Sci—Fri PM: Delivery — 06: Checking MLC Leaf Gap Consistency for Accurate Dynamic IMRT Delivery by Measurements of the Dosimetric Leaf Gap Using Two Methodologies

This study is to evaluate the methods of checking MLC leaf gap consistency by measuring dosimetric leaf gap (DLG) using both ionization chamber and electronic portal imaging device (EPID). We have derived DLG from measurements of sliding window (SW) fields using an ionization chamber placed at central axis in a solid water phantom. These measurements were performed at four different gantry angles (0, 90, 180 and 270 degrees) and using different number of MLC control points. EPID measurements of SW fields were used to derive the DLG distribution in 2 dimensions. Systematic leaf gap errors of ±0.2 mm, and ±0.5 mm were introduced to test the sensitivity of using ion chamber to detect leaf gap variations. Similar errors were introduced in four off‐axis areas in each SW field to test the sensitivity of using EPID to detect them. The DLG is ∼0.5mm greater for SW fields only specified at start and end positions (2 control points) than those with control points specified every 1 cm or less. Ion chamber measurements show that standard deviation of DLG at different gantry angles is 0.04mm. The ion chamber measurements detected correctly the leaf gap changes of ±0.2 mm and ±0.5 mm, while EPID detected the leaf gap errors introduced in the four isolated off‐axis areas. Both ion chamber and EPID based DLG measurements are able to detect small changes of leaf gaps in dynamic MLC delivery. EPID measurements of SW fields are useful to check leaf gap consistency in 2D.