Purpose/Objective: The determination of beam width for selected energy and focus combinations is frequently performed at the Heidelberg Ion-Beam Therapy Center (HIT). For beam width adjustment a scintillating screen detector (SD, in-house production) is used and radiographic film is employed for the subsequent quality assurance. As the width measurement results of these detectors show deviations, an iterative beam adjustment process is used. To improve and speed up this process and to replace

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... e irradiation-and evaluation-time-intensive film measurements, a comparison of 5 different detectors is currently performed. Materials and Methods: Vertically scanned carbon ion line fields with a beam width of 6-10mm were used to compare lateral beam width results from radiographic Xomat V film (Kodak), a SD, a flatpanel detector (FPD, XRD 0822 without scintillator, PerkinElmer) and a multiwire chamber (MW, MWPC Type I, Siemens) against those from a linear array of 12 pinpoint ionization chambers (IC, TM31015, PTW) as the gold standard in absolute dosimetry. For the first 3 detectors background corrected .tiff images containing the accumulated measurement of the whole scanned irradiation field were used as the result. As the MW integrates over time-increments of 400 µs, the beam width was calculated for each of these time-increments and the final result subsequently averaged over all single measurements. For the IC results, serving as the reference, the array was shifted in 2mm steps perpendicular to the scanned field and the irradiation repeated for each array position. For all detectors, the lateral beam width was obtained as the FWHM value and complementary calculated from a single Gaussian fit. Results: The comparison of the beam width measured with different detectors compared to IC measurements resulted in the maximum deviations presented in Tab. 1.