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

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5.2 Phantom setup, CT scanning and preparation within Eclipse For the field-related verification process, no special phantom is necessary; it is convenient to arrange the array between plates of water equivalent material. We use a sandwich setup of water equivalent PTW RW3 plates with a stack of 3 cm below and 5 cm above the array (see Fig. 5). The phantom arrangement is CT scanned then in exactly the same way as it is later used for the verification measurements. To achieve an adequate spatial resolution during the following verification dose calculations, it is essential to scan the phantom with a sufficiently small slice thickness. We have scanned the phantom with a slice thickness of 2 mm (Fig. 6). The scanned phantom is imported via DICOM to Eclipse. Directly after import, it is convenient to define a user origin within Eclipse exactly at the effective measuring point of the central ion chamber of the array. 2D array RW3 Figure 5: For the field-related verification the 2D- ARRAY is simply located between RW3 plates. 3 cm RW3 material are below, 5 cm above the array. The effective measuring point is located in the middle of the chamber area and 5 mm below the surface of the 2D-ARRAY. By that, all fields, which are verified afterwards, are automatically positioned correctly with their isocenter at the effective measuring point of the central ion chamber. Note that it is necessary to perform the scanning and import process only once. Within Eclipse, the phantom with the correctly placed user origin can then be defined as the default phantom, which is always used for verification. 5.3 Transfer of the patient treatment fields and dose calculation Eclipse supports both the plan-related and the field-related approach for the verification of IMRT plans. After an IMRT plan is accepted for treatment, the planner has to use the function “create verification plan”, to start the verification process. Here, the user can select between several possibilities (Fig. 7). For the field-related approach with the 2D-ARRAY, the following selections are necessary (Fig. 7a): • Each patient field must be placed into separate verification plans • The couch, gantry and collimator angles must be reset to 0° Then the verification method has to be defined (Fig. 7b), which is “Phantom” in our case. The user can then select the correct verification phantom, which then can be set as default. 6 / 16 Figure 6: The scanned phantom arrangement after import to Eclipse. The phantom was scanned with 2 mm slice thickness. The user origin is marked by a red cross.
a b Figure 7: Creation of the verification plan (both plan- and field-related) within Eclipse. The correct options for the fieldrelated approach with the 2D-ARRAY are selected. For each field of the patient plan, a new verification plan is now created. The field isocenters are automatically positioned at the correct place if the user origin was properly defined (see 5.2). The created verification fields have exactly the same dosimetrically relevant treatment parameters as the real patient fields. Any changes are suspended by Eclipse. Thus, it is guaranteed that the verification fields really reflect the behavior of the real patient treatments. After the correct transfer of the treatment fields to the verification phantom, the planner can calculate the 3D dose distribution of each field. In Fig. 8, the dose distribution of an IMRT field of a complex head and neck treatment plan within the verification phantom is depicted. Note that the frontal view already shows the 2D dose distribution within the effective measuring plane of the 2D-ARRAY. 5.4 Export of the verification dose plane The dose comparison in VeriSoft requires a DICOM dose export of the 2D dose distribution within the effective measuring plane of the ion chambers. To do that, the following steps are necessary: • The frontal dose view in Eclipse must be adjusted in such a way that it shows the correct measuring plane. If the user origin was defined as discussed in section 5.2, the y-coordinate of that plane is y=0. • The frontal dose view must have the focus. The easiest way to guarantee for that is to enlarge the frontal dose view (Fig. 9a). • The plane export is started by a right-click (Fig. 9a). • Within the dose export wizard, the “absolute” dose export must be selected and the dimensions of the dose plane must be correctly set (Fig. 9b). The Eclipse dose export wizard automatically positions the center of the exported dose matrix at the field isocenter (see Fig. 9b). This function is important for the later dose comparison within VeriSoft as discussed in section 5.7 and was not available in earlier Eclipse versions. To avoid confusion with the cryptic DICOM file names, it is convenient to save all DICOM plane dose files within a special folder, which is given the patient’s name, for example. 7 / 16 Figure 8: Dose distribution of an IMRT field within the verification phantom. The dosimetrically relevant treatment parameters are identical to the patient plan and cannot be changed by the user.
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The 2D-ARRAY seven29 A new way of dosimetric verification of - PTW

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