Poster — Wed Eve—27: Feasibility of Using Amorphous Silicon EPIDs for QC of MLCs

The correct delivery of dynamic IMRT treatments is dependent on proper functioning of the MLCs. Recent publications suggest that an error of 0.3 mm in leaf position could lead to a 2% error in absolute dose to the target volume. QC tests that allow quick checks of the reproducibility of leaf positions and speeds would provide additional confidence in the correct functioning of the MLCs on a frequent basis. The feasibility of utilizing amorphous silicon EPIDs for QC of MLCs was evaluated with aS1000 EPIDs. Portal images of the MLC test patterns were acquired and analyzed in Matlab. Portal images of a picket fence pattern (1 cm steps separated by standard gaps between leaf ends) were acquired at different times and collimator and gantry angles (0° and 90°). The consistency in MLC leaf positions was evaluated by calculating the second derivative of image intensity profiles, allowing analysis of an image in 1–2 minutes. The measured standard gap was consistent within 0.5 mm, indicating reproducibility over time, and at gantry angles of 0° and 90°, indicating no measurable gravitational effects. Portal images of a wedge intensity pattern were acquired at various MUs to investigate effects of MLC acceleration/deceleration. Analysis of the wedge intensity pattern demonstrated a linear relationship between dose delivered and response of the EPID indicating consistency in leaf speed. QC tests developed will allow for daily robust evaluation of the performance of MLCs at different gantry and collimator angles without using films.