SU-E-J-262: Segmentation in Therapy: Impact of Display

Purpose: To determine the calibration of monitors used for structure delineation in radiotherapy and to investigate the impact of window/level on the delineations. Method and Materials: hotometers were used to measure ambient light and luminance for 12 monitors used for delineating structures in radiotherapy. Luminance was determined using AAPM TG-18 patterns TG18-LN12-01 to TG18-LN12-18. The DICOM grayscale display function (GSDF) was calculated for each monitor. To investigate the impact of window and level eight observers each outlined two sets of structures (four structures per set) on CT simulation images. In the first set window and level were initialized at random values and the observers adjusted as needed. In the second set window and level were locked at preselected values for all observers. Results: Ambient light variability at different displays was 24% and was higher than radiology reading rooms. Monitor luminance curves were substantially different than the corresponding GSDF. Monitors demonstrated higher contrast than the GSDF in the lower third of driving levels and lower contrast in the upper third of driving levels. Increased contrast at low driving levels may be to compensate for high ambient light which would otherwise reduce contrast. Abrupt changes in luminance and loss of all contrast at both upper and lower driving levels was observed. Interobserver variability was 379HU for window and 217HU for level. Outline variability was lower when the window and level settings were similar and when they were preselected. Structures without sharp boundaries (e.g., partial volume and a wide window value) resulted in substantial interobserver variability even when window/level were preselected. Conclusion: Substantial variation in monitor calibration and ambient light exists in radiotherapy. Further, the substantially different window/level contributes to outline variability. Future work will quantify the contributions of monitor calibration and window/level on outline variability and accuracy