Premium
Wide field array calibration dependence on the stability of measured dose distributions
Author(s) -
Simon Thomas A.,
Simon William E.,
Kahler Darren,
Li Jonathan,
Liu Chihray
Publication year - 2010
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.3442028
Subject(s) - calibration , dosimetry , stability (learning theory) , field (mathematics) , physics , optics , computational physics , materials science , nuclear medicine , mathematics , statistics , medicine , computer science , machine learning , pure mathematics
Purpose The aim of this work was to simulate the effect of dose distribution changes on detector array calibrations and to explore compensatory methods that are used during calibration measurements. Methods The array calibration technique that was investigated is known as wide field (WF) calibration. Using this method, a linear array [ y ‐axis (65 detectors) of the IC PROFILER ™ (Sun Nuclear Corporation, Melbourne, FL)] is calibrated with three measurements ( α , θ , and λ ); each measurement uses the same radiation field, which is larger than the array. For measurement configuration θ , the array is rotated by 180° from its position in α ; for λ , the array is shifted by one detector from its position in θ . The relative detector sensitivities are then determined through ratios of detector readings at the same field locations (using θ and λ ). This method results in error propagation that is proportional to the number of detectors in the array. During the procedure, the calibration protocol operates under three postulates, which state that (a) the beam shape does not change between measurements; (b) the relative sensitivities of the detectors do not change; and (c) the scatter to the array does not change as the array is moved. The WF calibration's sensitivity to a postulate (a) violation was quantified by applying a sine shaped perturbation (of up to 0.1%) to α , θ , or λ , and then determining the change relative to a baseline calibration. Postulate (a) violations were minimized by using a continuous beam and mechanized array movement during θ and λ . A continuously on beam demonstrated more stable beam symmetry as compared to cycling the beam on and off between measurements. Additional side‐scatter was also used to satisfy postulate (c). Results Simulated symmetry perturbations of 0.1% to θ or λ resulted in calibration errors of up to 2%; α was relatively immune to perturbation ( < 0.1 % error). Wide field calibration error on a linear accelerator with similar symmetry variations was ±1.6%. Using a continuous beam during θ and λ with additional side‐scatter reduced the calibration error from ±1.6% to ±0.48%. Conclusions This work increased the reproducibility of WF calibrations by limiting the effect of measurement perturbations primarily due to linear accelerator symmetry variations. The same technique would work for any array using WF calibration.