Program Information
A General Fill Factor Definition Serving to Characterise the MLC Misalignment Detection Capabilities of Two-Dimensional Detector Arrays
T.S. Stelljes1,2*, H.K. Looe11,2 , D. Harder3 , B. Poppe1,2 , (1) Clinic for Radiation Therapy, Pius-Hospital, Oldenburg, Germany,(2) WG Medical Radiation Physics, Carl von Ossietzky University, Oldenburg, Germany, (3) Prof. em., Medical Physics and Biophysics, Georg August University, Goettingen, Germany
Presentations
SU-E-T-35 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall
Purpose:
To present a general definition of the fill factor realistically characterizing the “field coverage”, i.e. the MLC misalignment detection capabilities of a detector array.
Methods:
According to Gago-Arias et al.¹ the fill factor of a 2D array is defined as the ratio of the area enclosed by the FWHM of the fluence response function KM(x) of a single detector and its cell area defined by the detector spacing. More generally - accounting also for the possible overlap between FWHM’s of neighboured detectors - the fill factor is here defined as that fraction of the sum of the detector cell areas in which a defined MLC misalignment Δ is detectable when the induced percentage signal changes exceed a detection threshold d. Ideally the generalized fill factor may reach 100 %. With user code EGS_chamber and a 2 MeV photon slit beam 0.25 mm wide, both types of the fill factor were calculated for an array with total cell area 100 cm² for chamber widths 1-9 mm, using Δ=1mm, d=5%.
Results:
For single chamber width 5 mm, fill factors were 0.49 (FWHM) and 0.61 (generalized). For chamber width 2 mm the FWHM fill factor was 0.13 whereas the generalized fill factor was 0.32. For chamber widths above 7 mm, the FWHM fill factor exceeds unity, and the general fill factor is exactly 1.00.
Conclusions:
An updated fill factor definition is introduced which, as a generalization of the FWHM-based definition, more closely estimates the performance of small array chambers and gives a realistic value in the case of overlapping sensitive areas of neighboured chambers.
References:
¹A. Gago-Arias, L. Brualla-Gonzalez, D.M. Gonzalez-Castano, F. Gomez, M.S. Garcia, V.L. Vega, J.M. Sueiro, J. Pardo-Montero, "Evaluation of chamber response function influence on IMRT verification using 2D commercial detector arrays," Phys. Med. Biol. 57, 2005-2020 (2012).
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