have the same dose). The user has to define two acceptance values: The maximum acceptable local dose difference (in %) and the maximum acceptable distance to concurrence (in mm). In VeriSoft, the gamma method is implemented according to the method of Depuydt et al. [4], which adapts the Low algorithm to the use of discrete data sets. For our analyses, we use always 3% dose difference and 2 mm distance to agreement. Calculated dose value are not analyzed for dose values below 5% of the maximum. Fig. 15 shows the resulting gamma evaluation. The acceptance criteria are not met for only a few points below the 10% isodose. This is a typical result. Two factors are responsible for higher deviations with very low doses: • The measuring device has a higher relative measuring error. • For very low doses, the dose of a dynamically collimated IMRT field is mainly determined by leaf transmission, which is not calculated as precise as the dose within the open part of the field by the treatment planning system. We normally accept these dose deviations, if they only show up in low dose areas. In chapter 6, more verification examples of IMRT fields are discussed 11 / 16 Figure 15: Gamma index plot of the dose distribution of Fig. 13a. The red dots indicate points where the acceptance criteria (3%, 3 mm) are not reached. All these points are located within low-dose regions.
Page 1 and 2: Application note The 2D-ARRAY seven
Page 3 and 4: 3. Dosimetric verification of treat
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Page 13 and 14: 6. Some clinical examples In Berlin
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The 2D-ARRAY seven29 A new way of dosimetric verification of - PTW

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