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SU-E-T-129: Rescaling of IMRT Verification Deviations from Detector Arrays into the Patient
B Poppe1,2, H Looe1,2, G Heilemann1,2
1Carl von Ossietzky University Oldenburg Pius Hospital, Oldenburg.
Purpose:
To discuss a method of "rescaling" failures detected in a Gamma-Index analysis with detector arrays into the patient as a re-evaluation method in IMRT verifications.
Methods:
In a homogeneous phantom, plane signals measured with ionisation chamber arrays during IMRT field-by-field verifications can be understood as resulting from the convolution of the incident photon fluence with two spatially invariant kernels (dose deposition in the measurement plane and spatial detector response). In principle, Gamma-Index deviations between planned and the measured plane signal profiles can therefore be "back-projected" into a deviation of the photon fluence profile from the planned one which can be "forward-projected" as a dose deviation at test points in the patient. Assuming this model for each field under investigation, all Gamma-Index failures in a certain array region can thus be "rescaled" as deviations of the patient dose and evaluated by with patient related tolerances. The method is evaluated in the prostate and H&N region by use of the 2D-ARRAY729 and VeriSoft5.0. The software offers the possibility to project the position of the ionization chambers onto the patient's CT. Thereby, the field specific analysis of deviations between measurements and plan is supported by suitable imaging procedures. Simulated Gamma-Index failures (3mm/3%) have been evaluated according to the method described above and compared with direct dose calculations.
Results:
The degree of consistency between relative dose deviations predicted from 2D-ARRAY evaluations and corresponding relative dose deviations calculated within the patient is found to be in an acceptable range. The applicability in regions of inhomogeneity boundaries (air-tissue) and out of field regions was not included in this study and will be analyzed in the future.
Conclusions:
In combination with the described visualization tools, the method offers the possibility to re-evaluate the dose deviations inside the patient when Gamma-Index failures have been detected with 2D-arrays.
